Better reporting standards are needed to enhance the quality of hop testing in the setting of ACL return to sport decisions: a narrative review

Food for thought on a new study (and infographic) about better hop hop reporting “Cite: Better reporting standards are needed to enhance the quality of hop testing in the setting of ACL return to sport decisions: a narrative reviewhttps: // bjsm / content / early / 2020/06/10 / bjsports-2019-101245 with subsequent infographics.

1️⃣ The first point the authors mention is that many studies do not report the test order, and whether this can affect the test results due to fatigue. But in my practice, it may be more important to test jump tests before strength tests so that the patient is not tired. Now, for example, I use MyJump2 App and I can see a decrease in jump height after each attempt. At Aspetar we have a 6 week test battery where the patient undergoes jump testing on a force platform and subsequent isokinetic testing. I would imagine that the jump tests would be affected if we did it the other way around. What do you guys think of the order of tests?

2️⃣The next point is reporting which leg will be tested first. Most people do not test the leg operated first and compare LSI.

3️⃣The third point is the break between tests where between 30 sec and 3min have been reported in included studies. Testing can be of lasting size, and if you have many tests, it can take a long time. For logistical reasons at a clinic, you would probably see shorter breaks. Maybe you should include test days in the rehabilitation plan where the patient needs to book a double session? What do you do at the clinics, find in testing time-consuming?

4️⃣The fourth point is which test result to take. Someone takes the best attempt based on 3 attempts. Others take the average of 3 attempts. What do you do? I personally would take the best result, but I have no specific considerations about it.

5️⃣Fifth point is about standard landing landing. The study mentions “Only six studies mentioned any landing requirements with descriptions ranging from ‘stable’, ‘controlled’, ‘without losing balance’ and either ‘hold for 2s’, or ‘hold for 2-3s’.” Personally, I use controlled landing. But it is a slightly vague wording. Maybe holding for 2s would be better? what do you think?

6th Sixth point is familyization and practice trials.
In relation to hop tests, I think there must have been a lot to gain from familyization. But could send the tests to patients 2 weeks before and ask them to practice. If you have practical tests in the same session, you may risk retiring the patient. What do you think about that?

7️⃣The seventh point is the standardization of heating. The study recommends general cardio activity (eg, stationary cycling or jogging performed at approximately 60% of maximum perceived effort) and 5 min task-specific activities such as squats, lunges, practice jumps / hops, etc.

Other considerations they make are the patient’s mental readiness on the test day, how to measure (for example, measure from toe to heel LSI in a hop test). In addition, studies may also consider reporting minimal detectable change (MDC) to exclude noise in the test.

This post has been meant as any thoughts I have made after reading the article and what challenges I see in daily practice.


Flatting the curve. Effect on 16 weeks Squat training and 8 weeks detraining. 1981 study

A good old 1981 paper on squat training and detraining after a 24 weeks intervention. (Link to article).

Showed elite group of weightlifter showed a huge improvement in squat training (30%). As soon as they stopped squatting and training there was a rapid strength decline loosing about 15% of there strength over a 4 weeks period, however this decrease plateau for another 4 weeks at a level there still significant higher than pre intervention.

A experimental group trained with barbells three times per week with a program consisting mainly of dynamic squat exercises about 75% of the
contractions were concentric, and 25% was eccentric.

The training. was progressive with weekly increasing loads (80-100% concentric and 100-120% eccentric of 1 RM) and an increasing number
of lifts (16-22 per exercise).
The concentric contractions included one to six repetitions per set and the eccentric contractions of one to two repetitions per set (3-4 s in duration). The training period lasted 16 weeks and was follow by a 8-week detraining period, therefore the total duration being 24 weeks.

To prevent injuries and make the training program more interesting, light concentric exercises for the trunk, arms, and legs were included in each training session. During the detraining period strength training was terminated totalily, but the subjects maintained their normal daily activities.


Early phase of ACL rehabilition

Contents of Article

  1. Aim
  2. Introduction
  3. Functional anatomy
  4. Mechanism
  5. Clinical examination
  6. Rehabilitation plan after anterior cruciate ligament reconstruction
  7. Pre-surgery / pre-op / pre-rehabilitation
  8. Post op ‘Early phase’
  9. References
  10. About the Author and disclaimer


The aim with this blog series is to share some thoughts about rehabilitation, reconditioning and return to sport and performance following an Anterior Cruciate Ligament (ACL) injury.

We hope that it will inspire both health professionals and athletes recovering from ACL injuries. 


An ACL injury is a devastating injury for athletes of all levels and often sidelines them from their sport. When the ACL ruptures, the knee often becomes unstable with impaired limb function and decreased quality of life (QoL). The goal for the rehabilitation process is to allow the athlete to return to their pre-injury strength and function, while also minimizing the risk of re-injury and possible early onset of osteoarthritis. Luckily, ACL injuries are NOT a common injury in sports, with an overall incidence rate 1.52 per 10.000 athletic exposure (AE) across different sports (Montalvo et al., 2018).

An ACL injury commonly requires extensive rehab before returning to their pre-injury strength level. Having a plan and following a roadmap can optimize the rehabilitation process, which is crucial for a successful long-term return to their sport.

Athletes with ACL injuries tend to have very high expectations prior to ACLR, which do not match average outcomes. In a study by Feucht et al. in 2014, all of the athletes expected to return to normal knee function within 12 months of surgery, 91% expected to return to sport within one year of surgery and 98% expected little to no increased risk of knee osteoarthritis after ACLR.

Even though ACL reconstruction (ACLR) is the most common treatment for ACL injuries, surgery does not always result in a return to pre-level activity. A study published by Ardern et al. (2014) showed that only 63% of athletes returned to pre-injury level, 44% returned to competitive sports and approximately 65% did not return to pre-injury level after 12 months. However, 80% of the ACL patients in this study returned to some form of sport within 1 to 2 years post-op (Ardern et al., 2014). Among elite athletes, return to sport (RTS) rates are much higher with 83% returning to pre-level sport within 13 months post-op ACLR (Lai et al., 2018), and 94% of elite footballers (Ekstrand, 2010).

With the low numbers of RTS in mind, ACLR is not always the right treatment for ACL injuries, and RTS depends on many multifactorial contextual factors.

In fact, one prospective RCT by Frobell et al. (2013) found that by delaying the ACLR at least 10 weeks, 50% of the athletes did not need a ACLR at all. After 5 years, no difference was found between ACLR or those treated conservatively with rehabilitation alone. This has been further investigated by Grindem (2018) who proposed a prediction model for a more accurate two year prognosis for those who were treated with the non-surgical method. Athletes who were older, women, and individuals with better knee function early after ACL rupture were more likely to have successful two-year outcomes. These RTS numbers drop further if the athlete has a revision ACLR (Grassi, 2015).

Therefore, clearing the athlete for RTS is not a straight forward procedure, which has also been highlighted in a consensus statement on RTS (Ardern, 2016) which explains how its multifactorial, biological, psychological and social factors might influence treatment and outcomes (Ardern, 2016). RTS criteria will be discussed in a future blog post.

This information should be delivered in a way that is comprehensible to the patient to make informed, shared decisions before deciding to pursue ACLR or conservative rehabilitation (Feucht, 2014).

Functional anatomy

The ACL is located in the center of the middle of the knee and runs between femoral condyle and into the tibial bone. The ACL is the primary restraint to anterior translation of the tibia relative to the femur and secondary to internal rotation. The restraints from the ACL help with controlling the glide in the knee joint throughout the movement in various range of motions. Damage to the ACL can disrupt this system, resulting in aberrant movement during activities and causing a general feeling of instability.


The fairly common description of an ACL injury involves a ‘non-contact’ episode where the athlete typically describes a twisting sensation occurring during change of direction (pivoting) or landing from a jump. The athlete often reports a ‘popping’ or ‘snapping’ sound associated with pain and swelling shortly after the injury.

A common injury pattern has been observed during multidirectional activities (football, soccer, rugby, basketball) with up to 85% of the ACL injuries being non-contact injuries (Walden, 2015). The injury typically appears during 1) change-of-direction maneuvers such as jumping, cutting and pivoting movements, 2) re-gaining balance after kicking or 3) landing with hyperextended knee. 

The ACL will typically rupture during a forceful valgus moment at 5-30 degrees of knee flexion, where the knee is closer to the midline of the body than the ankle. While the tibia is moving forward and rotating internally, the forces from the foot-contact to the ground is compressing the tibia against the femur and elongating the ACL more than tolerated. The rupture occurs in approximately 17 to 50 milliseconds after this initial ground contact (Walden et al., 2015; Jordan et al., 2017).

An injury to the ACL often occurs concomitantly with damage to other knee joint structures such as the meniscus, articular cartilage and medial and collateral ligament which can influence the rehabilitation time and surgical procedure.

Clinical examination

  • History: An ACL rupture should always be suspected if the patient reports the previously mentioned injury mechanism combined with hearing or feeling a “pop” in addition to knee swelling and if the athlete has a perception of an “unstable” knee.
  • Test: Several clinical tests can be used to detect an ACL rupture. The Lachman test is the most accurate clinical diagnostic test with a pooled reported sensitivity of 85% and specificity of 94%. A positive result with the pivot shift test is a very clear indication of an ACL rupture (98% specificity). A negative test, however, is not sufficient to rule out possible injury (24% sensitivity). The anterior drawer test has high sensitivity and specificity for chronic ACL ruptures (92% sensitivity and 91% specificity), but lower accuracy for acute cases. (Filbay, 2019). All of the above tests measure a form of knee laxity.
  • Magnetic resonance imaging (MRI): If the above history and clinical test indicate an ACL injury, MRI can be used to determine if the ACL is ruptured. ACL ruptures often occur with concomitant ligament sprains, meniscus tears, bone marrow lesions, articular cartilage injuries, and intra-articular fractures with a prevalence of 30% and 42%, respectively for medial collateral ligament (MCL) injuries and meniscal tears. The rates of concomitant lateral collateral ligament (LCL) and posterior cruciate ligament (PCL) injuries are generally lower (Frobell, 2006).

Rehabilitation plan after anterior cruciate ligament reconstruction

Over the past few decades, rehabilitation protocols after ACL injuries have moved away from strict time-based protocols to more criteria-based guidelines. Programs are often individualized to the person’s injury, training status/experience and expectation for the rehab. Progressing from one phase to the next only occurs when the patient meets specific clinical milestones. One of the first milestones is to regain full range of motion (knee extension and flexion) pain-free, optimizing muscular strength and function and being familiar with basic post-operative exercises and expectations (patient education). However, creating a long-term rehabilitation plan can be challenging, therefore short-term milestones built on a combination of time-frame and criteria-based decision-making are advisable for building up a plan for your athlete. These time-frames are often called early phase, intermediate phase, late/advanced phase (Filbay, 2019), sports specific, return to participation, return to sport and return to performance (Ardern, 2016).

Timeline and criteria-based road map 

The figure below is an example of what a simple roadmap can look like:

Range of motionMotor controlMuscle strength
Early phaseDaily/weekly measurable skills and task: E.g. pain, swelling, range of motion. Early phase exercises
Milestone / CriteriaMilestone / CriteriaMilestone / Criteria
Intermediate phaseFunctional test, intermediate level exercises, preparing for return to run and jump
Milestone / CriteriaMilestone / CriteriaMilestone / Criteria
Advance/late phaseFunctional test, advanced level exercises, return to running, jumping and preparing for sport.
Milestone / CriteriaMilestone / CriteriaMilestone / Criteria
Main goal – Return to Sport

Pre-surgery / pre-op / pre-rehabilitation

The rehabilitation process prior to the ACL surgery is termed “preoperative rehabilitation” or just pre-op. 

In the first days and weeks after the injury, the focus is to restore full range of motion, control swelling and pain, and good quadriceps activation. Later in the pre-op, the goal is to either prepare the athlete for surgery or nonoperative management.

After the injury, the athlete can be subclassified into:

  1. coper (athlete that can resume previous recreational activities without reconstruction)
  2. non-coper (athlete that requires ACLR because of recurrent give-away episodes in activities of daily living)
  3. adaptors (athletes that can manage without reconstruction by modifying/lowering their activity level).

Thoma et al. (2019) showed that almost half (45%) of initial potential noncopers became potential copers after a 5 week pre-op neuromuscular strength training program, while only 13% of initial potential copers became noncopers after the training program.

The program consisted of progressive strengthening, plyometric, and neuromuscular exercises (Eitzen, 2010).

Stationary cycleContinuous warm-up at your preferred resistance10 min
TreadmillContinuous warm-up at your preferred speed. Walking or running10 min
Elliptical trainerContinuous warm-up at your preferred resistance10 min
Single-limb squatMaintain knee-over-toe position3 × 8
Step upMaintain knee-over-toe position2 x 10
Squat on BOSUMaintain knee alignment and core stability. Squat quickly down and up2 x 20
Single-limb leg pressStart in 90° knee flexion3 x 6 (+2)
Single-limb knee extensionStart in 90° knee flexion4 x 6 (+2)
SquatSquat slowly down to 90° knee flexion, stop, lift quickly up again3 × 8 (+2)
Leg curlLift quickly up, stop, and then slowly down to full extension3 × 8 (+2)
Hamstring on FitballOne foot on top of the ball, lift back and pelvis up, pull ball towards you3 × 6
Single-leg hopHop up on step, stop, continue down and directly 1 hop forward with a soft controlled landing1 × 15
Progressively perturbation trainingIncludes balance and stability exercises on custom-made roller board, rocker board, and platform, and involved perturbation of the support surface that allowed altered forces and torques to be applied to the injured limb in multiple directions in a controlled manner 

Eitzen et al. (2009) have also shown that quadriceps strength limb symmetry with a deficit below 20% is a significant predictor of knee function two years after an ACL injury. Similarly, another systematic review from Ashewaiver et al. (2016) analyzed 451 patients (15-57 years), showing that a 3-14 weeks pre-op period with an average training of three times per week improved the outcomes of patients with ACLR.

A part of getting the athlete ready for surgery could be obtaining a list of objective and subjective data that can be used to guide the rehabilitation. These test could involve: muscle testing (with hand held dynamometer (HHD), nordbord/physiometer or isokinetic device), jump testing (using force platform, mobile app like MyJump2 or use measuring tape and stopwatch to record limb symmetry in distance- or numbers for single leg jump for distance, 6-m jump test, triple cross-over test, cross-hop for distance, side hop, countermovement jump, functional testing a numbers of single leg squat, % of RM in back squat, deadlift etc. 

My recommendation is to choose a handful of tests that are available, time-saving and fit in our facilities. Furthermore, questionnaires like Tegner scale, IKDC2000, KOOS. ACL-RSI, Tampa Scale of Kinesiophobia (TSK-11) can be useful to track the rehabilitation.

In study by Ardern et al. (2013), it was suggested that a score of ≥ 56 points on the ACL-RSI scale increased the odds by four of RTS, which may help to identify at-risk athletes with lower points.

Post op ‘Early phase’

Following ACL surgery, a period of protected loading is recommended due to the graft healing, pain, swelling, limit range of motion and muscle control in order to limit muscle atrophy and avoid arthrogenic muscle inhibition. Acute management should adhere to the general principles of POLICE (Protection, Optimal Loading, Ice, Compression and Elevation) to ensure joint protection, removal of pain and swelling, meanwhile maintaining a gradual restoration of function.  

The athletes are presented with a set of surgical restrictions set by the surgeon depending on the surgical procedure and the surgeon clinical experience. These restrictions are often based on graft-type where bone-to-bone (BTB) and hamstring tendon is among the most common for first choice ACLR. Comorbidities of knee joint structures may involve meniscus meniscectomy, meniscus repair, lateral tenodesis (ALL), cartilage damage and in rare cases high tibial osteotomy. Each surgery is slightly different and can influence the progression in the first phase of rehabilitation and therefore it is important to fully understand the surgical procedure.

In the early phase rehab, training should focus on patient education, pain/swelling, range of motion (ROM), motor control and muscle strength to minimize the loss of muscle strength and volume. The goals through the early phase can be visualized as below.

Range of motionMotor controlMuscle strength
Early phaseDaily/weekly measurable skills and task: E.g. pain, swelling, range of motion. Early phase exercises
Criteria based rehab (Ardern, 2018, Wilk 2018)Milestone: Have minimal/trace effusion or zero swelling. (Measure with stroke test and knee circumference). Have minimal or no pain (0-2 NRS). Full active extension, 120 degrees active knee flexion.Walking without crutches, walking on stairs. Stationary bike. Drive car.Full quadriceps activation without quadriceps lag in single leg raise. Restore balance and perform mini squat weight shift. Ability to hold terminal knee extension during single leg standing without support 10 sec. 
Intermediate phaseFunctional test, intermediate level exercises, preparing for return to run and jump
Milestone / CriteriaMilestone / CriteriaMilestone / Criteria
Advance/late phaseFunctional test, advanced level exercises, return to running, jumping and preparing for sport.
Milestone / CriteriaMilestone / CriteriaMilestone / Criteria
Main goal

In the early phase we recommend beginning training with low to moderate intensity (e.g. 12-20 RM in multiple set) and slowly progressing to support more optimized muscle hypertrophy and strength.

In addition, a focus on non-weightbearing exercises such as lumbopelvic hip exercise, upper body and cardio can be used to stimulate the exercise program. In addition, modalities such neuromuscular electrical stimulation (NMES) or blood flow restriction can be used to enhance muscle activation and growth. Furthermore, cross-education training with heavy strength training of the non-injured leg, shortly after ACLR has demonstrated improvement in quadriceps muscle strength at 8 weeks when perform 3 or 5 days per week compared with control which did not receive cross-educational training (Papandreou et. al. 2012). Similar result has been reported after 24 weeks when trained 3 times per week (Harput, 2019), however Zult et al. did not got these results. One explanation could be that the intervention group only performed cross-education training group twice a week. This illustrate, that there might be a dose-response relationship in cross-education training, recommend at least 3 times per week. Although further research should be conducted to investigate the long-term effects of cross-education training in ACLR rehabilitation.

Post op ‘Early phase’ exercises

Before beginning rehabilitation consider following restriction.

  • Follow surgical advice
  • ACLR: Full weightbearing, no brace
  • ACL + lateral tenodesis: Full weight-bearing + brace 0-90° for 4-6 weeks. Be careful with lateral weighted exercises.
  • ACL + Meniscus repair: Protected weight-bearing + brace 0-90° for 4-6 weeks (different protocols).
  • POLICE protocol for management of pain and swelling.

0-2 weeks


  • Control pain, swelling, range of motion (0-105°+ (Wilk, 2018)), restore balance, minimize loss of muscle strength.
  • Criteria to phase 2 includes: closed wound, full active knee extension, 120°+ knee flexion, little to no effusion, active dynamic gait pattern without crutches, ability to hold terminal knee extension during single leg (Arderen, 2018, Gokeler, 2017).
Patella mobilizationPatella mobilization (Superior/inferior and medial/lateral directions).1-2 min
Knee flexionTowel slide, sitting on a chair and passive flexion and extension assisted by the “good” leg, wall slide3 set x 15 reps+. multiple times per day
Static Quads-activationContract m. quadriceps in several seconds (see if you can lift the heel from the surface)3 set x 15 reps+. multiple times per day
Straight leg raisesLift up your leg without lag of extension3 set x 15 reps+. multiple times per day
Supermanprone straight leg raise extension3 set x 15 reps per day
Prone assisted hamstring curlsProne position and flex your assisted by the opposite foot under the ankle3 set x 15 reps per day
Side lying hip abduction/clamsBe careful if the patient has undergo meniscus repair3 set x 15 reps per day
Side lying hip adductionStart in week 2-3 if not pain.3 set x 15 reps per day
Calf: Seated calf raisesBed based/seated plantar flexor strengthening.3 set x 15 reps+. multiple times per day
Balance exercisesChallenge the oculomotor, vestibular and proprioceptive system. Single leg balance with open, close eyes, rotational head movement eg. other body part.3 set x 15 reps+. multiple times per day
Terminal Knee Extension 3 set x 15 reps per day
Squat 3 set x 15 reps per day
Step up 3 set x 15 reps per day
LungesACL in front starting week 23 set x 15 reps per day
Upper body driven core exercises  
Gait re-education (March walk, hurdle walk, side walk)  
Cardio: SkiErg, Concept2 Rowing  
Neuromuscular Electrical Stimulation (NMES)  


  1. Waldén M, Krosshaug T, Bjørneboe J, et al, 2015, Three distinct mechanisms predominate in non-contact anterior cruciate ligament injuries in male professional football players: a systematic video analysis of 39 cases
  2. Jordan, M.J., Aagaard, P., & Herzog, W. (2017). Anterior cruciate ligament injury/reinjury in alpine ski racing: a narrative review. Open access journal of sports medicine.
  3. Eitzen, I., Moksnes, H., Snyder-Mackler, L., & Risberg, M. A. (2010). A Progressive 5-Week Exercise Therapy Program Leads to Significant Improvement in Knee Function Early After Anterior Cruciate Ligament Injury. Journal of Orthopaedic & Sports Physical Therapy, 40(11), 705–721. doi:10.2519/jospt.2010.3345
  4. Ardern CL, Taylor NF, Feller JA, et al., Fifty-five per cent return to competitive sport following anterior cruciate ligament reconstruction surgery: an updated systematic review and meta-analysis including aspects of physical functioning and contextual factorsBritish Journal of Sports Medicine 2014;48:1543-1552.
  5. Frobell, R. B., Roos, H. P., Roos, E. M., Roemer, F. W., Ranstam, J., & Lohmander, L. S. (2013). Treatment for acute anterior cruciate ligament tear: five year outcome of randomised trial. BMJ, 346(jan24 1), f232–f232. doi:10.1136/bmj.f232
  6. Grindem, H., Wellsandt, E., Failla, M., Snyder-Mackler, L., & Risberg, M. A. (2018). Anterior Cruciate Ligament Injury—Who Succeeds Without Reconstructive Surgery? The Delaware-Oslo ACL Cohort Study. Orthopaedic Journal of Sports Medicine, 6(5), 232596711877425. doi:10.1177/2325967118774255
  7. Eitzen, I., Holm, I., & Risberg, M. A. (2009). Preoperative quadriceps strength is a significant predictor of knee function two years after anterior cruciate ligament reconstruction. British Journal of Sports Medicine, 43(5), 371–376. doi:10.1136/bjsm.2008.057059
  8. Alshewaier, S., Yeowell, G., & Fatoye, F. (2016). The effectiveness of pre-operative exercise physiotherapy rehabilitation on the outcomes of treatment following anterior cruciate ligament injury: a systematic review. Clinical Rehabilitation, 31(1), 34–44. doi:10.1177/0269215516628617 
  9. Logerstedt et. al., Single-legged hop tests as predictors of self-reported knee function after anterior cruciate ligament reconstruction: the Delaware-Oslo ACL cohort study, 2012 Am J Sports Med.
  10. Andrea Reid, Trevor B Birmingham, Paul W Stratford, Greg K Alcock, J Robert Giffin; Hop Testing Provides a Reliable and Valid Outcome Measure During Rehabilitation After Anterior Cruciate Ligament Reconstruction, Physical Therapy, Volume 87, Issue 3, 1 March 2007, Pages 337–349,
  11. Filbay SR, Grindem H, 2019, Evidence-based recommendations for the management of anterior cruciate ligament (ACL) rupture, Best Practice & Research Clinical Rheumatology, https://
  12. Frobell, R. B., Lohmander, L. S., & Roos, H. P. (2006). Acute rotational trauma to the knee: poor agreement between clinical assessment and magnetic resonance imaging findings. Scandinavian Journal of Medicine and Science in Sports
  13. Feucht, M. J., Cotic, M., Saier, T., Minzlaff, P., Plath, J. E., Imhoff, A. B., & Hinterwimmer, S. (2014). Patient expectations of primary and revision anterior cruciate ligament reconstruction. Knee Surgery, Sports Traumatology, Arthroscopy, 24(1), 201–207. doi:10.1007/s00167-014-3364-z
  14. Ardern, C. L., Glasgow, P., Schneiders, A., Witvrouw, E., Clarsen, B., Cools, A., … Bizzini, M. (2016). 2016 Consensus statement on return to sport from the First World Congress in Sports Physical Therapy, Bern. British Journal of Sports Medicine, 50(14), 853–864. doi:10.1136/bjsports-2016-096278
  15. Ardern CL, Ekås GR, Grindem H, et al2018 International Olympic Committee consensus statement on prevention, diagnosis and management of paediatric anterior cruciate ligament (ACL) injuriesBritish Journal of Sports Medicine 2018;52:422-438.
  16. Wilk, K.E., and A.Arrigo, Rehabilitation: Common Problems and Solutions, 2018, Clinics in Sports Medicine Volume 37, Issue 2, April 2018, Pages 363-374
  17. Montalvo AM, Schneider DK, Yut L, et al“What’s my risk of sustaining an ACL injury while playing sports?” A systematic review with meta-analysisBritish Journal of Sports Medicine 2019;53:1003-1012.
  18. Ekstrand, J. (2010). A 94% return to elite level football after ACL surgery: a proof of possibilities with optimal caretaking or a sign of knee abuse? Knee Surgery, Sports Traumatology, Arthroscopy.
  19. Grassi A, Zaffagnini S, Marcheggiani Muccioli GM, et al, 2015, After revision anterior cruciate ligament reconstruction, who returns to sport? A systematic review and meta-analysis, British Journal of Sports Medicine
  20. Ardern, C. L., Taylor, N. F., Feller, J. A., Whitehead, T. S., & Webster, K. E. (2013). Psychological Responses Matter in Returning to Preinjury Level of Sport After Anterior Cruciate Ligament Reconstruction Surgery. The American Journal of Sports Medicine,
  21. Thoma, L. M., Grindem, H., Logerstedt, D., Axe, M., Engebretsen, L., Risberg, M. A., & Snyder-Mackler, L. (2019). Coper Classification Early After Anterior Cruciate Ligament Rupture Changes With Progressive Neuromuscular and Strength Training and Is Associated With 2-Year Success: The Delaware-Oslo ACL Cohort Study. The American Journal of Sports Medicine, 036354651982550. doi:10.1177/0363546519825500
  22. Papandreou M, Billis E, Papathanasiou G, Spyropoulos P, Papaioannou N. Cross-exercise on quadriceps deficit after ACL reconstruction. J Knee Surg. 2013;26(1):51–58. doi:10.1055/s-0032-1313744
  23. Harput G, Ulusoy B, Yildiz TI, et al. Cross-education improves quadriceps strength recovery after ACL reconstruction: a randomized controlled trial. Knee Surg Sports Traumatol Arthrosc. 2019;27(1):68–75. doi:10.1007/s00167-018-5040-1
  24. Zult T, Gokeler A, van Raay JJAM, et al. Cross-education does not accelerate the rehabilitation of neuromuscular functions after ACL reconstruction: a randomized controlled clinical trial. Eur J Appl Physiol. 2018;118(8):1609–1623. doi:10.1007/s00421-018-3892-1

what is plyometric training

What is plyometric training

Plyometric training is apply optimal force (strength) and velocity (speed) in the correct direction within the shortest time (efficacy).

What is the benefits of plyometric training

  1. Increase explosive strength due to an improve rate of force development
  2. Increase reactive strength due to greater storage and reutilization of elastic energy
  3. Improved ability to transfer force through the joints and minimize energy leaks

Force velocity relationship (picture is taken from science for sport)

The maximal force often takes more than 300ms to develop

Speed strength (midzone) takes less than 250ms to develop

Speed exercises is characterized high velocity low force with less than 100ms.

Plyometric is in the spectrum of speed training.

Force-Velocity Curve | Science for Sport

Rate of force development

If we can train the RFD for the untrained person, we can make him to create more force in shorter time, we often also get the athlete maximum strength up.

Speed Development Part 3: How to Train to Increase Rate of Force ...

What are the mechanism that underpinning the performance benefits of plyometrics

The ability to express stretch shortening cycle is what underpins all the capacities optimize rate of force development and their by maximizing the force velocity characteristics exhibited within your sport.

Movement utilizing a stretch-shortening cycle have been shown to increase performance by 10-15% compared to movements that do not.

Performance enhancement from plyometric

Skærmbillede 2020-03-31 kl. 09.02.33

Injury reduction from plyometric

Plyometric decrease risk of injury through increased tolerance to stretch loads at various speeds, loads and direction.



Eitzen et al. A progressive 5-week exercise therapy program leads to significant improvement in knee function early after anterior cruciate ligament injury.

Eitzen I, Moksnes H, Snyder-Mackler L, Risberg MA. A progressive 5-week exercise therapy program leads to significant improvement in knee function early after anterior cruciate ligament injury. J Orthop Sports Phys Ther. 2010;40(11):705–721. doi:10.2519/jospt.2010.3345


FINDINGS A 5-week progressive exercise therapy program in the early stage after ACL injury led to significantly improved knee function before the decision making for reconstructive surgery or further nonoperative management.

IMPLICATION Short-term progressive exercise therapy programs should be incorporated in the early stage after ACL injury, to optimize knee function before ACLR or as a first step in the preparation to return to previous activity without surgery.

CAUTION The participants in this study had an ACL tear with no symptomatic concomitant injuries; therefore, results cannot be generalized to all patients with ACL injury. The results of this study are further dependent on motivated patient with high compliance to the exercises program

Skærmbillede 2020-02-26 kl. 18.38.19Skærmbillede 2020-02-26 kl. 18.38.23


Daniel Menzel ACL story

Daniel Menzel ACL story
Post op
0-4 weeks
🏈 Quads activation
🏈 Straight leg raise (SLR)
Week 5-8 Core
🏈 russian twist
🏈 abs
🏈 Side bridge
Week 9-12
🏈Mini band walk for glute
🏈BW Single leg R.DL
🏈Stepping over hurdles
🏈Legs swings
🏈High Knees
Week 13-16
🏈 Leg extension
🏈 single leg hip extension/single leg hip thrust
🏈Balance exercise on a bosu ball
🏈 Nordic Hamstring
🏈 hanging swiss ball curl
Week 17-20
🏈 gymnastic
🏈 S-rung
🏈 Trapbar deadlift
🏈 stiff legged deadlift
🏈 DB2 Step up
Week 21-24 foot drills
🏈 drill 1
🏈 drill 4
🏈 drill 5
🏈 drill 7
Week 25-28 on field pre drills
🏈 1
Week 29-32
Week 33-36
Week 37-40
🏈Change of direction
🏈Jumping landing
🏈Being tackled
🏈Hitting the ground
🏈Training under fatique

Changes in rectus femoris architecture induced by the reverse nordic hamstring exercises

Changes in rectus femoris architecture induced by the reverse nordic hamstring exercises, Diego ALONSO-FERNANDEZ, Rosana FERNANDEZ-RODRIGUEZ, Rocío ABALO-NÚÑEZ, Department of Special Didactics, Faculty of Science Education and Sport, University of Vigo, Vigo, Spain; Galicia Sur Health Research Institute (IIS Galicia Sur), Vigo, Spain; Department of Functional Biology and Health Sciences, Faculty of Physiotherapy, University of Vigo, Vigo, Spain, DOI: 10.23736/S0022-4707.18.08873-4

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Injuries and mechanical stimuli alter the muscle architecture and, therefore, its function. The changes in the architecture of the rectus femoris (RF) induced by an eccentric training protocol with reverse nordic hamstring exercises (RNHE) have never been studied. Therefore, the aim of the present study was to determine the architectural adaptations of the RF after an eccentric training with RNHE, followed by a subsequent detraining period.


Twenty-six subjects performed a first week of control, 8 weeks of eccentric training, concluding with a 4-week period of detraining. The architectural characteristics of the RF were evaluated using 2D ultrasound at rest (pretest: week 1), after the training (post-test: week 9), and at the end of the detraining period (retest: week 13).


At the end of the training period, a significant increase in the muscle fascicle length (FL) (t=-8.96, d=2.22, P<0.001), muscle thickness (MT) (t=-8.76, d=2.219, P<0.001), pennation angle (PA) (t=-9.83, d=2.49, P<0.05) and cross-sectional area (CSA) (t=-13.06, d=3.06, P<0.001) was observed. After the detraining period FL, MT, PA and CSA showed a significant decrease.


The eccentric training with RNHE may cause changes in the architectural conditions of RF, which, in addition, are also reversible after a 4-week detraining period. The adaptations produced by RNHE may have practical implications for injury prevention and rehabilitation programs, which include the changes in muscle architecture variables.


Knee Extension Deficit in the Early Postoperative Period Predisposes to Cyclops Syndrome After Anterior Cruciate Ligament Reconstruction: A Risk Factor Analysis in 3633 Patients From the SANTI Study Group Database

Delaloye, J.-R., Murar, J., Vieira, T. D., Franck, F., Pioger, C., Helfer, L., … Sonnery-Cottet, B. (2020). Knee Extension Deficit in the Early Postoperative Period Predisposes to Cyclops Syndrome After Anterior Cruciate Ligament Reconstruction: A Risk Factor Analysis in 3633 Patients From the SANTI Study Group Database. The American Journal of Sports Medicine, 036354651989706. doi:10.1177/0363546519897064 



Cyclops syndrome is characterized by a symptomatic extension deficit attributed to impingement of a cyclops lesion within the intercondylar notch. The syndrome is an important cause of reoperation after anterior cruciate ligament reconstruction (ACLR). It has been suggested that remnant-preserving ACLR techniques may predispose to cyclops syndrome, but there is very limited evidence to support this. In general terms, risk factors for cyclops syndrome are not well-understood.


To determine the frequency of and risk factors for reoperation for cyclops syndrome in a large series of patients after ACLR.


Case-control study; Level of evidence, 3.


A retrospective analysis of prospectively collected data was performed, including all patients who underwent primary ACLR between January 2011 to December 2017. Patients undergoing major concomitant procedures were excluded. Demographic data, intraoperative findings (including the size of preserved remnants), and postoperative outcomes were recorded. Those patients who underwent reoperation for cyclops syndrome were identified, and potential risk factors were evaluated in multivariate analysis.


A total of 3633 patients were included in the study, among whom 65 (1.8%) underwent reoperation for cyclops syndrome. Multivariate analysis demonstrated that preservation of large remnants did not predispose to cyclops lesions (odds ratio [OR], 1.11; 95% CI, 0.63-1.93). The most important risk factor was extension deficit in the early postoperative period. If present at 3 weeks postoperatively, it was associated with a >2-fold increased risk of cyclops syndrome (OR, 2.302; 95% CI, 1.268-4.239; P < .01), which was increased to 8-fold if present 6 weeks after ACLR (OR, 7.959; 95% CI, 4.442-14.405; P < .0001). None of the other potential risk factors evaluated were found to be significantly associated with an increased frequency of cyclops syndrome.


Failure to regain full extension in the early postoperative period was the only significant risk factor for cyclops syndrome after ACLR in a large cohort of patients. Other previously hypothesized risk factors, such as preservation of a large anterior cruciate ligament remnant, did not predispose to the development of this debilitating postoperative complication.



The term cyclops syndrome is used to describe the clinical scenario of a symptomatic extension deficit attributed to impingement of a cyclops lesion within the intercondylar notch. The major importance of cyclops syndrome arises from the resultant morbidity. Patients experience loss of extension with snapping and catching while walking. Additionally, the associated gait disturbance is poorly tolerated, and there is some evidence to suggest that altered sagittal plane mechanics during the loading response phase of gait predisposes the patient to early medial compartment degeneration. Rates of reoperation to regain full extension after the development of cyclops syndrome are between 1.9% and 8.4% of ACLR.

The precise etiology and risk factors for the development of cyclops syndrome are incompletely defined.1 However, Pinto et al recently identified extension deficit in the early postoperative period as an important risk factor.


The final study population was composed of 3633 patients divided into 2 groups: patients with or without postoperative cyclops syndrome. Cyclops syndrome was identified in 65 patients (1.8%), who all recovered full knee extension after arthroscopic debridement of the lesion.

After univariate analysis, the following factors reached the 25% threshold of correlation with cyclops syndrome and were included in a multivariate analysis: knee extension deficit at 3 and/or 6 weeks postoperatively, body mass index .25, and the presence of bimeniscal lesions.

Indeed, patients who had a knee extension deficit at 3 and 6 weeks postoperatively had a .2-fold and 8-fold increase in the risk of postoperative cyclops syndrome, respectively, as compared with patients who presented without extension deficit (Figure 3).

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The main findings of this study were that extension deficit in the early postoperative period was the most important predictor for cyclops syndrome, while preservation of large ACL remnants did not confer any increased risk of developing this complication. A further important finding was that the overall frequency of reoperation for cyclops syndrome was 1.8%.

It is not entirely clear where in the literature the concerns regarding an association between remnant preservation and cyclops syndrome have arisen, because comparative studies have not demonstrated this. However, several authors have postulated that the presence of some portion of the ACL remnant could explain the pathogenesis. Jackson and Schaefer suggested that remaining tissue around the tibial tunnel may be part of the tissue that forms the cyclops lesion. Therefore they recommended a thorough debridement around tibial tunnel. This opinion is shared by other authors as well, but is based only on personal experience rather than any scientific evidence. Delince et al observed that cyclops lesions were located at the base of the ACL graft and seemed to develop from the fibers of the ACL stump left behind. Finally, Wang hypothesized that cyclops lesions were due to an inflammatory proliferative process for which one of the possible stimulating factors could be an ACL remnant. However, no study could clearly demonstrate a correlation between ACL remnant preservation and cyclops syndrome.

It is our opinion that the findings of the current study provide a firm rebuttal against theoretical concerns that large remnants can predispose to cyclops syndrome. This study identified that the most important risk factor for the development of cyclops syndrome was knee extension deficit in the early postoperative period. This confirms the findings of Pinto et al, who reported that extension deficit at postoperative 3 and 6 weeks was associated with a significantly increased risk of cyclops syndrome. In that case-control study, 45 patients with cyclops syndrome were matched to random controls. The study design of Pinto et al did not allow an evaluation of risk factors for cyclops syndrome or a determination of whether early postoperative extension deficit remained an important predictor of cyclops syndrome in multivariate analysis. The current study confirms that extension deficit in the early postoperative period is not only a significant risk factor for cyclops syndrome but also the most important predictor. These results support the work of Jackson and Schaefer, who suggested that it is not the cyclops lesion that causes this early extension deficit but rather the extension deficit that promotes the development of the nodule in the intercondylar notch.

Despite all these precautions, 1.8% of our patients still developed cyclops syndrome, confirming that the cyclops lesion must have formed sometime after the patient was discharged from the hospital. This observation is in keeping with the findings of Gohil et al, who performed MRI evaluation at 2, 6, and 12 months after ACLR. In their prospective study including 48 patients, the initial development of cyclops lesions, as noted on MRI, was typically between 6 and 12 months. Interestingly, and consistent with our findings, the authors noted that patients who eventually had cyclops syndrome already had an extension deficit at postoperative 2 months but without MRI evidence of any cyclops lesion at that stage. The authors concluded that the early postoperative loss of extension was multifactorial and could promote the development of scar tissue in the intercondylar notch.

On the basis of the aforementioned evidence, it is apparent that the early extension deficit observed in patients who go on to develop cyclops syndrome is very unlikely to be the result of a mechanical problem. It therefore logical to suggest that the observed extension deficit that leads to an increased risk of cyclops syndrome could be related to arthrogenic muscle inhibition (AMI). AMI is a frequent but underrecognized cause of extension deficit after ACLR. It is caused by changes in the discharge of articular sensory receptors (attributed to inflammation, pain, and swelling), which in turn alter neurological pathways resulting in a hamstring contracture and a quadriceps activation failure (Figure 4). Clinically, this manifests as a passive and active extension deficit. However, further study is required to evaluate whether proven therapeutic modalities (cryotherapy and hamstring fatigue exercises) for AMI are able to reduce the risk of developing cyclops syndrome.


Failure to regain full extension in the early postoperative period was the only significant risk factor for cyclops syndrome after ACLR in a large cohort of patients. Other previously hypothesized risk factors, such as preservation of a large ACL remnant, did not predispose to the development of this debilitating postoperative complication.


Rate Of Force Development after Anterior Cruciate Ligament Reconstruction.


Rate Of Force Development after Anterior Cruciate Ligament Reconstruction.

The aim of this blogpost is to share some thoughts on the Rate of Force Development (RFD) following anterior Cruciate Ligament reconstruction (ACLR).  


Anterior Cruciate Ligament reconstruction

An ACL injury can be devastating for athletes of all levels.

Up to 85% of ACL injuries are non-contact injuries which occur during pivoting actions, in particular change-of-direction maneuvers (jumping, cutting, and pivoting). An ACL will typically ruptures at approximately 17 to 50 milliseconds post initial ground contact in 5-30 degrees of knee flexion during a forceful valgus moment (1).


The predominant complaints following ACL injury are knee laxity, instability and in some cases pain. These factors can prevent athletes from participating in their specific sport at previous levels of intensity and competition.


Surgery is not necessary in all cases and is not a guarantee for return to previous sporting level. A systematic review and meta-analysis showed that 81% returned to some kind of sport, but only 55% returned to a competition level (2). In addition the risk of sustained a second ACL rupture or knee injuries in was higher (3). Despite this, elite athletes appears to return to their pre-injury level of sport within 6 and 13 months of surgery (4). 


To determine safe and successful return to sport following surgery, the athlete should pass a number of modifiable criteria which have been clinical proven to be risk factors for potential ACL re-injury. Some of these criteria are based on: Limb symmetry index (LSI) on 90% difference in m. quadriceps deficit hamstring strength, a hamstring to quads ratio (H/S-ratio) at 60% executed on the biodex isokinetic test with 60 degree/second (meaning the velocity is constant regardless of the force applied), hop tests (Single hop, triple hop, triple crossover hop), on field training and running t-test. (4). Kyritis et al. demonstrated that athletes who returned to football without meeting this criteria were four times more likely to encounter re-ruptures (4). 


In addition to the functional criteria, a time oriented model has shown to further reduce the risk of re-injury. For every month of delayed return to sport up until 9 months post ACL reconstruction, the rate of knee reinjury was reduced by 51% (6). Despite this,  Welling et al. found only a limit number (11%) meet the return to sport criteria after 9 months post anterior cruciate ligament reconstruction (7). 


Although muscular strength as measured by 90% LSI in peak force is a common criteria standards, previously literature has demonstrated that maximal force although important, is only one measure of readiness to return to play. Peak force does not reveal is the time to develop peak force, also known as rate of force development (RFD) (10). 


What is RFD

RFD measures the ability to generate explosive muscle force and is defined as the speed at which the contractile elements of the muscle can develop muscle force (9,13) from a low or resting level through a neural drive. In other words, RFD is the muscle activation (RFD = Force / Time) and is not determined by the actual movement itself.


Subsequently, we can categorize RFD into different ranges of time-intervals. Early phases are typically measured after 0-50ms (also called initial RFD or starting strength), and the late torque development, 0-100ms, 0-250ms, and a later phase at <250ms (9,13) where peak force is reach during a maximal voluntary contraction (MVC).(Figure 1). 

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Figure 1 (11)


This could have important clinical relevance as the estimated time of ACL injury, as mentioned previously, occurs in range of 17-50ms after initial ground contact. Therefore it can be hypothesized that early RFD  is more important than MVC when assessing muscle muscle stabilization of the knee joint in the context of most sporting pivoting playing situations.


One study by Angelozzi et al. (8) observed RFD in ACL rehabilitation, and found a significant deficit in m. quadriceps concentric RFD at 6 months post-ACLR which was shown to first returned after 12 months. Baseline RFD was measured for 30% (RFD30), 50% (RFD50) and 90% (RFD90) of MVC (Figure 2). At 6 months post-reconstruction, the average RFD30 value for the involved side was only 80% of the baseline value whereas at 12 months, the mean RFD30 value for the involved side was 98% of the baseline value. For RFD50 at 6 months, the mean value was 77%, and after 12 months it was 93%. The average RFD90 was 63% at 6 months and 91% after 12 months.

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(Figure 2 (8))


RFD Hamstring / quadriceps ratio (RFD H/Q ratio)

Another important risk factor in rehabilitation of the ACL is the balance between hamstring and quadriceps muscles, also referred to as the hamstring / quadriceps H/Q ratio. This ratio is important as the contraction force of m. quadriceps during knee extension produces substantiel anteriorly directed shear force of the tibia relative to the femur at extended joint angles of 0–35 degrees of flexion (14) Therefore, a rapid co-activation of hamstrings, together with ACLR itself, can counteract and reduce this anterior shear force, which may be protectable factor the ACL. 


In a study Jordan, M. Aagaard, P. and Herzog W.  looked a elite alpine skiers and ACL injuries. As mentioned beforehand, the ACL mechanism often occurs after 60ms in a rapid increase in knee flexion 26 to 63 degrees in a combination of knee valgus and a tibial internal rotation. So there is external forces acting above the knee, and those external forces are causing a anterior draw load on tibia and a passive strain in the ACL where is being ruptured. So there is a major discrepancy between injury time (60ms) and maximal strength (250-600ms), and therefore we need to look at the rapid hamstring and quadriceps force production (explosive strength) as this timeline is critical to active stabilize that knee joint. What the study found was a clear deficit in a skier with ACLR in knee extensor torque (Nm) in both peak force and rate of force development.

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To protect the ACL, it can be hypothesized that the neuromuscular system of the hamstring 

 In a cross-section study by Zebis and her colleagues, it was observed in a group of female football players, that a lower RFD H/Q in the initial phase of contraction, could be a possible risk factor for sustaining future ACL injury. Interestingly, the MVC H/Q ratio did not differ significantly amongst tested players.


In addition, another study by Ishøi et al. found an association between early hamstring RFD (0-100ms) with various measures of sprint performance in youth elite football players. 


Therefore, as a clinician, it can be speculated that including initial phase of RFD H/Q, rather than MVC H/Q strength ratio, throughout the rehabilitation could provide additional information regarding the underlying impairments in force (11) and may be a substantial factor for both injury reduction and a quicker return to performance (figure 3). 


(figure 3 (12))

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Stretch shortening cycle (SSC).

In continuation of RFD, the time to perform an exercises/movement can also be explored. This is referred to as the stretch shortening cycle (SSC). 


Simply explained, the stretch-shortening cycle (SSC) is where the muscle undergoes an initial lengthening “pre-stretch” (eccentric phase), followed by an isometric transitional period (amortization phase), leading in a shortening concentric action. In the pre-stretch, elastic energy is stored in tendons while muscle spindles are stimulated. In the following concentric contraction, this energy is transferred through the tendon, and a greater force is produced. It seems that this transfer of energy is influenced by the  stiffness of the tendon, and a stiffer tendon transfer restored energy more efficient during the concentric contraction influencing both RFD and MVC (14,15).


You can imagine a limb just before contacting the ground. Just before contact there is a burst of muscle firing which pre tension the muscle and the tendon. This course some tension thoughout the muscle. So when the foot hits the ground there is minimal length change of the tendon and muscle (mechanical energy absorb) and after this rapid stretch, we will a dynamic shortening of the muscle (mechanical energy released).

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The SSC-movement can be subclassified as either fast-SSC: <250ms, or slow-SSC: >250ms. When the movement is slower, the muscles have longer time (300ms+) to form cross-bridges and thereby reach a higher MVC (8,9).


In contrast, during a fast-SSC movement, each movement takes 80-160ms (7). Here the athlete does not need to produce a high amount of force in every contraction. This might be more related to a side-cutting maneuver, where it have been found that the hamstrings activation is only 30–50% of MVC (13). 


The duration of the SSC can be measured within a single joint (eg. ankle, knee, hip) or entire lower limb, however it is important to note that introducing sports specific exercises with varying SSC durations may increase force and therefore improve performance and aid safe return to sport.


In general, the RFD has been reported to change depending on many factors such as muscle fiber type, muscle cross sectional area, fascicle length, penetration angle, motor unit recruitment, firing frequency, inter muscular coordination, muscle-tendon stiffness and training age. The figure below depicts some of those possible factors and how clinicians can enhance it in clinical practice (Figure 5). Exploring  these components in further details is beyond the scope of the blogpost (12,15).

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Figure 5 (12)



The aim of this blogpost was to share thoughts and explore literature pertaining to RFD in adjunction to maximal strength during ACL rehabilitation.  

Additionally it can be hypothesized that incorporating resistance training with different SSC timings aiming to increasing RFD in the early phase of contraction may be beneficial in both performance enhancement, injury prevention and the overall athletic development during the rehabilitation process. 

Future studies should focus on assessment of RFD as a predictor of return to play, and RFD measurement in the fatigued athlete. 




  1. Waldén M, Krosshaug T, Bjørneboe J, et al, Three distinct mechanisms predominate in non-contact anterior cruciate ligament injuries in male professional football players: a systematic video analysis of 39 cases
  2. Ardern CL, Taylor NF, Feller JA, Webster KE. Fifty-five per cent return to competitive sport following anterior cruciate ligament reconstruction surgery: an updated systematic review and meta-analysis including aspects of physical functioning and contextual factors. Br J Sports Med. 2014;48(21):1543-1552.
  3. Fältström, A., Kvist, J., Gauffin, H., & Hägglund, M.. Female Soccer Players With Anterior Cruciate Ligament Reconstruction Have a Higher Risk of New Knee Injuries and Quit Soccer to a Higher Degree Than Knee-Healthy Controls. The American Journal of Sports Medicine, 2018
  4. Lai CCH, Ardern CL, Feller JA, et al. Eighty-three per cent of elite athletes return to preinjury sport after anterior cruciate ligament reconstruction: a systematic review with meta-analysis of return to sport rates, graft rupture rates and performance outcomes, Br J Sports Med 2017
  5. Kyritsis P, Bahr R, Landreau P, et al, Likelihood of ACL graft rupture: not meeting six clinical discharge criteria before return to sport is associated with a four times greater risk of rupture, Br J Sports Med 2016;50:946-951.
  6. Grindem H, Snyder Mackler L, Moksnes H, et al., Simple decision rules can reduce reinjury risk by 84% after ACL reconstruction: the Delaware-Oslo ACL cohort study, Br J Sports Med 2016
  7. Welling, W., Benjaminse, A., Seil, R. et al. Knee Surg Sports Traumatol Arthrosc (2018) 26: 3636.
  8. Angelozzi M, Madama M, Corsica C, Calvisi V, Properzi G, McCaw ST et al. Rate of force development as an adjunctive outcome measure for return-to-sport decisions after anterior cruciate ligament reconstruction. J Orthop Sports Phys Ther 2012;42(9):772-80.
  9. Aagaard et al., Increased rate of force development and neural drive of human skeletal muscle following resistance training, J Appl Physiol 93: 1318–1326, 2002
  10. Hsieh CJ, Indelicato PA, Moser MW, Vandenborne K, Chmielewski TL. Speed, not magnitude, of knee extensor torque production is associated with self-reported knee function early after anterior cruciate ligament reconstruction. Knee Surg Sports Traumatol Arthrosc. 2015;23(11):3214–3220)
  11. Andersen, L. L., Andersen, J. L., Zebis, M. K., & Aagaard, P. (2010). Early and late rate of force development: differential adaptive responses to resistance training? Scandinavian Journal of Medicine & Science in Sports, 20(1)
  12. Maffiuletti NA, Aagaard P, Blazevich AJ, Folland J, Tillin N, Duchateau J. Rate of force development: physiological and methodological considerations. Eur J Appl Physiol. 2016;116(6):1091-116.
  13.  Zebis, M.K., Andersen, L.L., Ellingsgaard, H., & Aagaard, P. (2011). Rapid hamstring/quadriceps force capacity in male vs. female elite soccer players. Journal of strength and conditioning research, 25 7, 1989-93.
  14. Witvrouw, E., Mahieu, N., Roosen, P., & McNair, P. (2007). The role of stretching in tendon injuries. British journal of sports medicine, 41(4), 224-6.
  15. Bojsen-Møller, J., Magnusson, S. P., Rasmussen, L. R., Kjaer, M., & Aagaard, P. (2005). Muscle performance during maximal isometric and dynamic contractions is influenced by the stiffness of the tendinous structures. Journal of Applied Physiology, 99(3), 986–994.
  16. Bencke J, Aagaard P and Zebis MK (2018) Muscle Activation During ACL Injury Risk Movements in Young Female Athletes: A Narrative Review. Front. Physiol. 9:445. 
  17. Ishøi L, Aagaard P, Nielsen MF, Thornton KB, Krommes KK, .,The Influence of Hamstring Muscle Peak Torque and Rate Of Torque Development for Sprinting Performance in Football Players: A Cross-Sectional Study., Int J Sports Physiol Perform. 2018 Nov 14:1-27