How much force can the anterior cruciate ligamant (ACL) tolerate?

Woo et. 1991, Am J Sport Med., has demostrated that ACL from health younger person (aged 22-35, see table 1) can tolerate 2160±157 (N) in the anatomical orientation.

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The reconstructed ACL has shown similar strengths compared with the healthy ACL, although these values can change considerably depending on graft type, donor’s age, and donor characteristics (e.g, autograft versus allograft, patellar tendon versus hamstrings graft, etc.) (Escamilla et al, 2012, J Engineering in Medicine,)

Unfortunately, it is not known how much force to the graft site is too much and how soon force can be applied to the healing tissues after reconstruction.


?? “This raising questions regarding elongation of the graft when providing load too early. From a biomecanichal point, we can might use these force data to guide exercises recommendations??

Who can answer?

Escamilla and colleagues, wrote in 2012 a review to summarize the available literature on ACL loading during commonly used therapeutic exercises.

He found that the peak ACL strain occurs at knee angles of less than 30 degrees. Therefore, if the rehabilitation goal is to minimize ACL loading, such as during the early phases after ACL reconstruction surgery, training both non weight-bearing exercises (NWBE) and weight-bearing exercises (WBE) at higher knee angles (i.e. 50–100) is recommended, compared with training these exercises at lower knee angles (i.e. 0–50).  Peak ACL strain is typically greatest at around 10–15 knee flexion, and gradually decreases between 15–50 knee angle, and between approximately 50–90 knee angles there is minimal or no ACL strain.

Therefore from a practical point, the ACL strains during the isometric seated leg extension, do not produce ACL strain below 60 degrees angles.

More detailed

The peak ACL strain was not significantly different between squatting with or without 136N of external resistance. It can be concluded from these weight-bearing data that increasing resistance during the squat, may not increase ACL strain. The explanation may be due to more muscle recruitment patterns under higher loads, such as recruiting the hamstrings to a greater extent (perhaps owing to changes in technique, such as a greater forward trunk tilt). Muscle force from the hamstrings helps unload the ACL owing to their posterior directed force on the leg.

In contrast, open kinetic chain, like seated Leg extension do not recruit hamstrings to unload the ACL), and there the strain on the ACL will increase when adding load.

Interestingly, performing a seated knee extension with no external resistance (quadriceps activation only), produced the same amount of ACL strain compared to a one leg sit-to-stand or stair climbing, but in the functional exercise, the recruiting of hip and thigh musculature (e.g. quadriceps, hamstrings, and gluteals) occurs. These muscles helps stabilize the knee and protect the ACL Therefore, functional WBE minimize ACL strain to a greater extent compared with the NWBE and this could be a rational for safely begin functional multi-joint exercises after the ACL reconstruction.

Squatting typically resulted in minimal or no ACL tensile force, and one-leg squatting producing slightly greater ACL loading compared with two-leg squatting. A progressively increasing the forward trunk tilt during the squat tends to increase hamstring activity and decrease quadricep activity, both which result in ACL unloading at knee angles of less than 60. Also, squatting with the heels off the ground, which typically results in increased forward knee movement beyond the toes, resulted in over three times the ACL loading compared with squatting with the heels on the ground. In addition, ACL loading was significantly greater in the one-leg squat, in which the knees moved forward beyond the toes 10+- 2 cm, and in lunges exercise using a short step, in which the knees moved forward compared with a lunge with a long step which the knees did not move forward beyond the toes.

The highest ACL tensile forces occurred during maximal-effort isokinetic seated knee extension exercises, in which ACL tensile force was approximately 40% greater at a slower 60/s speed, compared with faster 180/s speed.

Rapid deceleration activities, such as one-leg landing from a jump, or running and cutting movements, have been shown to generate very high ACL loading and are often implicated in ACL injuries. For example, during a running plyometric exercise involving a single-leg landing and rapidly coming to a stop, high deceleration forces are produced that result in approximately 1300 N of ACL tensile force. Therefore explosive deceleration plyometric exercise should not be performed until the later stages of ACL rehabilitation after the ACL graft has healed, revascularized, and strengthened adequately.

In contrast, a two-leg drop jump from a 60 cm platform only resulted in approximately 250 N of ACL tensile force, which is similar to the ACL loading that occurred during the NWBE seated knee extension..


This review allows the clinician to select specific therapeutic exercises to progress ACL loading safely.

As a personally note, maybe we should not be so afriad of loading our patient isometric early on in 90-60 degree with the resistance pad position more proximal on the leg compared with a more distal position, if no sign of pain or swelling occurs. This isometric exercise can be done while the patient is training leg extension with the non-involved leg. Furthermore, we should have in mind, that the peak ACL loading occurs at knee angles of between 10–15 and progressively decreases between 15–60 knee angles.

  1. Am J Sports Med. 1991 May-Jun;19(3):217-25. Tensile properties of the human femur-anterior cruciate ligament-tibia complex. The effects of specimen age and orientation. Woo SL1, Hollis JM, Adams DJ, Lyon RM, Takai S.
  2. Cruciate ligament loading during common knee rehabilitation exercise, 2012, Journal of Engineering in Medicine,Rafael F Escamilla, Toran D MacLeod, Kevin E Wilk, Lonnie Paulos and James R Andrews C, 2012, Journal of Engineering in Medicine


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