How much pain do you accept when progressing your ACL patient? One study suggests less than 5 before load was increased with both groups.

Perry MC, Morrissey MC, King JB, Morrissey D, Earnshaw P. Effects of closed versus open kinetic chain knee extensor resistance training on knee laxity and leg function in patients during the 8- to 14 week post-operative period after anterior cruciate ligament reconstruction. Knee Surg Sports Traumatol Arthrosc. 2005;13:357-369. http:dx.doi.org/10.1007/s00167-004-0568-7

Three sets of 20 repetition maximum (RM) were used in each session until the beginning of week 4 of the 6-week intervention period whereupon a switch to three sets of six RM was instituted.

For all resistance exercise, a progressive resistance (i.e. load) programme was used with the load used dependent on knee pain and capacity of the target muscle group to move the load. Load was increased when the pain level was less than 5 (on a 0–10 scale reported verbally with 10 being the worst pain ever experienced and 0 being no pain) and the muscle group could successfully lift the load through the available range of motion for the target number or repetitions.