Don’t be afraid of knee extensions post-ACL

Traditionally open chain knee extension exercises post-operatively have frightened medical professionals and patients alike. This was due to limited research in this area, and fear regarding strain on the new ACL graft. In the last 10 years, comprehensive research has demonstrated that we can introduce knee extensions much earlier than originally thought. Earlier introduction, in the first four weeks post-operatively, has a significant affect on quadriceps muscle recovery, and sets you up well for the strength accumulation and return to sport phases of rehabilitation.

The research paper by Escamilla et al (2012) highlights that knee extensions are not the only exercise that strains that ACL. For example, squatting 0-90 degrees with or without 30lbs/13kg resistance had the same amount of strain on the ACL as knee extension through 0-90 degrees with 10lb/4kg of resistance – with the peak knee angle of strain being 10 degrees knee flexion for both exercises. Performing the Lachman Test for ACL integrity (surgeons/physios test alike) with 34lbs/15kg of pressure created just as much strain as the abovementioned exercises.

What is the most interesting is that 12RM knee extensions (0-90 degrees) had peak shear forces of 248N at 15 degrees knee flexion, BUT walking across level ground had peak shear forces of 355N at 15 degrees knee flexion à Walking puts more strain on the graft than open chain knee extension exercises. What does this tell us? Don’t fear knee extensions post ACL reconstruction!

The latest ASPETAR clinical guideline on ACL reconstruction (Jan 2023) summarised recent research on the affects of open chain knee extension exercises.

Two studies investigated the effect of adding open kinetic chain exercises early (4 weeks) in the rehabilitation protocol compared with later (12 weeks). The protocol in one study started with seated knee extension with no resistance at week 4 from 90° to 40° of knee flexion, at week 5 from 90° to 20° and at week 6 from 90° to 0°. The other study initiated the open kinetic chain protocol with seated knee extension at week 4 from 90° to 45° of knee flexion and maintained this until 12 weeks. The findings from these studies highlighted that patients may start open kinetic chain exercises in limited range of motion (90°−45° of knee flexion) from the fourth week after surgery without compromising knee stability.

Nine studies explored the differences between open and closed kinetic chain exercises after the 8-week mark, in ACL reconstruction rehabilitation. Across all the studies, there was no significant difference in anterior tibial laxity between open and closed kinetic chain exercises. No differences were reported in subjective knee function, range of motion, atrophy, or functional activities between open and closed kinetic chain exercises. The evidence did highlight the need for both open and closed kinetic chain exercises post-ACLR for regaining quadriceps strength, and long-term knee function. It is worth noting that open chain exercises might induce more anterior (patellofemoral) pain compared with closed chain exercises – something your Physiotherapist will monitor closely.

In summary a combination of closed and open kinetic chain exercise may lead to significantly better quadriceps strength and earlier/more successful return to sports, without any increase in laxity, compared with closed chain alone.


Kotsifaki R, et al. Br J Sports Med 2023;57:500–514. Aspetar clinical practice guideline on rehabilitation after anterior cruciate ligament reconstruction. doi:10.1136/bjsports-2022-106158

Escamilla R, el al. J Orthop Sports Phys Ther 2012;42(3):208–220. ACL Strain and Tensile Forces for Weight Bearing and Non—Weight-Bearing Exercises After ACL Reconstruction: A Guide to Exercise Selection. doi:10.2519/jospt.2012.3768






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