Still Think It's The Thigh Muscles?

I have seen many patients who have had an anterior cruciate ligament (ACL) tear over the years.  Some patients chose not to go under the knife but most had reconstruction done. ALL of them were taught to focus their strengthening on their quadriceps (or thigh) muscles.

So not surprising that a recently published study by Alzobi et al (2026) found that patients who did not opt for surgery exhibited progressive hamstring muscle atrophy.

A total of 1,207 thighs were examined. There were 92 with ACL tears and the other 1,115 acted as controls. The average age of the subject group was 61± 9 years.

Over 4 years, the ACL deficit thighs were smaller by an average of 28.18 mm, all due to progressive hamstring atrophy. The differences ranged from 13.92 to 42.43 mm smaller. The sartorius muscle also atrophied by an average of 3.02 mm.

There were no significant differences in the quadriceps or adductor cross sectional area. hamstring force was decreased significantly whereas quadriceps force showed no significant change.

The researchers concluded that muscle deterioration occurred in the posterior thigh muscles (hamstrings) with minimal changes in the front thigh muscles (quadriceps) over time. And for ACL deficient knees, it is really important to target long term rehabilitation strategies focusing on hamstring preservation. 

We already know that one of the reasons women sustained ACL tears is that their hamstrings were significantly weaker than their quadriceps muscle strength.

If you have been reading our previous blog articles, you already know that the quadriceps (thigh muscles) and gastrocnemius (calf muscles) increases load on the ACL due to anterior shearing forces at the tibia (shin bone). This is especially so when the knee is straightened (Maniar et al, 2022).

The hamstrings and soleus (deeper calf muscles) help to unload the ACL by generating posterior tibial shearing forces (Maniar et al, 2022).

So for those of you who have torn your ACL, whether or not you choose to go for surgery, make sure you focus on your hamstrings and soleus muscles instead.

References

Alzobi O, Mohajer B, Fleuriscar J et al (2026). Thigh Muscle Changes In The ACL-Deficient Knee: A 4-Year Lonitudinal MRI Study of 1,207 Patients. JBJS Am. 108(3): 219-226. DOI: 10.2106/JBJS.25.0064

Maniar N, Cole MH, Bryant AL et al (2022). Muscle Force Contributions To Anterior Cruciate Ligament Loading Sports Med. DOI: 10.1007/s40279-022-016743

Quads And Squats

We saw a few patients who tore their Anterior Cruciate ligament (ACL) in our clinic this past week. I have written many articles on the ACL before. Once you've torn your ACL, it usually takes 9 to 12 months before you can return to your sport, not to mention the financial cost. 

I came across an interesting article summarizing the evidence of the relationship between muscle forces acting on the ACL. This is important since muscles around the knee can increase and decrease the strain and mechanical loads on the ACL. This presents opportunities for preventive intenventions. 

Subsequently, our staff had interesting discussions regarding that article (referenced at the end of this post). The article demonstrated the forces acting on the knee joint and what can cause injuries there, specifically, what can hurt the ACL. 

The article reviewed muscle and and ACL loads during knee bending as well as weight bearing tasks like walking, lunging, landing, jumping and sidestep cutting.

Ready for the results? The quadriceps (thigh muscles) and gastrocnemius (calf muscles) increases load on the ACL due to anterior shearing forces at the tibial (shin bone). This is especially so when the knee is straightened.

The hamstrings and soleus (deeper calf muscles) helps to unload the ACL by generating posterior tibial shearing forces. For the hamstrings to 'protect' your ACL, your knee has to be bent at least 20 to 30 degrees.

R gluteus medius

The gluteus medius muscle was demonstated to consistently prevent the knee from collapsing inwards (knee valgus movement) and thus unloading the ACL, better than any other muscle.

Surprised? 

Patients who have been been told to strengthen their quadriceps (especially after ACL reconstruction) were really surprised when we told them. Make sure you focus on your hamstrings and soleus muscles instead.

The muscle to rule them all is of course the gluteus medius. Our patients will now understand why we always ensure their gluteus medius muscle is strong to prevent knee pain, Achilles tendon and of course ACL injuries.

Now you know.

Reference

Maniar N, Cole MH, Bryant AL et al (2022). Muscle Force Contributions To Anterior Cruciate Ligament Loading Sports Med. DOI: 10.1007/s40279-022-016743