Almost all gyms that have machines have the Knee extension machine for quadriceps strengthening, but is it a good exercise to do? Not in my opinion, especially if you have had an ACL repair….
It is not my favorite machine for a number of reasons:
1. Not a functional exercise, I prefer to use standing or “closed chain exercises to help with quadriceps strength: squats, lunges, even the leg press.
2. The last 30 degrees of extension (when the bar is almost straight out) puts a significant amount of shear force through the tibia in the posterior direction, stressing the ACL. Why stress a ligament that is already so commonly injured?
3. No co-contraction of the hamstrings to help with the above stated posterior shear forces of the tibia to help the ACL.
4. Most of us have over developed quadriceps as compared to our hamstrings anyway and this just adds to the strength imbalances. Women especially tend to be “quad dominant” already so may not need to further isolate their quadriceps.
5. If you have a patellar tracking issue already or Chrondromalacia (softening of the cartilage) this exercise could flare things up due to the extra compression on the patellar-femoral joint (basically the knee-cap).
On the positive side, if you are a Body Builder is a great exercise to get those quads even bigger!! Let me know if you have any thoughts or questions~ Ed Deboo, PT, Integrative Physical Therapy, Bellingham, WA