When using a leg press machine for reps, what should the range of motion be? And specifically, should I lock out my knees or not?
No, the knees should not be locked out.* See caveat.
The two primary reasons:
- Once the knees are locked all the weight is transferred directly to the joint, which can be disasterous
- Not locking out increases the time under tension.
With appropriate loading and a controlled lift the risk of injury is relatively low.
There's two aspects: the joint load itself, and the "snap" into the locked position. The "snap" may introduce a dynamic load to the joint, while the joint load is a static stress.
Every single joint that locks out was designed to do that by evolution. There’s simply no supporting research no scientific peer reviewed papers suggesting that locking out under normal training conditions with proper execution proves to lead to long term wear and tear. If locking out was that bad for the joints then it wouldn’t be a rule in powerlifting? Olympic lifters lock out their elbows doing clean and press with a lot of weight and they lock out. When you lock out you simply finish the range of motion as then you are actually training your joints through their full range of motion and then it will actually make the ligaments around the knee joint much more stronger and healthier and then you won’t be at risk for an injury out side of the gym in normal day to day activities such as if you are somehow forced to hold a load in that locked out position and you aren’t used to that end range of motion then there is a higher risk of injury. Now to the point of “injury” while “locking out” the thing what is dangerous is HYPEREXTENSION. That is completely different to locking out. If you have highly lax joints or simply ego lifting and locking out at a velocity that you aren’t personally used to then yes you can get injured.