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.