Should I lock my knees doing Leg Press?

Question

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?

Answer 1

No, the knees should not be locked out.^{* See caveat.}

The two primary reasons:

1. Once the knees are locked all the weight is transferred directly to the joint, which can be disasterous
2. Not locking out increases the time under tension.

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* Caveat

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.