Should you lock out on your exercises? It depends. Your goals and purpose will answer for you. Let’s start by talking about what happens when you lock out.
Locking out is the process where at the end range of motion of an exercise (usually at the completion of the concentric muscle contraction) you take the working joint(s) to its fully extended position. Once the joint reaches this position, the muscles are relieved of the load that is being moved and the force/resistance is thereby shifted to the joint. The upside to locking out is that you have the ability to rest for a short period of time before completing your next rest period. The downside, however, is that this places a lot of stress on the joints. For this reason, I believe that most exercises should not be performed to full lock out.
I would say that 95% of the population is not interested in competition and that there are 3 prominent goals for most: Increase fitness/health, Add muscle mass, and Increase strength. All 3 of these goals can be enhanced by not locking out. One of the keys to increasing muscle mass and strength levels is to increase your time under tension. As stated earlier, when you lock out a joint, the tension placed is moved from the muscle being trained and shifted to the joint. With that in mind, it would be prudent to end a repetition just prior to lock out in order to maintain maximal time under tension. Not only will the muscles be subjected to more work, but the joints will be spared.
When I believe that locking out should be done:
There are two instances that I believe locking out should be utilized: during max lift attempts and during competition or competition prep. Max lifts obviously require maximum effort to be completed and therefore the muscle cannot maintain the tension of the lift for an extended period of time. It is ideal to have the joint locked out when un-racking the lift and also at the completion of the lift. Since most programs only include a max lift 1-4x per month, this shouldn’t contribute too much if any joint discomfort.
The other time locking out is not only acceptable, but more likely crucial, is during competition. Whether it is Olympic weightlifting, power lifting, or CrossFit, competitions require a full lock out in order for a repetition to be complete. Since I am a big believer in the idea that you will compete the way you train, you should also perform most lifts during your training to competition standards. It is advisable, however, to use sleeves and wraps if possible to minimize joint stress.