Should You Lock Out When Lifting?
When it comes to whether you should lock out (i.e. fully extend your joint, be it your knee, elbow or shoulder) there is no clear and concise answer.
A quick Google search will show you that guys on bodybuilding, powerlifting and general fitness forums have been arguing over this exact point for the better part of two decades.
Frankly I don't think there's a blanket answer when it comes to locking out.
What YOU should do depends on a few factors including the exercise in question as well as your training goals.
If you're a powerlifter and you opt to not lock out on your exercises when it comes to competition time you're going to be disqualified for not completing a full repetition... be it on your bench, deadlift or squat. As such it makes sense for powerlifters to lock out while training - if you only perform three quarters of a rep you'll only build strength and fluidity in three quarters of the rep - this is counterproductive to your goals.
On the other side of the coin...
If you're an enhanced bodybuilder and time under tension is the aim of the game when it comes to your training then you're probably going to be selective on the exercise you lock out on (if you lock out on any at all)
Pros For Locking Out While Lifting
You'll Build Strength In The Full Range Of The Exercise
It makes sense, unless you're constantly performing a full range of motion on your exercises (which includes the lock out) you won't build strength in the full range of motion of the exercise.
As mentioned, if you're a powerlifter this is a huge reason why you should be locking out on your lifts, on the other hand if strength isn't a concern to you and you're more interested in achieiving hypertrophy of the muscle on isolation based exercises this won't apply to you.
You'll Strengthen Your Joints
Here's the catch-22... locking out does place additional stress on your joints but at the same time we must place stress on our joints to strengthen them.
Unless you're regularly locking out on your exercises you won't be forcing your joints to adapt to this stress.
No, I'm not telling you to load up 315lbs and try and lock it out as many times as you can to target your joints, simply lock out each rep as normal and over time your joints will adapt to this.
Every Rep Will Count
In the eyes of a powerlifter a rep doesn't count unless you've locked out the exercise.
If you want to attempt a guiness world record you must lock out for the repetition to count.
Essentially in the eyes of any competition or governing body you must lock out to complete the entire repetition for it to count.
Want to eliminate all debate over whether your set/rep/exercise was legitimate? Lock out and they'll have nothing to say.
Biomechanically Your Joints Were Made To Lock Out
iomechanically, joints are made to lock out. It is in this locked out position when they are in the most stable position to support loads.
Some individuals also believe that the joint isn't taking on any additional load whether it is locked out or not (more research is required here).
Cons For Locking Out While Lifting
Stopping just shy of lock out ensure tension remains on the targeted muscle group instead of being transferred to the joint - time under tension is one of the factors contributing to progressive overload. This is why you'll rarely see any serious or professional bodybuilder locking out on an exercise as they know this is disruptive to theur goals - time under tension = growth and locking out eliminates this.
Coming Down On A Locked Joint Is Dangerous
The risk of injury when landing on a locked joint is extremely high - coming down from box jumps is a great example of this.
You've always been told to land from a jump with 'soft knees' as the knee joint is at its weakest under load when at a full extension.
As we've discussed above whether you should lock out your shoulders and elbows etc. on lifts is still up for debate and depends on your goals, but you should never ever come down on a locked joint.
You'll Put More Stress & Wear On Your Joints
In order to strengthen a joint we must place a degree of stress on that joint but at the same time constantly placing stress on your joints is a sure-fire way to wear them out.
A guy that locks out on his bench press for years and years may have stronger joints than the guy that focuses on performing three quarters of each repetition (and stops before the lock out) but years down the track the guy who didn't place all of that extra stress on his joints will have better mobility and overall joint health.