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  Don't lock out! (elbows, knees)

When using weights or other strength-building tools, don't lock out your elbow and knee joints at the extremity of the movement. 

That is, don't extend them to their absolute limit, where the load is eventually being carried by the joint itself: 

- firstly, it's your muscles that are meant to be taking the load,

- secondly, it causes wear on the joints, with ensuing (eventual) pain.  It's worse if you don't follow good form, or even make a one-time mistake, and/or do it with too much load.  Arthritic pain may remind you of this folly - later, and frequently!