Once you start hurting and you do not understand why. It’s simple! What the general population doing as a workout, we can call just: let’s damage the body forever. Many people, specially the beginners or the re-starters are don’t know where is their limitation and they just kill their joints with amazing overloads and techniques. Often, due to the force arm, the joints are much more powerful than the muscles. With the wrong technique, we just put the joints in place of the muscles and the surrounding ribbon system.
The joint pain is a bit like crawling cancer: we don’t want to understand why, but it is quite clear. It’s a fact that we should take care of the mechanical load of the joints, not just the muscles. The two things are not the same. We don’t have the same load on our joints as our muscles. Neither the magnitude of the forces nor the direction of the forces. We usually charge the joints at a different angle than the muscles.
Certainly, many people know the feeling when the lower biceps begin to hurt the biceps instead. This is a standard example, but the squirming knee and the shoulder damaged by bad foot pressure are all talkative examples of how the movement should be coordinated and concentrated with a weight, velocity, and movement range that maximizes muscle mass and does not mow the joints. This, of course, also means that we will not go down to low repetition numbers unless we can perform repetitions very slowly and controlled.
Do not forget: the build of true strength is takes time. Our goal is to develop strength and muscle, not to kill the joints.