Very true...

People will get bored of me saying this but this is why isolation is very bad. Look at how muscle groups manipulate the Skeleton:



Muscles work by shortening during contraction and pulling on tendons which in the case of the elbow joint either extend the joint by having the tricep pull the forearm or flex the joint by having the bicep pull the forearm.

If one is too tensed and too short it causes imbalance which leads to injury, and fatigue because other muscles in the group are having to work harder to compensate.

In the case of each individual muscle... to much contraction and not enough stretching means that the muscle cells are not at their maximum length and the muscle does not contract as much as it should, so it doesn't achieve the strength it is capable of pulling.

Theres an animation of the elbow joint flexing and extending... I'll find it to give you a better idea of how it works

Muscle work by shortening from a length... if the length isn't there... the contraction is shorter... guess what that means?