Our muscles act as the motors of the movement system. A signal from the nervous system initiates a muscle force.
A muscle pulls (internal force) on its attachment points. This is called tension.
However, the muscle force is not solely dependent on the signal from the nervous system. The force is also dependent on the actual length of the muscle, which can be determined by some sort of external force.¹
When a muscle shortens, various protein filaments slide past each other forming tiny interlocking cross bridges. The cross bridges allow for the buildup of muscle force.
When muscle is very short, there is too much overlap among the filaments to allow much tension to build up, and when muscle is very stretched, there is too little overlap for the development of appreciable tension.²
At intermediate muscle lengths, many cross-bridges can be formed and tension can be great.²
Think of this as a strip of velcro. There is an optimum amount of overlap to produce tension.
[Tom Purvis, explaining length-tension relationship in his DVD, Developing Advanced Insights]
At extreme joint angles, when the muscles tend to be maximally stretched or contracted, the forces that can be produced are smaller than at intermediate joint angles.²
This is why greater force can be generated when the joint is in the middle of its range of motion.
The complication: since motion of body segments depends not only on the muscle forces but also on the forces of the environment, actual muscle forces become unpredictable as the environment is typically unpredictable.¹