Question

explain how one muscle (such as the biceps brachii for example) can perform different strengths of contraction.

Answer 1

Muscles can perform at different force of contraction, because of the property known as motor unit recruitment, which is the successive activation of the same and additional motor units with increasing strength of voluntary muscle contraction.

A motor unit consists of one motor neuron and all of the muscle fibers it stimulates, all the muscles have number of motor units and the fibers belonging to a motor unit are spread across,. Each muscle fiber is contacted by only one motoneuron, but a motoneuron can contact many muscle fibers. When a motor neuron is activated, all of the muscle fibers innervated by the motor neuron are stimulated and they contract, but the activation of one motor neuron will produce a weak force, but the activation of more motor neurons will result in more muscle fibers being activated, and therefore a stronger muscle contraction and a stronger force, so higher the recruitment higher the force produced.

This was first proposed by a scientist known as Henneman, who proposed that  as more force is needed, motor units are recruited in a precise order according to the magnitude of their force output, with small units being recruited first,folllowed by larger motor neuron, thus from slow twitch fibere to fast twitch fiber, thus exhibiting a task based recruitment.

Another important factor which determines force production is the frequency with which muscle fibers are stimulated, this is known as motor unit firing rate, generaly  the motor unit firing rate of each individual motor unit increases with increasing muscular effort until a maximum rate is reached.