The ribht answer is a.
* Muscle contraction results from the coordinated contraction of each of the muscle cells.
There are four phases during the contraction of a "typical" muscle cell:
- the excitation or the stimulation which corresponds to the arrival of the nervous message on the muscular fiber;
- the excitation-contraction coupling which groups together all the processes making it possible to transform the nerve signal received by the cell into an intracellular signal towards the contractile fibers;
- the contraction itself;
- the relaxation which is the return of the muscular cell to the state of physiological rest.
* There are two types of muscle contraction:
isometric contraction when the length of the muscle does not change when contracting (supports a load);
anisometric contraction (or isotonic) when there is a change in the length of the muscle, with a rapprochement of the points of insertion of the muscles it is concentric and eccentric when these move away;