Since the 50s we known that the characteristics (speed and force) of your muscles are determined by the properties of the motoneurons. This implies to stop blaming your parents for giving you bad muscles and to start blaming them for giving you bad motoneurons. A THREAD

From Buller et al. J Physiol, 1960

In the preceding paper (Buller, Eccles & Eccles, 1960) evidence was presented which suggested that the differentiation of slow muscles of the cat hind limb to a large extent failed to occur after certain operative procedures on the spinal cord.

This finding indicates that in some way the central nervous system controls muscle differentiation. A more analytic investigation into this postulated influence of nerve on muscle has been accomplished by dividing and cross-uniting nerves to fast and slow muscles,

so that motoneurones formerly innervating the fast muscle come to innervate the slow muscle by virtue of the regenerative outgrowth of their fibres, and vice versa for the motoneurones of the slow muscle.

The effect of this crossed innervation on the speed of muscle contraction has been tested at varying times after the cross-union. These investigations have also been carried out on animals subjected to the operative procedures on the spinal cord (Buller et al. 1960).

Cross-union experiments allow in addition an investigation into the possible effects in the reverse direction, i.e. of speed of muscle contraction on the conduction velocity of the axons and the after-hyperpolarization of the motoneurones which innervate it.

RESULTS:
The invariable result of the crossunion operation: a considerable acceleration of the contraction of soleus (slow) muscle reinnervated by FDL nerve fibres, and, complementarily, a considerable slowing of the contraction of the FDL muscle reinnervated by soleus nerves.

All of these results imply that the activity of the motoneurons shape the characteristics of the muscle. We recently proved that by looking at the in-vivo population of the motoneuron, we can predict the speed of the joint in humans during voluntary contractions.