The two key elements when modeling the effects of concentric or isometric muscular contractions on fiber mechanical loading, and therefore the hypertrophic stimulus, will be (1) whether fiber is activated, and (2) its shortening velocity.

(A more advanced model would include the length of the fiber at the point when peak force is produced during the contraction, and the subsequent elastic deformation of the fiber, but this is less important. Also, the model would not describe the mechanical tension involved in the eccentric phase of any dynamic movements, although this is not as serious a problem as we might anticipate, because recruitment is much lower in his phase in normal strength training)

Muscle fibers increase exponentially with increasing motor unit recruitment number, and there are formulae available for estimating this. For example: https://www.instagram.com/p/BlICiKRnOgT/?taken-by=chrisabeardsley

Motor unit recruitment in unfatigued muscle increases with increasing force levels, and this has been modeled a lot. For example: https://www.instagram.com/p/BlICiKRnOgT/?taken-by=chrisabeardsley

Fatigue also increases motor unit recruitment, and this has been modeled recently here: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5473583/

Muscle-tendon unit shortening velocity can be modeled from joint angular velocity. Ideally, we would will need at least two adjustments to reach fiber shortening velocity from muscle-tendon unit shortening velocity because of (1) the series elastic element that is the tendon, and (2) the effects of fiber rotation and subsequent architectural gearing. https://www.instagram.com/p/BgoH3CyHj3Y/?taken-by=chrisabeardsley