Thanks!

It is true that some researchers originally speculated that the density (number per unit cross-sectional area) of myofibrillar strings in parallel inside each muscle fiber could vary. This would affect the strength of a muscle fiber (and therefore of a muscle) relative to its size, since it is the myofibrillar strings that are responsible for producing force.

This hypothesis arose in order to explain why strength increases by far more than muscle size after strength training, and why strength increases by more after strength training with heavy loads than after strength training with light loads, despite similar gains in muscle size.

Specifically, it was suggested that strength training with heavier loads might cause more myofibrillar hypertrophy, and less sarcoplasmic hypertrophy, compared to strength training with lighter loads.

However, this probably does not happen.

Research has shown that the number of myofilaments in a muscle fiber increases in proportion to its cross-sectional area after strength training. Also, the force that a single muscle fiber can exert relative to its cross-sectional area tends to remain constant after strength training, when the fiber is measured in vitro.

Also, we now know that strength training increases strength by more than size because there are many other adaptations that contribute to an increased ability to produce force, and these adaptations are preferentially stimulated when using heavy weights. So the hypothesis that myofibrillar and sarcoplasmic hypertrophy can occur independently is unnecessary.