How does strength training produce muscle growth?

To understand exactly how the volume-matched, proximity-to-failure-matched very light load (20% of 1RM) strength training program was unable to produce the same amount of muscle growth as the heavier load programs, we need to start right at the very beginning.

How does strength training cause muscle growth?

There are three proposed mechanisms by which muscle fibers increase in size (mechanical loading, metabolic stress, and muscle damage), but only mechanical loading has a strong weight of evidence behind it. Conceivably, the other mechanisms may be effective simply because they also stimulate mechanical loading on individual fibers (but we do not know for sure yet).

For an individual muscle fiber to experience mechanical loading during a strength training exercise, it needs both:

to be activated; and to contract at a speed that is slow enough to allow enough actin-myosin bindings to form simultaneously (the number of simultaneous actin-myosin bindings affects the force produced by a fiber, and this is determined by the contraction velocity).

When lifting heavy loads — most motor units are quickly recruited, and the weight is heavy enough that a fast bar speed is impossible. Thus, heavy loads automatically cause (1) activation of fibers attached high-threshold motor units, and (2) enough actin-myosin bindings to form simultaneously.

When lifting light loads to failure — few motor units are initially activated, but as metabolites accumulate and cause fatigue, high-threshold motor units are recruited in order to compensate for the reduced capacity for force production in the working muscle fibers. Similarly, bar speed is initially quick, but as metabolites accumulate and cause fatigue, bar speed involuntarily slows down. By the end of a set to failure, light loads also cause (1) activation of fibers attached high-threshold motor units, and (2) enough actin-myosin bindings to form simultaneously.

Strength training, either with heavy loads, or with light loads to failure, therefore imposes mechanical loading on individual muscle fibers, and therefore brings about hypertrophy.

(N.B. deliberately slowing down bar speed with light loads does not work, because it reduces motor unit recruitment even though it increases the number of actin-myosin bindings that form simultaneously).