Final answer:
Electrical stimulation strengthens muscles primarily through wave summation, where successive stimuli are combined for a stronger contraction due to increased calcium release, and motor unit recruitment, where more muscle fibers are activated for greater force generation.
Step-by-step explanation:
Electrical stimulation is thought to strengthen muscles through two primary mechanisms: wave summation and motor unit recruitment. Wave summation occurs when successive stimuli are added together, resulting in a stronger muscle contraction. This is because a second stimulus triggers the release of more calcium ions while the muscle is still contracting from the first stimulus. These additional calcium ions become available to activate more sarcomeres, enhancing contraction.
Another mechanism is motor unit recruitment. The amount of force a muscle can produce is proportional to the number of muscle fibers receiving an action potential from their controlling neuron. When a high force is required, more neurons are signaled, and more muscle fibers are activated, which is the essence of recruitment.
Muscle contraction in general involves the sliding filament model where ATP is used to power the cross-bridging of myosin heads into actin-binding sites, leading to muscle shortening and movement. Electrochemical stimulation of muscle fibers at the neuromuscular junction, initiated by neurotransmitter acetylcholine, is the start of this contraction process.