What is the Origin of High Threshold Recruitment?

This concept is directly derived from the Henneman Size Principle, a foundational law of motor unit physiology established in the mid-20th century, which states that motor units are recruited in an orderly, size-dependent fashion. The principle dictates that low-threshold, small units are activated first, followed by the high-threshold, large units as force demand increases. The term highlights the deliberate effort required to access these high-capacity fibers.

What is the Mechanism of High Threshold Recruitment?

The mechanism requires generating high levels of intramuscular tension, which can be achieved through lifting heavy external loads or moving submaximal loads with maximal intended velocity. This intense demand necessitates a maximal excitatory input to the alpha motor neurons, allowing them to reach the high firing threshold of the Type II motor units. The recruitment of these fibers is the primary mechanical signal that upregulates muscle protein synthesis and drives the chronic adaptations in muscle mass and strength.

High Threshold Recruitment ∞ Area

High Threshold Recruitment

Meaning

High Threshold Recruitment is the physiological process of selectively activating the motor units that innervate the largest, most powerful muscle fibers, typically the Type II or fast-twitch fibers, which possess a substantial firing threshold. These motor units are only engaged when the neural drive from the central nervous system is exceptionally high, typically due to heavy mechanical load or near-maximal effort. Clinically, achieving this recruitment is paramount for stimulating maximal muscle growth and strength adaptation.