Motor Unit / Muscle Fiber Recruitment

The two basic components of muscle force production are the number of muscle fibers activated and the firing rate of the motor nerve impulses. Muscle force can be increased by activating more muscle fibers, speeding up the firing rate, or both.

Because the firing rate of motor nerve impulses at slow speeds is not as fast as it is at high speeds, the greater muscle force produced at slow speeds is apparently due to greater recruitment of muscle fibers. The selective recruitment of fast-twitch and slow-twitch fibers does not seem to depend on speed: both fiber types are actively recruited during maximal muscular contractions, regardless of movement speed (Lesmes, Benham, Costill & Fink, 1983). Although the mechanism is not fully understood, it is logical that more muscle force can be produced at slow speeds because more fibers can be activated. (This is one of the five reasons the source gives for preferring slow weight-training technique.)

Recruitment also explains advanced systems: advocates of the exhaustion set system believe that more motor units will be recruited, giving a training stimulus not achieved when sets are not performed to exhaustion (Stowers et al, 1983). Slow, controlled resistance training is characterized by more muscle tension, more muscle force, and more muscle fiber recruitment — all of which enhance strength development (Westcott, 1987).

• Force depends on (1) number of fibers activated and (2) motor nerve firing rate.
• Increase force by recruiting more fibers, firing faster, or both.
• Greater force at slow speeds is attributed to greater fiber recruitment.
• Both fast- and slow-twitch fibers are recruited in maximal contractions, regardless of speed (Lesmes et al, 1983).
• Sets to exhaustion are thought to recruit more motor units (Stowers et al, 1983).

• muscle-contraction-nature — the all-or-none basis of recruitment.
• muscle-fiber-types — the fiber types being recruited.
• muscular-strength — recruitment as a determinant of force.
• exhaustion-set — recruiting more motor units via sets to exhaustion.