Ballistic contractions in man: characteristic recruitment pattern of single motor units of the tibialis anterior muscle.
Level 5 - mechanism / opinion, no new human data
Laboratory physiological investigation in healthy humans (level by design analogy).
PubMed 845820 · doi:10.1113/jphysiol.1977.sp011689
What was done
Electrophysiological recordings of single motor units were obtained with selective electrodes from intact tibialis anterior muscles in adult humans during voluntary isometric contractions. Investigators conducted parametric analyses comparing discharge patterns between slow ramp tracking contractions (force thresholds exceeding 8 kg) and brisk ballistic contractions reaching peak forces of 0.05 to 12 kg in less than 0.15 s. A method was developed to calculate ballistic force thresholds to evaluate recruitment order.
What was found
In slow ramp contractions, single motor units initiated firing at 5–15/sec and increased with rising force. In strong ballistic contractions, the same units fired in early bursts preceding force generation at instantaneous frequencies of 60–120/sec that subsequently slowed, a pattern absent during fast ramp tracking reaching 12 kg in 0.4 s. The recruitment order determined by ballistic force thresholds was virtually identical to that observed during slow tracking ramp contractions (correlation = 0.95). Force gradation in ballistic efforts occurred through both additional motor unit recruitment and increased firing rate.
Why it matters
The study demonstrated that the orderly recruitment of motor units is preserved even during rapid ballistic muscle contractions, with rapid force generation facilitated by initial high-frequency discharge bursts.
Limits
The abstract does not state the number of human participants or total motor units analyzed. Findings are limited to isometric contractions of a single lower-leg muscle (tibialis anterior) and do not describe dynamic, multi-joint movements.
Cited by
- contradicts Fast-twitch muscle fibers only recruit/activate with higher force production.