Hierarchical control of motor units in voluntary contractions.
Level 4 - case-series / case-control
Observational human neurophysiological study without a clinical comparator group (graded by design analogy)
PubMed 21975447 · doi:10.1152/jn.00961.2010
What was done
Investigators measured human motor unit firing rates during linearly force-increasing voluntary contractions to evaluate whether motor unit firing rates relate to recruitment thresholds as predicted by classical anesthetized animal stimulation models.
What was found
The abstract reports no exact numerical values, sample sizes, or error bounds. Qualitatively, during ramp contractions, firing rates increased as exponential functions. At all times and force levels, firing rates were inversely related to recruitment threshold (earlier-recruited units fired faster than later-recruited units), while the exponential time constants were directly related to recruitment threshold.
Why it matters
This study challenges the five-decade-old assumption derived from anesthetized animal experiments that higher-threshold motoneurons fire faster than lower-threshold ones, supporting instead a hierarchical control scheme optimized for force economy and fatigue prevention.
Limits
The abstract omits sample size, participant demographics, specific muscles tested, and precise quantitative metrics. Soma size relationships were inferred via the Henneman size principle rather than directly measured.
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