The increase in muscle force after 4 weeks of strength training is mediated by adaptations in motor unit recruitment and rate coding.
Level 3 - non-randomized controlled study
Controlled trial with a parallel control group; randomization is not stated in the abstract.
PubMed 30727028 · doi:10.1113/JP277250
What was done
The study evaluated motor unit discharge characteristics in the tibialis anterior muscle before and after 4 weeks of isometric strength training of the ankle dorsiflexors. Fourteen participants in a training group were compared to 14 control participants. High-density electromyography (128-electrode grids) was used to longitudinally track motor units during submaximal isometric ramp contractions up to 35%, 50%, and 70% of maximal voluntary force. Assessed outcomes included recruitment and derecruitment thresholds, motor unit discharge rates, interspike intervals, and input-output relationships.
What was found
After 4 weeks of training: - Normalized recruitment-threshold forces of motor units significantly decreased (P < 0.05). - Motor unit discharge rate during the plateau phase of submaximal contractions increased by 3.3 ± 2.5 pps across subjects and motor units (P < 0.001). - Discharge rates at recruitment and derecruitment were not altered. - The association between force and motor unit discharge rate during the ramp phase was unchanged.
Why it matters
This provides direct longitudinal human evidence that early strength gains are driven by neural adaptations, specifically decreased recruitment thresholds and higher motor unit firing rates.
Limits
The study tested a single muscle (tibialis anterior) during isometric contractions, which may not generalize to dynamic contractions or other muscle groups. The sample size was small (n = 28 total), and the abstract does not state whether subjects were randomized or provide demographic details such as age and sex.
Cited by
- supports Repeated practice and training improve the nervous system's ability to recruit neural resources necessary to perform the work.