Voluntary force production depends on muscle-tendon unit length, due not only to mechanical factors (e.g., force-length properties and moment arms) but also neural modulation mediated by afferent feedback. Whether muscle length-dependent modulation of motor unit (MU) discharge differs across MU recruitment thresholds remains unclear. This study compared MU firing rates of the tibialis anterior during submaximal contractions with two ankle joint positions, while grouping MUs by recruitment threshold. Eighteen healthy adults performed isometric dorsiflexion at 0° (shortened position) and 20° (lengthened position) plantar flexion. High-density surface electromyography was decomposed into individual MU discharge times. Maximal voluntary contraction (MVC) torque was measured in each position. Participants then performed ramp-up contractions to 50% MVC. Firing rates were separately assessed for the same absolute target and relative MVC torque. MUs were categorized as low- (0-15% MVC), mid- (15-30% MVC), or high- (30-45% MVC) threshold. High-threshold MUs discharged faster in the shortened than lengthened position under both absolute and relative torque comparisons, whereas low-threshold MU firing rates did not differ between positions. These findings indicate that a shortened muscle position preferentially enhances the firing rates of higher-threshold MUs during submaximal contractions, in agreement with neural compensation for reduced mechanical efficiency at shorter lengths.
Hirono et al. (Fri,) studied this question.
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