Motor unit firing patterns of lower leg muscles during isometric plantar flexion with flexed knee joint position.
Level 3 - non-randomized controlled study
Design analogy: prospective repeated-measures physiological crossover experiment in healthy humans.
PubMed 36368144 · doi:10.1016/j.jelekin.2022.102720
What was done
Twenty healthy participants performed isometric ramp contractions of the ankle plantar flexors and dorsiflexors up to 30% of maximal voluntary contraction (MVC) under two knee postures: extended (0°) and flexed (130°). High-density surface electromyography was recorded over the medial gastrocnemius (MG), soleus (SOL), and tibialis anterior (TA) muscles and decomposed to evaluate recruitment-threshold-dependent motor unit firing behavior across joint angles.
What was found
The torque-dependent change in motor unit firing rate (Δpps/Δ%MVC) was significantly higher with a flexed knee compared to an extended knee for MG motor units recruited at 15% MVC and SOL motor units recruited at 5% MVC (p < 0.05). No significant difference across knee angles was observed for TA motor units. Exact numerical values, baseline firing rates, and effect sizes were not reported in the abstract.
Why it matters
The findings illustrate how joint posture alters motor unit excitability in multi-joint and single-joint plantar flexors, demonstrating neuromuscular compensation when gastrocnemius mechanical leverage is compromised in flexed positions such as vehicle driving.
Limits
The study was small (n = 20) and conducted exclusively in healthy participants. Contractions were limited to static isometric ramps at a single submaximal target (30% MVC) and two discrete knee angles (0° and 130°), which does not capture the dynamic force demands of actual pedal operation. Specific quantitative firing rate metrics and variance were omitted from the abstract.
Cited by
- supports Seated calf raises isolate the soleus muscle because bending the knee reduces gastrocnemius involvement due to the gastrocnemius crossing the knee joint.