Motor unit discharge characteristics and conduction velocity of the vastii muscles in long-term resistance-trained men

Purpose: Adjustments in motor unit (MU) discharge properties have been shown following short-term resistance training, however MU adaptations in long-term resistance-trained individuals are less clear. Here, we concurrently assessed MU discharge characteristics and MU conduction velocity in long-term resistance-trained (RT) and untrained (UT) men.

Methods: MU discharge characteristics (discharge rate, recruitment and derecruitment threshold) and MU conduction velocity were assessed after the decomposition of high-density electromyograms recorded from vastus lateralis (VL) and medialis (VM) of RT (>3 years; N=14) and UT (N=13) during submaximal and maximal isometric knee extension.

Results: RT were on average 42% stronger (maximal voluntary force, MVF: 976.7±85.4 vs. 685.5±123.1 N; p<0.0001), but exhibited similar relative MU recruitment (VL: 21.3±4.3 vs. 21.0±2.3 %MVF; VM: 24.5±4.2 vs. 22.7±5.3 %MVF) and derecruitment thresholds (VL: 20.3±4.3 vs. 19.8±2.9 %MVF; VM: 24.2±4.8 vs. 22.9±3.7 %MVF; p≥0.4543). There were also no differences between groups in MU discharge rate at recruitment and derecruitment, or at the plateau phase of submaximal contractions (VL: 10.6±1.2 vs. 10.3±1.5 pps, VM: 10.7±1.6 vs. 10.8±1.7 pps; p≥0.3028). During maximal contractions of a subsample population (10 RT, 9 UT), MU discharge rate was also similar in RT compared to UT (VL: 21.1±4.1 vs. 14.0±4.5 pps, VM: 19.5±5.0 vs. 17.0±6.3 pps; p=0.7173). MU conduction velocity was greater in RT compared to UT individuals in both VL (4.9±0.5 vs. 4.5±0.3 m.s^-1; p<0.0013) and VM (4.8±0.5 vs. 4.4±0.3 m.s^-1; p<0.0073).

Conclusions: RT and UT display similar MU discharge characteristics in the knee extensor muscles during maximal and submaximal contractions. The between-group strength difference is likely explained by superior muscle morphology of RT as suggested by greater MU conduction velocity.