Motor Unit Discharge Characteristics and Conduction Velocity of the Vastii Muscles in Long-Term Resistance-Trained Men

Abstract

PurposeAdjustments in motor unit (MU) discharge properties have been shown after short-term resistance training; however, MU adaptations in long-term resistance-trained (RT) individuals are less clear. Here, we concurrently assessed MU discharge characteristics and MU conduction velocity in long-term RT and untrained (UT) men.MethodsMotor unit 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 vastus medialis (VM) of RT (>3 yr; n = 14) and UT (n = 13) during submaximal and maximal isometric knee extension.ResultsResistance-trained men were on average 42% stronger (maximal voluntary force [MVF], 976.7 ± 85.4 N 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 pps vs 10.3 ± 1.5 pps; VM, 10.7 ± 1.6 pps 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 with UT (VL, 21.1 ± 4.1 pps vs 14.0 ± 4.5 pps; VM, 19.5 ± 5.0 pps vs 17.0 ± 6.3 pps; P = 0.7173). Motor unit conduction velocity was greater in RT compared with UT individuals in both VL (4.9 ± 0.5 m·s−1 vs 4.5 ± 0.3 m·s−1; P < 0.0013) and VM (4.8 ± 0.5 m·s−1 vs 4.4 ± 0.3 m·s−1; P < 0.0073).ConclusionsResistance-trained and UT men 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.

Human motor unit discharge patterns reveal differences in neuromodulatory and inhibitory drive to motoneurons across contraction levels

“…However, the peak discharge rates were greater, and increased to a greater extent in TA, reaching values up to ∼30 pps on average during contractions to 70% MVF, compared to ∼17-18 pps in VL and VM at the same relative force levels. This is consistent with prior reports from studies using decomposition of intramuscular (Leong et al ., 1999) and surface EMG signals (Škarabot et al ., 2023b), whereby MU discharge rates of the knee extensors only reach ∼20-25 pps during maximal effort isometric contractions. The reason for differences in discharge rate between muscles remains unclear; the vastii motor pools might receive a large degree of tonic inhibitory input that is more constant or proportional in its pattern to excitatory input (see below), or the common input might be more evenly distributed in the vastii muscles due to the presumed smaller relative contribution of individual motor pools of the quadriceps to knee extension compared to the relatively greater contribution to TA motor pool to dorsiflexion.…”