Influence Of Divergent Resistance Training On Absolute And Relative Motor Unit Behavior And Input Excitation

PURPOSE: To compare the effects of 16 sessions of combined strength and hypertrophy lower body resistance training (SH) to strength focused only training (S) on maximal strength of the knee extensors, muscle cross-sectional area (mCSA) of the vastus lateralis (VL), and motor unit (MU) behavior. METHODS: 25 untrained males (age = 24 ± 6 yrs) were randomized into SH (n = 12) or S. Participants performed maximal voluntary isometric contractions (MVCs) and 70% MVCs of the knee extensors before (PRE) and after 6 weeks of training at the same absolute pre-training submaximal torque (POSTABS) and relative to post-training MVCs (POSTREL).Surface electromyography (sEMG) was recorded from the VL. VL mCSA was measured via ultrasonography. sEMG signals for the 70% MVCs were decomposed for analysis of MU: recruitment thresholds (RTs), action potential amplitudes (MUAPAMPS), and mean firing rates (MFRs). Y-intercepts (y-ints) and slopes were calculated for the MUAPAMP and MFR vs. RT relationships. Steady torque EMGRMS was normalized (N-EMGRMS) to testing MVCs. A 2-way mixed factorial repeated measures (MFRM) ANOVA (group [SH, S] x time [PRE, POST]) was used for MVC. An independent samples t-test examined mCSA change scores. 2-way MFRM ANOVAs (group x time [PRE, POSTABS, POSTREL]) were used for y-ints and slopes from the MUAPAMP and MFR vs. RT relationships, and N-EMGRMS. RESULTS: MVC increased at POST when collapsed across groups (p < 0.05). For mCSA, the change scores were greater for SH than S (p < 0.05). For SH, slopes from the MUAPAMP vs. RT relationship increased from PRE to POSTABS and POSTREL (p < 0.05). For S, the y-ints and slopes from the MFR vs. RT relationships increased and became more negative from PRE to POSTREL (p < 0.05), respectively. Moreover, the y-ints and slopes were greater for S than SH at POSTREL (p < 0.05). N-EMGRMS for POSTABS was lower than PRE (p < 0.05) when collapsed across groups. CONCLUSIONS: The similar increase in maximal strength for both groups may have resulted in lower input excitation necessary for POSTABS 70% MVC. However, the greater increase in mCSA for SH, may have resulted in recruiting MUs with potentially larger muscle fibers and greater force twitches to match POSTABS and POSTREL torque, whereas S relied on increased firing rates of the lower threshold MUs relative to RT to match targeted POSTREL 70% torque.