Alex A. Olmos1, Daniel J. Lawson1, Lyric D. Richardson1, Stephanie A. Sontag1, Brenden Roth1, Sunggun Jeon2, Allen L. Redinger1, Weston Franklin1, Tristan Ball1, Maria Parodi1, Bryson Trask1, & Michael A. Trevino1

1Oklahoma State University, Stillwater, Oklahoma; 2Northwestern State University, Natchitoches, Louisiana

PURPOSE: To compare the effects of 6 weeks of combined high- and low-load lower-body resistance training (HL) with high-load only training (H) on motor unit (MU) behavior and muscle cross-sectional area (mCSA) of the vastus lateralis (VL), and maximal strength of the knee extensors. METHODS: Twenty-three untrained males (age = 24 ± 6 yrs) were randomized into the HL (n = 12) or H (n = 11) group. Before (PRE) and after training (POST), participants performed maximal voluntary isometric contractions (MVCs) of the knee extensors on a dynamometer followed by a submaximal trapezoidal muscle action at 40% MVC. Surface electromyography (sEMG) was recorded from the VL. VL mCSA was measured via ultrasonography. sEMG signals for the 40% 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. Five separate 2-way mixed factorial repeated measures ANOVAs (group [H vs. HL] x time [PRE vs. POST]) were used to examine potential differences in MVC, y-ints and slopes for the MUAPAMP and MFR vs. RT relationships. An independent samples t-test examined mCSA change scores. RESULTS: For MVC, there was no two-way interaction (p > 0.05) or main effect for group (p > 0.05). There was a main effect for time (p < 0.05). MVC increased at POST collapsed across groups. For mCSA, the change scores were greater for HL than H (p < 0.05). For the y-ints and slopes from the MUAPAMP vs. RT relationship, there was a two-way interaction (p < 0.05). For HL, the slopes increased (p < 0.05) and y-ints decreased (p < 0.05) from PRE to POST. For the slopes from the MFR vs. RT relationships, there was no two-way interaction (p > 0.05) or main effects for group or time (p > 0.05); there was a two-way interaction for the y-ints (p < 0.05). For H, the y-ints increased from PRE to POST (p < 0.05). CONCLUSION: Maximal strength increased similarly for both groups, whereas HL had greater increases for mCSA. MU adaptations differed between groups post-training. During the 40% MVC, the H group increased firing rates across the recruitment spectrum to match the targeted torque, whereas the HL relied on recruiting MUs with potentially larger muscle fibers and greater force twitches as firing rates relative to RTs were unchanged.

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