(20) YOGA IMPROVES KNEE EXTENSOR STRENGTH BUT DOES NOT INFLUENCE MECHANICAL BEHAVIOR OR MUSCLE SIZE IN ACTIVE ADULTS

Purpose: Mechanomyography (MMG) is described as the mechanical counterpart to neural activation and is influenced motor unit (MU) behavior. The purpose was to examine the mechanical behavior of the vastus lateralis (VL) before (PRE) and after (POST) a yoga intervention. Methods: Twenty-one healthy, recreationally active adult males (N = 13) and females (N = 8) were assigned to YOGA (X̅ ± SD; N = 11, age = 22.9 ± 4.5 yrs, body mass = 80.4 ± 15.3 kg) or Control (CON) (N = 10, age = 21.5 ± 3.5 yrs, body mass = 71.3 ± 12.1 kg) groups. YOGA completed 8 weeks of Hatha Vinyasa yoga, attending 2x/week (16, 1-hour sessions) while continuing their regular physical activity. CON continued their regular physical activity for 8 weeks without the addition of yoga. At PRE and POST 8-week treatment, panoramic ultrasound imaging examined the VL for muscle cross-sectional area (mCSA) and echo intensity (EI). Then, subjects performed isometric voluntary contractions (MVC) of the knee extensors, followed by an isometric trapezoidal muscle action at 50% MVC for the respective visit. A miniature accelerometer was affixed to the skin above the muscle belly of the VL to collect MMG signals. Simple linear regression models were applied to the log-transformed MMG_RMS vs. torque relationships for the linearly increasing segment of the muscle action. For the plateau, MMG_RMS was normalized (N-MMG_RMS) to the MVC MMG_RMS. Repeated measures ANOVAs (2x2; group x time) were used for analyses. When appropriate, follow-up analyses were used with Bonferroni corrections. Effect sizes were reported as partial eta squared (ηp²) and Hedge's g. α was set at 0.05. Results: An interaction for MVC (p = 0.010; ηp² = 0.301) showed an increase for YOGA (PRE = 232.0 ± 103.3 Nm, POST = 258.4 ± 111.8 Nm; p = 0.010, g = 0.30), but not for CON (PRE = 202.6 ± 51.2 Nm, POST = 203.1 ± 60.0 Nm; p = 0.907, g = 0.01). There was no interaction (p = 0.279; ηp² = 0.061), nor time (p = 0.836; ηp² = 0.002) or group (p = 0.208; ηp² = 0.008) effect for mCSA. There was no interaction (p = 0.787; ηp² = 0.004), nor time (p = 0.625; ηp² = 0.013) or group (p = 0.935; ηp² < 0.001) effect for EI. There was no interaction (p = 0.087; ηp² = 0.146), nor time (p = 0.730; ηp² = 0.006) or group (p = 0.289; ηp² = 0.059) effect for N-MMG_RMS for the plateau. There was no interaction (p = 0.328; ηp² = 0.050), nor time (p = 0.560; ηp² = 0.018) or group (p = 0.927; ηp² < 0.001) effect for the b terms during the linearly increasing segment. There was no interaction (p = 0.255; ηp² = 0.068), nor time (p = 0.829; ηp² = 0.003) or group (p = 0.440; ηp² = 0.032) effect for the a terms during the linearly increasing segment. Conclusions: Hatha Vinyasa yoga can improve knee extensor strength when added to physical activity compared to physical activity alone, despite no changes in muscle size or quality. As a result, YOGA had to produce greater torque during POST tests. There was no change in the gain (a terms) or the rate of change between MMG_RMS and torque (b terms) during the linearly increasing segment—suggesting there was no additional recruitment of higher threshold MUs to modulate torque. Instead, torque may have been modulated by an increase in MU firing behavior as previously reported (Sontag et al. 2024), but MMG_RMS methods may not be a sensitive enough measure for the magnitude of strength improvement observed in this study. Practical Applications: Even in an already active lifestyle, the addition of a yoga practice may further promote improvements in strength. Acknowledgements: None