(02) YOGA INFLUENCES STRENGTH AND MOTOR UNIT BEHAVIOR BUT NOT MUSCLE SIZE IN ACTIVE ADULTS

Purpose: To investigate the effects of adding an 8-week yoga intervention to physical activity compared to physical activity maintainance on knee extensor strength, and muscle size, muscle quality, and motor unit (MU) behavior of the vastus lateralis (VL).

Methods: Twenty-four healthy, adult males and females participated in this study. The yoga group (YOGA) (X̅±SD; N=13, age=23±4yrs) participated in 8 weeks of Hatha Vinyasa yoga 2x/week (16, 1-hour sessions) while continuing their regular activity. The control group (CON) (N=11, age=21±3yrs) continued their regular activity for 8 weeks. Subjects reported various physical activity histories prior to the study. Before (PRE) and after (POST) the 8 week treatment period, panoramic ultrasound imaging was performed on the VL for muscle cross-sectional area (mCSA) and echo intensity (EI). Subjects then performed 3 isometric maximal voluntary contractions (MVCs) of the knee extensors, followed by an isometric trapezoidal muscle action at 50%MVC. The highest torque output from each visit determined MVC and the torque level for the 50%MVC. Surface electromyographic (EMG) signals recorded from the VL for the 50% MVC were decomposed. Recruitment thresholds (RT)(%MVC), MU action potential amplitudes (MUAP_AMP)(mV), and mean firing rates (MFRs)(pulses per second [pps]) were calculated for each MU. MUs with accuracies >90% were used for analyses. Linear regression models were performed to calculate the slopes and y-intercepts for the MUAP_AMP and MFR vs RT relationships. EMG amplitude (EMG_RMS) at steady torque for the 50%MVC was normalized (N-EMG_RMS)(%) to MVC EMG_RMS. Repeated measures ANOVAs (2x2; group x time) were used for analyses. When appropriate, follow-up analyses included paired and independent samples t-tests 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 (p=0.041, ηp²=0.18) for MVC showed an increase only in YOGA (PRE=221.1±98.0 Nm, POST=241.4±110.4 Nm; p=0.024, g=0.19). There was an interaction (p=0.004, ηp²=0.31) for the slopes of the MFR vs RT relationships where at PRE YOGA (0.34±0.15 pps/%MVC) was greater than CON (-0.47±0.13 pps/%MVC; p=0.034, g=0.92), YOGA decreased from PRE to POST (-0.42±0.16 pps/%MVC; p=0.047, g=0.19), and CON increased PRE to POST (-0.39±0.08 pps/%MVC; p=0.044, g=0.74). An interaction (p=0.006, ηp²=0.30) for the y-intercepts of the MFR vs RT relationships showed YOGA increased PRE (24.0±4.5 pps) to POST (27.6±6.2 pps; p=0.038, g=0.66). There were no interactions, time, or group effects for mCSA, EI, N-EMG, or the MUAP_AMP vs RT relationship coefficients (p>0.05).

Conclusion: Adding Hatha Vinyasa yoga to current activity improved knee extensor strength compared to activity maintenance alone.  For YOGA, no change in muscle size or quality, N-EMG_RMS, and MUAP vs. RT relationships in conjunction with the greater y-intercepts and more negative slopes for the MFR vs RT relationships at POST suggests that increases in maximal strength may be due to increased excitability of the lower threshold MUs. For CON, the more positive slopes for the MFR vs RT relationships may be a result of their regular activity, but there were no maximal strength related changes. PRACTICAL APPLICATION: Consistent yoga practice added to a recreationally active lifestyle has the potential to increase excitability of the lower threshold MUs that could result in greater maximal force production.

Acknowledgements: None