Author(s): MATSUMOTO, T., TAKAMARU, Y., NAKAYAMA, C., SAWADA, S., MACHIDA, S., Institution: MURATA MANUFACTURING CO., LTD., Country: JAPAN, Abstract-ID: 2784

INTRODUCTION:
Conventional electromyographic (EMG) power spectral density (PSD) analyses are highly influenced by inter-individual variability, limiting detailed assessment of muscle fiber recruitment. This study aimed to examine the effects of habitual bodyweight resistance training (RT) on muscle fiber recruitment in older adults using the Wasserstein-2 distance (W2), which captures global spectral changes beyond conventional band-based frequency analyses.
The W2 distance provides a mathematically consistent metric for quantifying distances between distributions [1]. This property enables the construction of group-representative distributions and allows quantitative and visual evaluation of the direction and magnitude of frequency-domain shifts across conditions, facilitating accurate characterization of spectral shifts and shape changes that are difficult to assess using traditional EMG PSD analyses [2].
METHODS:
Forty community-dwelling Japanese older adults (64-84 years; 16 men, 24 women) participated. Individuals performing RT at least twice per week were classified as the training group, whereas those not performing RT were assigned to the non-training group. Participants completed four motor tasks. Group-representative distributions were calculated using the W2 metric based on rectus femoris EMG PSD for each combination of task, sex, and training status. Between-group comparisons were then conducted.
RESULTS:
Substantial W2 differences between concentric and eccentric contractions were consistently observed, indicating that eccentric contractions allow efficient fast-twitch fiber recruitment under low-load conditions. Compared with the non-training group, the training group demonstrated greater slow-to-intermediate fiber recruitment. This pattern was confirmed as a significant between-group difference in women in the comparison between half-squat descent and squat-hold conditions (p = 0.016), and a similar tendency was observed in men (p = 0.083). These patterns were consistent with reduced recruitment thresholds.
CONCLUSION:
The W2 metric, derived from global EMG spectral distributions, detects between-group differences in muscle fiber recruitment patterns with greater sensitivity and robustness than conventional band-averaged methods.
This framework provides a novel approach for task-specific muscle function assessment and individualized exercise intervention in older adults.
[1] Panaretos, V. M., & Zemel, Y. (2019). Statistical aspects of Wasserstein distances. Annual review of statistics and its application, 6(1), 405-431.
[2] Matsumoto, T., Takamaru, Y., Nakayama, C., Sawada, S., & Machida, S. (2025). Muscle Fiber Recruitment Characteristics in Trained Older Adults: An EMG Frequency Analysis During Voluntary Contraction. Journal of Functional Morphology and Kinesiology, 10(3), 355.