Author(s): ICHINOSE, H.1, KOBAYASHI, Y.1, MAEO, S.1, NISHIZAWA, N.1, SHINZAKI, S.1, KANEHISA, H.2, ISAKA, T.1 , Institution: RITSUMEIKAN UNIVERSITY, Country: JAPAN, Abstract-ID: 2752

INTRODUCTION:
Intramuscular fat increases with inactivity regardless of age (Manini et al., 2007). However, the influence of daily physical or muscle activity and exercise training on intramuscular fat in upper-limb muscles remains poorly understood. In elbow flexor muscles, the dominant arm exhibited significantly greater muscle size than the non-dominant arm (Tanton et al., 2009), which may reflect greater daily use of the dominant arm. Given that physical activity influences intramuscular fat, fatty infiltration may also reflect differences in the amount of daily muscle activity. Therefore, the aim of this study was to examine differences in intramuscular fat between dominant and non-dominant arms, as well as longitudinal changes following resistance exercise training.
METHODS:
Nine healthy young females and males (19–26 years, BMI 19.6–24.5 kg/m2) underwent Dixon magnetic resonance imaging to assess muscle volume and fat fraction (%FF) of three elbow flexor muscles (biceps brachii, brachialis, and brachioradialis) in both the dominant and non-dominant arms. Participants then completed a 12-week unilateral incline curl training program on a randomly assigned arm at 70% of one-repetition maximum, performing 5 sets of 10 repetitions per session, twice weekly. After the intervention, magnetic resonance imaging of the trained arm was repeated to evaluate changes in muscle volume and %FF.
RESULTS:
At baseline, muscle volume was significantly greater in the dominant arm than in the non-dominant arm for all three elbow flexor muscles (P ≤ 0.008) and the combined muscle volume (258.0 ± 104.4 cm3 vs 236.4 ± 97.8 cm3, P = 0.003), whereas %FF showed no between-arm differences in either the individual muscles (P ≥ 0.231) or the mean %FF across the three elbow flexors (11.1 ± 1.5% vs 10.2 ± 2.7%, P = 0.375) (n = 9). Before and after the training intervention, muscle volume significantly increased in all three muscles (P ≤ 0.019) and the combined muscle volume (293.8 ± 128.7 cm3 to 261.9 ± 108.4 cm3, P = 0.001), but no significant changes were observed in %FF in the biceps brachii (11.6 ± 2.0% to 10.8 ± 3.3%, P = 0.347), brachialis (12.0 ± 1.7% to 11.6 ± 2.1%, P = 0.612), and brachioradialis (10.7 ± 2.4% to 10.6 ± 2.8%, P = 0.801), or in the mean %FF (11.4 ± 1.8% to 11.0 ± 2.6%, P = 0.474) (n = 8).
CONCLUSION:
Elbow flexor muscle size showed significant between-arm differences (dominant > non-dominant) at baseline and increased after resistance training, whereas intramuscular fat showed neither between-arm differences nor training-induced changes. These findings suggest that intramuscular fat in elbow flexors may be less sensitive to differences in daily muscle activity or resistance training than muscle volume.