Author(s): KAWAKAMI, E., SAWADA, S., LIU, Z., NAITO, H., MACHIDA, S., Institution: JUNTENDO UNIVERSITY, Country: JAPAN, Abstract-ID: 1765

INTRODUCTION:
Muscle echo intensity (EI), measured using ultrasound imaging, is widely used as a noninvasive indicator of skeletal muscle quality [1,2]. Previous studies have reported inconsistent effects of resistance training on EI [3]. Because EI may reflect not only intramuscular composition but also water distribution within the muscle tissue, changes in EI following training may be influenced by fluid dynamics. In recent years, bodyweight-based resistance training (BWRT) in older adults has been reported to be effective in improving muscle mass, muscle strength, and physical function, with increases in muscle thickness assessed using ultrasonography also being documented [4,5]. Therefore, in this study, we aimed to examine whether changes in EI are associated with changes in fluid distribution indices assessed using bioelectrical impedance analysis (BIA) after a 6-week BWRT program in older adults. We hypothesized that if training-induced changes in EI are primarily driven by alterations in intramuscular fluid dynamics rather than structural muscle degeneration, changes in EI would be associated with changes in fluid distribution indices.
METHODS:
Twenty-eight community-dwelling Japanese older adults (mean age: 73.2 years; 9 men and 19 women) participated in this study. The BWRT program (1 supervised session and 1 home session/week) consisted of six exercises targeting the lower limbs, trunk, and upper limbs, including split squats and squats, for 6 weeks. Muscle thickness and EI of the rectus femoris and vastus intermedius were assessed using ultrasonography. EI was quantified from 8-bit grayscale images using the ImageJ software. Fluid distribution indices [extracellular water/intracellular water (ECW/ICW)] in the lower limbs were measured using BIA. The pre- and post-intervention changes were analyzed using paired t-tests. Associations between the changes in EI and fluid indices were examined using Spearman’s rank correlation coefficients. Statistical significance was set at p<0.05.
RESULTS:
No significant increase in muscle thickness of the anterior thigh was observed following training; however, the EI significantly increased in both muscles (p<0.001). The lower limb ECW/ICW also significantly increased after training (p=0.003), but changes in EI did not significantly correlate with changes in ECW/ICW in either the rectus femoris (r=-0.073) or vastus intermedius (r=0.261).
CONCLUSION:
The BWRT resulted in increases in both EI and extracellular fluid indices; however, these changes were not correlated. These findings suggest that changes in EI following short-term BWRT may occur independently of segmental fluid redistribution, as measured using BIA.
References:
1. Oranchuk DJ, et al. J Sport Health Sci. 2024. 13(6):820-840.
2. Stock MS, et al. Eur J Appl Physiol. 2021. 121(2):369-380.
3. Wong V, et al. J Ultrasound. 2020. 23(4):457-472.
4. Ozaki H, et al. J Sports Sci Med. 2020.19(4):721-726.
5. Sawada S, et al. BMC Geriatr. 2021.21(1):464.