Author(s): KAWAMA, R., MIKATA, S., OKUDA, Y., SATO, K., TAKAHASHI, K., KITAMURA, M., WAKAHARA, T. , Institution: DOSHISHA UNIVERSITY, Country: JAPAN, Abstract-ID: 641

INTRODUCTION:
Human skeletal muscles have adaptive plasticity in response to resistance training, and hypertrophic adaptation requires high energy and protein costs. Recent evidence has shown that hypertrophy of recruited muscles can occur at the expense of non-recruited muscle atrophies under limited energy and protein availability [1], suggesting that these resources are preferentially allocated to recruited muscles. This finding raises a further question: does recruiting multiple muscle groups during whole-body resistance exercise attenuate the hypertrophic response of targeted muscles, potentially due to the distribution of limited nutritional resources across a greater number of recruited muscles? Here, we investigated the hypertrophic responses of targeted muscles (e.g., pectoralis major and triceps brachii) after resistance training programs using multiple exercises (bench press, squat, and stiff-leg deadlift) or a single exercise (bench press only).
METHODS:
Forty healthy young males were divided into training group performing multiple exercises (ME), training group performing a single exercise (SE), and control group (CON). The ME performed bench press, full squat, and stiff-leg deadlift, whereas SE performed only bench press at 40% of one-repetition maximum (1RM) with 3 sets of 10 repetitions at two weekly sessions for eight weeks. Before and after this period, we assessed 1RM for the bench press, full squat, and stiff-leg deadlift as well as volumes of the pectoralis major, triceps brachii (long head and medial + lateral heads), and individual lower limb muscles. Moreover, energy and protein intakes were monitored using nutrition-tracking apps, whereas basal and activity energy expenditures were estimated using a triaxial accelerometer during the intervention period. The 1RM and volume were analyzed using mixed-effects models (time × group) and analysis of covariance with adjustments for baseline values, dietary intakes, and energy expenditure, and semi-partial correlation analysis.
RESULTS:
There were increases in 1RM of the bench press and volumes of the pectoralis major and medial and lateral heads of the triceps brachii in SE (all p ≤ 0.001), but not in ME or CON. Moreover, the relative changes in these variables were approximately 2–3 times greater in SE than ME (p = 0.001–0.038) and CON (p = 0.001–0.008). In ME, relative increase in volume of the pectoralis major was associated with the protein intake normalized to body mass (p = 0.017, r = 0.669).
CONCLUSION:
Multiple resistance exercises attenuate hypertrophic responses of targeted muscles and strength gain compared with a single exercise. Additionally, the attenuation of hypertrophic responses in a preferentially recruited muscle was most pronounced, especially in the participants with limited protein availability. These findings highlight that training multiple muscle groups simultaneously is “not always better” for maximizing muscle hypertrophy and strength gains.

Ref.
1. Van vossel et al. Med Sci Sports Exerc (2024)