Author(s): SHINZAKI, S., NAKATA, K., MAEO, S., KINOSHITA, M., EIHARA, Y., NISHIZAWA, N., KANEHISA, H., ISAKA, T., Institution: RITSUMEIKAN UNIVERSITY, Country: JAPAN, Abstract-ID: 2141

INTRODUCTION:
The abdominal muscles, particularly the deep muscles such as the internal oblique and transversus abdominis, play a crucial role in stabilizing the trunk, maintaining posture in response to external perturbations, and enabling the limbs to generate significant force. Therefore, developing effective training methods for the abdominal muscles, including the deep muscles, is essential for improving quality of life and sports performance. One such training method involves performing resistance exercises in an unstable environment using specialized equipment (e.g., sling-based push-ups, Maeo et al. 2014; Kohiruimaki et al. 2019) or using free weights with asymmetrical loads (e.g., asymmetrical bench press, Saeterbakken et al. 2020). However, it is beneficial to propose a more practical and safer method for implementation. Therefore, this study examined the muscle activity of agonist and abdominal muscles during resistance exercise using a conventional machine performed with asymmetrical versus symmetrical loads.
METHODS:
Fifteen healthy young men performed chest presses using a chest press machine (Life Fitness) under four different load conditions combining 30% and 70% of their one-repetition maximum (1RM) load: two symmetrical (Right-Left: R30-L30, R70-L70) and two asymmetrical (R30-L70, R70-L30) conditions. Muscle activity during the exercise was measured from the pectoralis major, lateral head of the triceps brachii, rectus abdominis, and internal oblique on both sides (eight muscles in total) by using surface electromyography (EMG). The EMG values were normalized to those obtained during maximal voluntary contractions of each muscle (% EMGmax).
RESULTS:
Muscle activity of the agonist muscles (pectoralis major and triceps brachii) was higher with a higher load in all conditions (e.g., right pectoralis major, 37.3% vs 76.2% [2.0-fold] in R30-L30 vs R70-L70, p<0.001). On the other hand, the muscle activity of the abdominal muscles was the highest in the lower load side of the asymmetrical conditions, and this was prominent in the internal oblique (e.g., right internal oblique, 10.7% vs 26.4% [2.5-fold] in R70-L70 vs R30-L70, P<0.001).
CONCLUSION:
The internal oblique muscle activity was the highest in the lower load side of the asymmetrical conditions even compared to when lifting a higher load under symmetrical conditions, despite the lower total weight lifted. This supports the notion that the internal oblique plays a crucial role in stabilizing the trunk and suggests that its activity is specifically enhanced during exercises performed in an unstable environment. Thus, performing resistance exercises using a conventional machine with asymmetrical loads may serve as a practical and safe training method to strengthen the deep abdominal muscles.
REFERENCES:
Maeo et al. (2014) BMC Research Notes, 7:192.
Kohiruimaki et al. (2019) Int J Sports Med, Nov;40(12):789-795.
Saeterbakken et al. (2020) Eur J Appl Physiol, 120:2517–2524.