Eccentric overload training promotes serial sarcomerogenesis to a greater extent than conventional resistance training

Eccentric exercise has been shown to increase serial sarcomere number (SSN) through sarcomerogenesis in both animal and human models. However, eccentric contractions rarely occur in isolation and are often used in conjunction with concentric movements, limiting the ability to evoke a maximal eccentric contraction. Eccentric overload training provides an increased eccentric stimulus, yet its impact on muscle morphology and mechanics is not widely understood. We compared the effects of eccentric overload training (ECC_OVERLOAD) and conventional resistance training (CONV) on morphological [SSN, fascicle length (FL), sarcomere length (SL), physiological cross-sectional area (PCSA), wet weight] and mechanical changes (torque, normalized torque, power, passive torque) pre- to post-training. Nineteen Sprague-Dawley (<i>n</i> = 10; ECC_OVERLOAD, <i>n</i> = 9; CONV) male and female rats (13-14 wk, 317 g) completed 4 wk (3× week) of training. SSN increased with training by 17% in the soleus (<i>P</i> < 0.001) and 6% in the medial gastrocnemius (<i>P</i> = 0.021) in the ECC_OVERLOAD group, compared with 2% and 4% in the CONV group. Peak plantar flexion torque increased ∼27% in the ECC_OVERLOAD and ∼21% in the CONV group (<i>P</i> < 0.001) but did not differ between groups (<i>P</i> = 0.318). There was a 26% increase in normalized torque for the ECC_OVERLOAD, as compared with 2.5% in the CONV group at 90°, demonstrating an interaction of training × group (<i>P</i> < 0.001). Power increased ∼9%, with an interaction of sex × training × group (<i>P</i> = 0.021) driven by increases in torque at peak power in the ECC_OVERLOAD male group. These findings indicate that overload training provided a more robust stimulus for longitudinal muscle remodeling than conventional resistance training.<b>NEW & NOTEWORTHY</b> Eccentric overload training promotes greater sarcomerogenesis than conventional resistance training in male and female rats.