Augmentation of single muscle fiber residual force enhancement and functional performance following 12 weeks of eccentric training in older adults

This study examined the effects of a 12-week eccentric resistance training program on body composition, physical function, and single muscle fiber contractile properties in older adults. Thirty-seven community-dwelling older adults (70.4 ± 3.3 years) completed supervised eccentric training twice weekly using an Eccentron device. Body composition, functional performance, and isokinetic strength were assessed before and after training, and vastus lateralis biopsies were analyzed to determine single-fiber active, passive, and residual force enhancement (RFE) properties. Body weight and BMI remained unchanged, whereas skeletal muscle mass ( p = 0.017), thigh girth ( p = 0.013) and calf girth ( p = 0.030) increased. Functional performance improved in the TUG ( p = 0.001) and stair-climb tests ( p = 0.020). At the single-fiber level, specific force increased in both type I ( p = 0.017) and type IIa fibers ( p = 0.012) and unloaded shortening velocity increased in type I fibers ( p < 0.001), without any change in fiber cross-sectional area. In type IIa fibers, both absolute ( p = 0.027) and relative residual force enhancement (RFE; p = 0.002) increased after training, with a comparable trend for relative RFE in type I fibers ( p = 0.052), indicating that history-dependent muscle properties remain modifiable with training. Importantly, this is among the first studies to demonstrate training-induced enhancement of RFE at the single-fiber level in older adults. These findings indicate that eccentric resistance training is accompanied by adaptations in single-fiber contractile properties, muscle strength, and functional performance in older adults, supporting its potential as an effective strategy for improving muscle function.