Cortical and spinal responses to short-term strength training and detraining in young and older adults in rectus femoris muscle.

Abstract

Strength training mitigates the age-related decline in strength and muscle activation but limited evidence exists on specific motor pathway adaptations. Eleven young (22-34years) and ten older (66-80years) adults underwent five testing sessions where lumbar-evoked potentials (LEPs) and motor-evoked potentials (MEPs) were measured during 20 and 60% of maximum voluntary contraction (MVC). Ten stimulations, randomly delivered, targeted 25% of maximum compound action potential for LEPs and 120, 140, and 160% of active motor threshold (aMT) for MEPs. The 7-week whole-body resistance training intervention included five exercises, e.g., knee extension (5 sets) and leg press (3 sets), performed twice weekly and was followed by 4weeks of detraining. Young had higher MVC (~ 63 N·m, p = 0.006), 1-RM (~ 50kg, p = 0.002), and lower aMT (~ 9%, p = 0.030) than older adults at baseline. Young increased 1-RM (+ 18kg, p < 0.001), skeletal muscle mass (SMM) (+ 0.9kg, p = 0.009), and LEP amplitude (+ 0.174, p < 0.001) during 20% MVC. Older adults increased MVC (+ 13 N·m, p = 0.014), however, they experienced decreased LEP amplitude (-0.241, p < 0.001) during 20% MVC and MEP amplitude reductions at 120% (-0.157, p = 0.034), 140% (-0.196, p = 0.026), and 160% (-0.210, p = 0.006) aMT during 60% MVC trials. After detraining, young and older adults decreased 1-RM, while young adults decreased SMM. Higher aMT and MEP amplitude in older adults were concomitant with lower baseline strength. Training increased strength in both groups, but divergent modifications in cortico-spinal activity occurred. Results suggest that the primary locus of adaptation occurs at the spinal level.