The training specificity versus structural adaptation paradox: Differential effects of isokinetic concentric and eccentric resistance training on muscle architecture and function in young men

It is unclear whether muscle functional adaptations to concentric (CON-RT) and eccentric (ECC-RT) resistance training are most specific to their exercise characteristics or the structural adaptations they evoke. In this study, the effects of effort- and volume-matched CON-RT and ECC-RT on regional hypertrophy, muscle architecture, and function were compared, and associations between the outcomes were explored. Twelve trained young men (25.5{+/-}3.6y) completed 18 isokinetic ankle-dorsiflexion exercise sessions over 6 weeks: CON-RT in one leg and ECC-RT in the other (2-4 sets, 6-10 maximal repetitions, 10{degrees}/s). Tibialis anterior size and architecture (ultrasound imaging) and maximum voluntary dorsiflexion function (isokinetic dynamometry) were assessed. Muscle thickness increased similarly between conditions and across proximal-distal regions (8%), pennation angle increased more in CON-RT (8%) than ECC-RT (4%), and fascicle length increased only after ECC-RT (7%). Functional adaptations were more closely associated with specific structural adaptations than with contraction mode, velocity, or angle. Isometric torque increased similarly in both conditions overall (8%) but CON-RT improved only at shorter muscle lengths and shifted the peak-torque angle leftward, whereas ECC-RT improved at both shorter and longer lengths and broadened the torque-angle plateau, which was associated with fascicle length increases. ECC-RT produced greater increases in both eccentric (13%) and concentric torques (17%) than CON-RT (3%, 9%, respectively), and changes were similar across velocities - contrary to the training specificity theory. Changes in pennation angle were associated with dynamic strength changes. These findings suggest that muscle function adapts to the structural changes induced by training, regardless of the training scheme used.