Six weeks of maximal eccentric knee extensor training affects muscle-tendon mechanics and muscle damage

Introduction: Strength training can influence muscle-tendon mechanics and architecture, and provide a protective effect from exercise-induced muscle damage; however more data are required describing the specific influence of eccentric training. Therefore, the aims of the present study were to examine the effect of 6 weeks of maximal eccentric knee extensor training on these properties.
Methods: Thirteen recreationally active participants (age = 28.3 ± 1.7 yr, mass = 73.5 ± 14.6 kg, height = 1.7 ± 0.1 m) volunteered for the study after giving written informed consent; ethical approval was granted from the University of Northampton. Training was performed twice weekly and consisted of 5 sets of 12 repetitions of 3-s maximal knee extensor isokinetic eccentric contractions at 30°·s-1 from 180° to 90° knee extension. Maximal isometric and eccentric knee extensor moment, range of motion (ROM), stretch tolerance, VL thickness and fascicle angle, and tendon and muscle-tendon unit (MTU) stiffness were measured using isokinetic dynamometry and real-time ultrasonography before and after the training. Creatine kinase (CK) concentration and delayed onset muscle soreness (DOMS) were also measured before and 24 h after a 20-min downhill run (16° decline). 
Results: A significant increase in isometric (21.6%; p<0.05) and eccentric (28.1%; p<0.01) strength, ROM (6.6°; p<0.01), stretch tolerance (83.6%; p<0.01), VL thickness (7.8%; p<0.01) and fascicle angle (9.2%; p<0.01) was found after training; changes in VL thickness and fascicle angle were strongly correlated (r=0.96: p<0.01). While no change was found in MTU stiffness (slope of the passive moment curve) using dynamometry (-5.8%; p=0.45), ultrasound data revealed a significant increase in VL tendon stiffness (8.7%; p<0.01). Although CK concentration after downhill running increased significantly before (103.2%; p<0.01) and after (42.1%; p<0.05) training, the increase in CK was significantly lower following the training (p<0.05), with subjects also reporting minimal DOMS within the knee extensors.
Discussion: The substantial increases in strength were accompanied by increases in both muscle size and pennation. Moreover, significant increases in ROM and stretch tolerance were observed, indicating a dual benefit to the eccentric training. An increase in tendon stiffness was observed, which will likely impact muscle-skeleton force transfer, and ultimately influence movement capacity. However, importantly this was not accompanied by a change in whole MTU stiffness, suggesting that MTU measurements may miss tissue-specific adaptations. Finally, the attenuated CK and DOMS response after training has clear implications for both muscular performance and injury risk. 
Contact: tony.kay@northampton.ac.uk