The human force : velocity relationship; activity in the knee flexor and extensor muscles before and after eccentric practice

The human voluntary force:velocity relationship frequently fails to demonst rate the expected high eccentric forces. Possible explanations include uniq ue activation strategies which might be affected by neural learning mechani sms. We investigated the effect of practicing eccentric contractions on (1) the force: velocity relationship of the human knee extensor muscles and (2 ) the extent of agonist and antagonist muscle activity. Eight healthy adult s [seven women, group mean age 31 (SEM 5) years +/-] practised twice a week for 4 weeks using their non-dominant legs. Each session comprised three is okinetic concentric and eccentric maximal voluntary contractions (MVC) at r andomised angular velocities of 100, 200 and 300 degrees . s(-1). Before an d after, the force:velocity relationship was determined bilaterally (angula r velocities 0-300 degrees . s(-1)). There were no significant differences in the forces generated or relative electromyogram (EMG) activity after pra ctice, although there was a trend for dynamic forces to increase. Beforehan d, the bilateral eccentric MVC forces were lower than isometric (P < 0.0025 ); afterwards they were broadly similar. The agonist EMG was similar during isometric and eccentric contractions, but lower during concentric (P < 0.0 3). Antagonist EMG activity showed considerable individual variation, was s imilar during all contraction types and tended to be greater during dynamic contractions. These data indicate that neither central learning mechanisms nor total muscle activation strategies underlie the human failure to produ ce the expected high eccentric voluntary forces in humans.