Electrically evoked eccentric and concentric torque-velocity relationshipsin human knee extensor muscles

The torque-velocity relationship, obtained during in situ conditions in hum ans, demonstrates a levelling-off of eccentric torque output at the isometr ic torque level, at least for knee extensor actions. In contrast, the in vi tro force-velocity relationship for animal muscle preparations is character ized by a sharp rise in eccentric force from isometric maximum. A force-reg ulating 'protective' mechanism has been suggested during maximal voluntary high-tension eccentric muscle actions. To investigate this phenomenon, maxi mal voluntary and three different levels of submaximal, electrically induce d torques were compared during isometric and low velocity (10, 20 and 30 de grees s(-1)) isokinetic eccentric and concentric knee extensor actions in 1 0 healthy, moderately trained subjects. Eccentric torque was higher than is ometric during electrically evoked, but not during maximal voluntary muscle actions. In contrast, concentric torque was significantly lower than isome tric for both maximal voluntary and submaximal, electrically evoked conditi ons. Comparisons of normalized torques (isometric value under each conditio n set to 100%) demonstrated that the maximal voluntary eccentric torque had to be increased by 20%, and the isometric by 10% in order for the maximal voluntary torque-velocity curve to coincide with the electrically stimulate d submaximal ones. These results support the notion that a tension-regulati ng mechanism is present primarily during eccentric maximal voluntary knee e xtensor actions.