Submaximal force accurately predicts submaximal work

Abstract

Several investigators have used work loops to evaluate neuromuscular function. Within a work loop, muscle must become activated, produce force while shortening, relax, and undergo passive lengthening as it would in many voluntary activities. However, to our knowledge investigators have only reported results from maximally activated work loops. Our purpose for conducting this study was to characterize submaximal work loop performance and its relation to submaximal isometric force. We isolated gastrocnemius of female PVG rats in situ with nerve and vascular supply intact. Achilles tendons were attached to a servo motor which controlled position and recorded force. We set onset and offset of the stimulation pulse train (130 Hz) to minimize negative work for the cycle. We imposed cycle frequencies of 2 and 4 Hz, which represent walking and running gait and excursion lengths of 2 and 4 mm, which are associated with gait and maximal flexion/extension. Current was adjusted to achieve tetanic forces that approximated 25%, 50%, 75% and 100% of maximum force. We used regression analyses to determine relationships of twitch and tetanic force with work. Our data indicated that work was highly related to tetanic force: R2=.98 for 2 Hz, both 2 and 4 mm excursions; R2=.90 and .93 for 6 Hz, 2 and 4 mm excursion respectively. Work was also highly related to twitch force: R2=.83 and .87 for 2 Hz, 2 and 4 mm respectively, and R2=.93 and .97 for 6 Hz, 2 and 4 mm respectively (p<.001 for all regressions). Despite many factors that influence work production such as force-length, force-velocity, activation and relaxation kinetics, and their interactions, it seems that work produced at various stimulation intensities can be predicted from tetanic force.