RICHARDS, C. T.; BIEWENER, A. A.; Harvard University; Harvard University: In vivo dynamic force-length properties in the Xenopus laevis plantaris muscle
In vivo plantaris activation and length were recorded by sonomicrometry and electromyography (EMG). Measurements of force were obtained from a calibrated stain gauge transducer fastened to the Achilles tendon. Peak force and strain were observed over a range of swimming speeds during burst and steady locomotion. The work loop method was used to calculate work and power for each propulsive cycle. In all conditions, the muscle actively developed force while shortening, generating positive work. Strain ranged from 4.3 to 30.2% (shortening) and from 0 to 4.7% (lengthening). Escape responses produced the highest cycle power (from 73.2 to 184.2 W/kg), whereas steady swimming ranged from 0.04 to 134 W/kg across several speeds (from 0.41 to 10.4 body lengths/s). Simultaneous EMG records from different sites demonstrated uniform timing of activation throughout the plantaris. Preliminary results suggest a correlation between average EMG burst amplitude and peak force (R2 = 0.74). Peak force and peak strain were also correlated (R2 = 0.73). These findings suggest that EMG and strain measurements may be used to predict work and power output in the plantaris muscle during swimming.
