Predicting maximum eccentric strength from surface EMG measurements

The origin of the well documented discrepancy between maximum voluntary and in vitro tetanic eccentric strength has yet to be fully understood. This study aimed to determine whether surface EMG measurements can be used to reproduce the in vitro tetanic force – velocity relationship from maximum voluntary contractions. Five subjects performed maximal knee extensions over a range of eccentric and concentric velocities on an isovelocity dynamometer whilst EMG from the quadriceps were recorded. Maximum voluntary (MVC) force – length – velocity data were estimated from the dynamometer measurements and a muscle model. Normalised amplitude – length – velocity data were obtained from the EMG signals. Dividing the MVC forces by the normalised amplitudes generated EMG corrected force – length – velocity data. The goodness of fit of the in vitro tetanic force – velocity function to the MVC and EMG corrected forces was assessed. Based on a number of comparative scores the in vitro tetanic force – velocity function provided a significantly better fit to the EMG corrected forces compared to the MVC forces (p ≤ 0.05), Furthermore, the EMG corrected forces generated realistic in vitro tetanic force – velocity profiles. A 58 ± 19% increase in maximum eccentric strength is theoretically achievable through eliminating neural factors. In conclusion, EMG amplitude can be used to estimate in vitro tetanic forces from maximal in vivo force measurements, supporting neural factors as the major contributor to the difference between in vitro and in vivo maximal force.