CONSIDERATIONS FOR MUSCLE ACTIVATION AND RATE OF FORCE DEVELOPMENT IN EXERCISE PHYSIOLOGY RESEARCH

Abstract

Introduction: Muscle activation under different contraction paradigms and rate of force or torque development are commonly investigated and interpreted phenomenon within neuromuscular and biomechanical research. However, there are several considerations which should be made when investigating these parameters in research from a methodological and interpretation standpoint. Different techniques of quantifying muscle activation during high-intensity vs. low-intensity fatiguing conditions are commonly studied in isolation, however, because each measure of muscle activation has limitations, it is important to use a multi-faceted approach to analyzing muscle activation during these different contraction conditions to obtain less biased results. This multi-faceted approach to estimating muscle activation includes analyzing surface EMG amplitude in combination with individual motor unit data. Rate of force development is a topic of high research interest, especially for its purported capacity for predicting vertical jump height. However, less interest has been given to rate of force or torque development during the previously mentioned multifaceted approach to analyzing muscle activation. The current work considers whether this multifaceted approach to estimating muscle activation is affected by rate of force development, and whether rate of force development is a robust predictor of vertical jump height in multiple regression models where other predictors are included. Methods: Two studies were performed which analyzed muscle activation, including the behavior and properties of individual motor units, during isometric voluntary contractions. The first study did this, during a high-intensity contraction in comparison to a series of moderate intensity contractions performed until volitional fatigue, and the second study, during contractions of equal intensity and duration, but with different rates of torque development. During both studies, surface EMG signals were analyzed in terms of root mean squared amplitudes, and in terms of individual motor unit action potential amplitudes, recruitment thresholds, and firing rates. All motor unit analysis was relationship based and performed on a subject-by-subject and contraction-by-contraction basis. The third study used multiple regression techniques to analyze the predictive capacity of different estimates of rate of force development on vertical jump height from a large sample of vertical jumps performed by collegiate athletes. Conclusions: The results of the first study indicated the multifaceted approach to analyze muscle activation revealed greater muscle activation was achieved during a high-intensity contraction in comparison to a moderate intensity contraction at the limit of voluntary fatigue. The second study indicated rate of torque development influences motor unit behavior during the commonly utilized isometric protocols such as were used in study 1. The results of the final study indicated that estimates of rate of ground reaction force development during vertical jumps is a significant moderate predictor of vertical jump height. However, when other predictors are included in the prediction model it appears the predictive capacity of rate of force development on vertical jump height rests solely on its shared variance with another predictor, peak ground reaction force. Therefore, rate of force development may be more important than previously thought in studies investigating muscle activation via individual motor unit behavior, and less important than previously thought in predicting vertical jump height.