Active Voice: Similar, Yet So Different - Neuromuscular Adaptations to Endurance Versus High-Intensity Interval Training

By Eduardo Martinez-Valdes, P.T., M.Sc., Ph.D., and Deborah Falla, P.T., Ph.D.

Eduardo Martinez-Valdes, P.T., M.Sc., Ph.D. Deborah Falla, P.T., Ph.D.
Viewpoints presented in SMB commentaries reflect opinions of the authors and do not necessarily reflect positions or policies of ACSM.

Eduardo Martinez-Valdes, P.T., M.Sc., Ph.D., completed his doctoral degree in clinical exercise science at the University of Potsdam, Germany. He currently works as a post-doctoral research fellow at the Centre of Precision Rehabilitation for Spinal Pain at the School of Sport, Exercise and Rehabilitation Sciences, University of Birmingham, UK. His research mainly focuses on the study of neuromuscular adaptations to training, muscle fatigue and experimentally induced pain by using advanced techniques of surface electromyography (high-density surface electromyography).

Deborah Falla, P.T., Ph.D., is professor in rehabilitation science and physiotherapy and director of the Centre of Precision Rehabilitation for Spinal Pain at the School of Sport, Exercise and Rehabilitation Sciences, University of Birmingham, UK. Her research uses electrophysiological measures to evaluate the control of human movement and how such movements are affected or adapted in response to various states (e.g., injury, fatigue, training, pain). She has published 150 papers and has given more than100 conference presentations, including 30 keynote lectures.

This commentary presents Dr. Martinez-Valdes’ and Dr. Falla’s views on the topic of a research article that they had published with their colleagues in the June 2017 issue of
Medicine & Science in Sports & Exercise® (MSSE).

Over the last decade, high-intensity interval training (HIIT) has become a popular alternative to endurance (END) exercise since it requires less time to produce favorable physiological adaptations. Previous literature has focused on HIIT-induced changes in aerobic metabolism and cardiorespiratory fitness; however, less attention has been placed on the neuromuscular adaptations accompanying HIIT. In our recent article published in the June 2017 issue of MSSE, we compared the neuromuscular adaptations following two weeks of either HIIT or END training, using high-density surface electromyography (HDEMG). With respect to conventional bipolar surface EMG, HDEMG covers a larger area of the muscle, allowing examination of regional differences in muscle activation, as well as the activity of individual motor units. Through HDEMG, it is possible to study the smallest functional units of muscle control.

Traditionally, recordings of motor unit behavior required invasive methods which imposed many limitations. These classic techniques can only identify a limited number of motor units at low force levels, providing partial and generally unreliable information about longitudinal changes in motor unit behavior following training interventions. Using HDEMG and a novel algorithm based on blind-source separation techniques, we could identify and follow a large population of motor units recorded from the vastus medialis (VM) and vastus lateralis (VL) muscles of 16 physically active participants before and after HIIT and END training interventions (8 participants in each group). Further, we could track these responses over a wide range of force levels (from low to high force threshold motor units), thereby allowing interpretation of changes in motor unit behavior to be made with greater certainty.

The study revealed that both HIIT and END training improved parameters of cardiorespiratory fitness similarly (i.e., VO2 peak and submaximal ventilation thresholds). However, the neuromuscular adaptations to training differed. HIIT induced a significant increase in knee extensor muscle strength which was accompanied by increased VM and VL motor unit discharge rates and HDEMG amplitude at high contraction levels (50 and 70 percent of the maximum voluntary contraction force [MVC]). In contrast, END training induced a marked increase in knee extensor endurance at a lower force level (30 percent MVC), without any influence on HDEMG amplitude or motor unit discharge rates.

Collectively, these findings show that, despite similar improvements in cardiopulmonary fitness, HIIT and END training induce differing adjustments in motor unit behavior. These results suggest that HIIT and END show specific neuromuscular adaptations. These adaptations possibly are related to their differences in exercise load intensity and training volume. Such findings have important implications for exercise prescription. However, further investigations examining longer HIIT interventions with varying work/rest ratios are required to fully appreciate the type of HIIT protocol which can maximize both an increase in muscle strength and cardiorespiratory fitness.