Effects of Concurrent Endurance and Explosive Type Strength Training on Neuromuscular Characteristics in Endurance Athletes

Abstract

Introduction: In previously untrained male and women subjects explosive type strength training results in great increases in the amount of neural input to the trained muscles contributing largely to the improvements in explosive force production (Häkkinen 1994). It has been suggested, that also in endurance athletes, during relatively short training periods of some weeks, the improvements in force and velocity characteristics might primarily come from neural adaptations, although no electromyographic (EMG) measurements on the muscles have been performed to support this suggestion (Paavolainen et al. 1999). Therefore, the aim of the present study was to investigate the effects of concurrent endurance and explosive strength training on EMG and force production characteristics of leg extensor muscles in well-trained endurance athletes. Methods: Six experimental (E) and 6 control (C) endurance athletes trained for 8 wk. The total training volume was kept the same in both groups, but 26% of training in E and 2% in C was replaced by explosive type strength training. It consisted of various concentric and stretch-shortening cycle exercises for leg extensor muscles performed with low loads but high velocities. A dynamometer was used to measure maximal bilateral isometric force and various force-time parameters of the leg extensor muscles. Three to five maximal isometric actions were performed at the knee and hip angles of 90°. EMG-activity was recorded from the vastus lateralis (VL), vastus medialis (VM) and rectus femoris (RF) muscles of the right leg. Results: Maximal isometric force did not change after 8-wk training but in E the average force-time curve changed in the absolute scale so that the times to reach lower force levels shortened (p < 0.01) and early forces (in 0–100 ms) increased (p < 0.01). Maximal average integrated EMG (AEMG) of the VL, VM and RF muscles increased in E during training and the changes were significant (p < 0.05–0.01) in the early portions of the isometric actions. No changes were observed in C. In E the relative increases in the AEMG of VL (0–100 ms) correlated with the relative increases in force (0–100 ms) (r = 0.82, p < 0.05). Discussion: The present 8-wk explosive type strength training resulted in considerable improvements in selected neuromuscular characteristics, although a large volume of endurance training was performed concomitantly. This supports previous findings (e.g. Paavolainen et al. 1999) that in well-trained endurance athletes training-induced improvements in neuromuscular characteristics may not be fully inhibited by concurrent explosive strength and endurance training. A specific training-induced shift in the “early” portions of the AEMG-time curve indicated that the improvements in explosive force production were largely due to neural adaptations. The importance of the neural component was further supported by the correlation between the changes in the early AEMG and the changes in explosive force production during the early part of the isometric force-time curve. No changes were observed in maximal isometric force during training which further supports the concept of specificity of training, since the training program did not include resistance exercises with high loads.