The effect of different forms of High-Intensity Intermittent running exercises on neuromuscular fatigue

Introduction Intermittent exercise has been found to cause a greater degree of neuromuscular fatigue than constant load exercise on a cycle ergometer. In this study we compare central and peripheral components of neuromuscular fatigue on knee extensor (KE) muscles following different forms of high-intensity intermittent running exercises. Methods 8 male subjects (24±5 yrs) completed four 16 min running trials. The mean running speed was the same across all trials (8.9 km/h). Subjects performed 1 trial at constant speed (CON) and 3 trials with three different forms of intermittent running; comprising of the same total and high-intensity distance (1173 m at >15 km/h) but different number of accelerations (LOW, 55; MID, 80; HIGH, 190). Blood lactate concentration and RPE were recorded at the end of each trial. To assess KE muscle contractile properties, passive mechanical responses to electrical stimulations, maximal voluntary isometric contraction, and maximal muscle activation levels were measured before and after completion of the running exercise. Peripheral KE neuromuscular properties were determined using 3 paired stimuli at 10Hz (low frequency) and 100Hz (high frequency). Maximal activation levels were established using twitch interpolation technique. EMG signal was also recorded during the test. Differences between the conditions were analyzed using ANOVA’s. Results RPE was higher after all intermittent trials compared to CON (p<0.05). Blood Lactate was significantly higher after LOW compared to all the other conditions, whereas CON was significantly lower than all intermittent trials (CON, 2.4±1.0; LOW, 8.1±3.2; MID, 5.8±2.7; HIGH, 4.7±2.4 mmol/L). No evidence of the presence of central fatigue was found, while peripheral fatigue was evident in all conditions. However, KE peak torque values obtained using 10Hz electrical stimuli decreased significantly more after HIGH compared to all other conditions, with the reduction being significantly lower after CON (CON, -31%, LOW, -35%, MID, -36%, HIGH, -39%). Discussion All trials induced peripheral fatigue, temporarily altering KE muscle contractile properties. HIGH increased the quantity of low-frequency fatigue compared to all other conditions; this might be explained by the greater number of accelerations. Differences in blood lactate concentrations suggest a dissimilar contribution of anaerobic glycolysis. In conclusion, different forms of high-intensity intermittent running exercise can significantly impact upon energetic system activation and the quantity of peripheral fatigue induced.