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Interval versus Continuous Training and Neuromuscular Fatigue

In the exercise column Train smart, recover smarter, you learned about the importance of optimal rest and nutrition in maximizing your body’s recovery from an intense exercise activity. Whether it is a strength or endurance activity, you are placing stress on your body to induce positive physiological adaptations. A recent study published in the European Journal of Applied Physiology, however, has shown that there is a significant difference in neuromuscular fatigue when comparing cyclists exercising at constant intensity with those exercising at varying intensity. These findings are important because the more you break down your muscles, the greater the stimulus to rebuild them.

Ten experienced cyclists were divided into two groups. One group rode at a constant 70% of their maximal aerobic power (MAP) for 33 minutes. The second group rode at both high and moderate power output levels. The intervals consisted of 200 %, 150 %, or 100 % MAP for 10, 15, and 20 seconds, respectively, followed by a recovery period at 50% MAP. The overall average power for the varied group was also 70% MAP.

Before and immediately following riding, cyclists were given a muscle test consisting of a maximal voluntary contraction of the knee. During this test, the knee extensors were directly stimulated with an electrical signal to measure the percentage of the total muscle capacity they were able to contract.

Results showed that the heart rate was slightly higher in the variable intensity group (162 vs. 157 bpm). Cyclists also reported that the variable trial felt more difficult, indicated by higher ratings of perceived exertion. Supporting this statement is the fact that lactate values were higher in the variable group. Lactic acid build up is what causes the burning sensation and fatigue during a high intensity work out. Lastly, while both groups experienced muscle fatigue as indicated by the muscle test, the degree of impairment was much greater in the variable group (26 % vs. 16 %) than the constant group. The ability to voluntary activate the muscle also showed a greater decrease in the variable group as compared to the control group (96 % vs. 97 %).

These results suggest that varying the intensity level of a workout will provide a stronger stimulus for physiological adaptation. However, without adequate time between workouts or competition, the decreased muscular function will lead to impairments in overall performance. If, however, variable intensity cardiovascular training is combined with a proper recovery period including rest and adequate nutrition, you will be giving your body what it needs to become stronger and better fit.

Note: A similar study published in 2007 showed that semi-elite triathletes showed no additional neuromuscular fatigue following a workout with varying power output. These results suggest that the benefit of a varying work out may only apply to non-elite athletes.

Written by Ryan Serrano with Eric Sternlicht, Ph.D. – Department of Kinesiology, Occidental College, Los Angeles CA.