A controversial training strategy is to take sets of a given exercise to muscular failure with submaximal loads. This means that a trainee performing exercise X (e.g., bench press) will perform as many reps as possibly with a given load (e.g., 50% 1RM) until his or her muscles literally do not allow the completion of another rep. Proponents of training to failure with submaximal loads have traditionally favored this style of training because it was a way to maximally recruit high threshold motor units without the use of heavy loads. The working theory was that as the muscles began to fatigue progressively throughout a set, more and more motor units would be recruited to accomplish more reps.
Until recently, it was unclear whether this theory carried any weight. In a new study published ahead of print in the Journal of Strength and Conditioning Research, a group of investigators tested the hypothesis that electromyography (EMG) activity and thus motor unit recruitment would not differ between submaximal versus near maximal loads taken to failure in the barbell back squat. The sample included 10 resistance trained young men (average squat 1RM = 310 lb). The researchers had each subject perform sets of squats with 50%, 70% and 90% of 1RM to failure. During each set, EMG of the thigh was measured and recorded for comparison. There was a linear relationship between EMG amplitude and %1RM. Performing a set to failure with 90% resulted in significantly greater motor unit recruitment than 70% and 50%, while 70% resulted in significantly greater recruitment than 50%. This was despite perceived ratings of exertion being no different between loads. Therefore, perceived effort does not appear to be a good measure of muscle activation levels.
The results of this study demonstrate that training with heavy loads is still the one of the most effective ways to recruit high threshold motor units. This is important for transfer to sporting movements such as sprinting and jumping. The implications of these results have on training for muscular hypertrophy are unclear. This is because training to failure with lighter loads may stimulate hypertrophy via other mechanisms, such as greater time under tension, great metabolite accumulation and greater cellular swelling. To sum, heavier loads are needed to effectively recruit high threshold motor units. Training with lighter loads to failure is not an appropriate substitute to accomplish the same task.
Looney, DP. et al. (2015) Electromyographical and Perceptual Responses to Different Resistance Intensities in a Squat Protocol: Does Performing Sets to Failure With Light Loads Recruit More Motor Units? Journal of Strength and Conditioning Research. In Press.