Intensity or Effort – part 3
Muscle activation during sub-maximal exercise
Let’s say you load a barbell with a weight that is your 8RM. You pick up the barbell and you curl it 8 times, at which point you are too fatigued to lift the barbell a 9th rep. The weight on the bar was sub-maximal throughout the exercise so it did not require the maximum level of force that would be needed to lift the much heavier weight you would use during a 1RM. Yet despite requiring sub-maximal force to curl the weight you did perform the exercise to the point of exhaustion. At failure your exercising muscles were fatigued to the degree of being unable to generate enough force to lift the bar for a 9th rep. At failure what was happening in your muscles? Specifically, were all your motor units active and firing during that 8th rep? This is a key question because conventional wisdom says “no, all the fibers are not firing” while others are claiming that “yes, all the fibers are active and producing force.” None of the previously cited ITT studies provide any insight so we have to look at other research.
Fuglevand and colleagues (1993) measured a 19% decrease in motor unit activation in subjects that maintained a static contraction at 65% of maximum until exhaustion (66 seconds average time to failure). “The major finding of this study was a failure of voluntary EMG to attain maximal levels during a sustained, submaximal contraction…”
Conversely, Gandevia and colleagues (1998) reported maximal or near maximal activation of the biceps muscle during a curl exercise performed “as fast as possible” (approximately 1 second per rep) for 10-12 repetitions to failure. “All maintained the capacity to attain maximal levels of activation.”
In contrast to Gandevia’s study, a recent study by Robbins and colleagues (2010) measured the acute effects of different loads and training patterns of the bicep muscle during a curl exercise (modified preacher curl machine). Using both 5RM and 10RM protocols they found significant decreases in voluntary muscle activation in three of the four protocols. Though a significant decrease was not measured in the fourth protocol a large effect size was measured, indicating “the presence of phenomena similar to those in the other three protocols… The results suggest that, within the context of this study, individuals were unable to maintain near full activation during the development of fatigue, which was induced by dynamic efforts of the elbow flexors…” They concluded, “Decreases in voluntary muscle activation were not specifically influenced by training load or pattern.”
Similarly, Babault and colleagues (2001) compared activation levels in the quadriceps during isometric, concentric, and eccentric contractions using the ITT method adapted for dynamic exercise. They reported that maximum activation levels during both concentric and eccentric contractions were significantly lower than during isometric contractions (During an isometric contraction the muscle does not shorten or lengthen. Isometric contractions are performed against an immovable object. Concentric contractions cause the muscle to shorten and in strength training language is referred to as “lifting the weight”. Eccentric contractions cause the muscle to lengthen and in strength training language is referred to as “lowering the weight”.) “The mean activation levels during maximal eccentric and maximal concentric contractions were 88.3 and 89.7%, respectively, and were significantly lower with respect to maximal isometric contractions (95.2%). As their results were markedly different from those of Gandevia, these researchers suggested that “The fact that nonisokinetic contractions were considered and that (Gandevia et al) did not ensure constant EMG activity during the full range of motion might in part explain the differences from our findings. Another reason for the different results could be that their study was performed on the elbow flexors.”
A previous study of activation level in the quadriceps muscle (Newman et al 1991) reported results similar to those in the Babault study. Newman and colleagues measured activation levels in the quadriceps at different repetition speeds and concluded “…that brief high-intensity dynamic exercise can cause a considerable failure of voluntary activation. This failure was most marked during isometric and the lower-velocity isokinetic contractions.”
What all the above research tells us is that during sub-maximal exercise motor unit activation is most often less than maximal even when the exercise is taken to the point of failure. The factors that seem to influence whether a muscle is fully activated at the point of repetition failure in sub-maximal exercise are similar to those that influence motor unit activation during a MVC; fatigue, activity, selected muscle, and speed of movement.
Muscle activation research shows that under many or most conditions motor units are not maximally activated during sub-maximal exercise carried to the point of failure. At this point it is tempting to proclaim that the issue is settled and that in the debate of Intensity or Effort the research clearly shows that it is Intensity. However, motor unit activation studies only provide an indication as to the effectiveness of any particular training method. In order to truly answer the question we have to have a look at the training research. If very heavy weight and low reps increase strength more than other weight and rep schemes it should be evident in the research, no matter what the motor unit activation research says.