Muscle fibre activation is unaffected by load and repetition duration when resistance exercise is performed to task failure

Abstract

Performing resistance exercise with heavier loads is often proposed to be necessary for the recruitment of larger motor units and activation of type II muscle fibres, leading to type II fibre hypertrophy. Indirect measures [surface electromyography (EMG)] have been used to support this thesis, although we propose that lighter loads lifted to task failure (i.e. volitional fatigue) result in the similar activation of type II fibres. r In the present study, participants performed resistance exercise to task failure with heavier and lighter loads with both a normal and longer repetition duration (i.e. time under tension). r Type I and type II muscle fibre glycogen depletion was determined by neither load, nor repetition duration during resistance exercise performed to task failure. r Surface EMG amplitude was not related to muscle fibre glycogen depletion or anabolic signalling; however, muscle fibre glycogen depletion and anabolic signalling were related. r Performing resistance exercise to task failure, regardless of load lifted or repetition duration, necessitates the activation of type II muscle fibres.

Higher effort, rather than higher load, for resistance exercise‐induced activation of muscle fibres

“…However, we should not disregard that the study by Morton et al . () had an acute design. Even though these findings might hypothetically suggest that hypertrophy of type I and type II muscle fibres would also be the same when training with low and high loads, there is still a need for future long‐term studies that would directly answer this question.…”

“…For example, would the results by Morton et al . () be replicated using even lower loads such as 10% or 20% 1RM? If future acute, as well as long‐term studies, confirm that load indeed is not an important determinant of muscle fibre activation and subsequently muscle fibre hypertrophy (provided that sets are carried out to the point of muscle failure), these findings may have considerable value in the prescription of resistance training.…”

“…A recent study published in The Journal of Physiology is a timely addition to the literature and provides intriguing information on this topic. Briefly, Morton et al (2019) demonstrated that both low-load (30% 1RM) and high-load (80% 1RM) lower-body resistance exercise performed to muscle failure results in different effects on time-under-load, training volume and surface electromyography amplitude. Despite these differences, the depletion of muscle glycogen and anabolic signalling were similar in both muscle fibre types regardless of the load used in the exercise session.…”

“…Based on the results of this study, it would be tempting to conclude that both low-load and high-load resistance training may, over the long term, produce similar hypertrophic effects on type I and type II muscle fibre hypertrophy. However, we should not disregard that the study by Morton et al (2019) had an acute design. Even though these findings might hypothetically suggest that hypertrophy of type I and type II muscle fibres would also be the same when training with low and high loads, there is still a need for future long-term studies that would directly answer this question.…”

“…In addition to future longitudinal studies, future acute research is needed to explore if there is a minimal threshold of external resistance necessary for eliciting the activation of both muscle fibre types. For example, would the results by Morton et al (2019) be replicated using even lower loads such as 10% or 20% 1RM? If future acute, as well as long-term studies, confirm that load indeed is not an important determinant of muscle fibre activation and subsequently muscle fibre hypertrophy (provided that sets are carried out to the point of muscle failure), these findings may have considerable value in the prescription of resistance training.…”