This study was well designed, which allowed the researchers to effectively investigate various questions. Most notably, does the amount of weight lifted affect changes in overall muscle size? According to this study, the answer is no.

In agreement with the summarized study, a meta-analysis of randomized controlled trials reported that low load resistance training (between 30% and 59% of 1RM) is just as effective as high load resistance training (at least 80% of 1RM) for muscle gain.^[1]

However, there may be a little more to the story. Certain conditions need to be met for low load resistance training to be as effective as high load resistance training. A crucial detail of the intervention in the summarized study is that each set was performed to muscular failure.

In an 8-week randomized controlled trial, the participants were assigned to perform resistance training with either 30% or 80% of 1RM.^[2] In addition, they performed each set to failure with one limb, whereas the other limb did not train to failure. This protocol was completed for both biceps curls and leg press.

The number of repetitions performed by the nonfailure limb was determined by the number of repetitions performed by the failure limb. Specifically, the average number of repetitions performed during each set across 3 sets to failure was multiplied by 60%, and then that many repetitions was performed each set with the nonfailure limb until the same number of total repetitions was completed. The researchers found that muscle size for both the arms and legs increased with 80% of 1RM, irrespective of whether each set was performed to failure. In the 30% of 1RM protocol, muscle size increased only in the limb that performed each set to failure.

A separate trial that was 12 weeks long used a similar design, except that the nonfailure protocol was different.^[4] In this study, the participants stopped their set when they felt sufficiently fatigued. In contrast to the aforementioned study, nonfailure training led to just as much muscle growth as failure training. The main explanation for this conflicting finding is that the nonfailure training was much more difficult in this study, which was evidenced by the performance of a similar number of repetitions between the failure and nonfailure limbs.

The need to train close to failure when using a light weight, if the goal is to maximize muscle growth, is proposed to be related to Henneman’s size principle, which refers to the recruitment of motor units (i.e., a motor neuron and all of the muscle fibers it innervates) during muscle contraction.^[5]

During the early stages of a set with a light weight, only low-threshold motor units — which are composed of type I slow-twitch muscle fibers — are recruited because the amount of force needed to produce muscle contractions is low. As the set continues and low-threshold motor units fatigue, high-threshold motor units — which are composed of more powerful type II fast-twitch muscle fibers — are recruited to sustain muscle contractions.^[6] Consequently, if a light weight is used and the set isn’t taken to failure, many fast twitch muscle fibers (which have a greater potential to increase in size than slow twitch fibers^[7]) will not be activated, resulting in a suboptimal stimulus for muscle gain.^[8] In contrast, when the weight is heavy enough, fast twitch fibers will be activated early in the set, and thus training to failure isn’t necessary to achieve a robust stimulus for muscle gain.

Evidently, light weights are as effective as heavy weights for increasing overall muscle size, but another question relates to the specific muscle fibers that change in size. In the context of maximizing muscle hypertrophy, it’s been proposed that varying the amount of weight lifted could be beneficial, with the idea that performing high-repetition sets with light weights is better for increasing the size of endurance-oriented type I fibers because this style of training exposes these fibers to a greater amount of time under tension.^[9]

The researchers behind the summarized study used several different methods to evaluate changes in muscle size, including DXA and ultrasound, and in the case of the vastus lateralis, muscle biopsies were collected to determine whether the amount of weight lifted influenced changes in specific muscle fibers. The changes in the size of different muscle fibers were similar between the heavier and lighter weights.

A meta-analysis of 5 randomized controlled trials also reported that lifting lighter weights did not increase the size of type I fibers to a greater extent than heavier weights.^[10] Mechanistically, these findings are consistent with the idea that when a set is performed to failure, all available motor units are recruited, resulting in equivalent activation of type I and type II muscle fibers.^[6]

However, when considering the use of training with light weights to maximize muscle gain, it may be important to ensure that the weights aren’t too light. Specifically, less than 30% of 1RM seems to be too low. In a 12-week randomized controlled trial, the participants trained one leg and arm using 20% of 1RM and the other limb with 40%, 60%, or 80% of 1RM.^[3] They performed 3 sets to failure with 20% of 1RM, then completed the same amount of work (sets multiplied by repetitions multiplied by weight) with the other limb.

The researchers found that muscle gain in both the leg and arm were greater with 80% of 1RM than 20% of 1RM, although there was no difference between 80% of 1RM and 40% or 60% of 1RM.

Despite how it may sound at first, it bears mentioning that training with light weights to failure isn’t for the faint of heart. Evidence indicates that training with a lighter weight produces greater discomfort and displeasure than heavier weights.^[11]^[12]

Consequently, lifting light weights to failure as a method to maximize muscle gain may be easier said than done. This style of training requires a substantial amount of effort every set to maximize benefits, so it may not be a practical method for everyone and could increase the risk of psychological burnout or reduce adherence to a resistance training plan over the long term.^[13] Additionally, some people may find the level of discomfort it causes to be intolerable, which makes it difficult to achieve muscular failure.^[14]^[11]

Another reason the researchers of the summarized study used this design is because they wanted to evaluate the variation in response to the resistance exercise intervention between participants, as well as within each participant. In agreement with previous studies, changes in muscle size varied widely between participants.^[15]^[16]^[17] Some participants even lost muscle mass during the study.

On the other hand, there was little variation within each participant, meaning that gains in muscle mass were similar between each side of the body and between the arms and the legs. Collectively, these findings suggest that intrinsic factors (i.e., a person’s biology) primarily dictate the response to resistance training. In other words, people who respond well to resistance training in the upper body will exhibit similar results in the lower body and vice versa, and the influence of resistance training variables (e.g., the amount of weight lifted) on muscle growth pales in comparison to intrinsic factors.

In support of this idea, a couple of trials have been conducted in which the participants performed a consistent training program with one limb, whereas for the other limb, resistance training variables (i.e., weight, volume, rest between sets, and the duration of eccentric contractions) were frequently manipulated. In both limbs, sets were performed close to muscular failure. Just like the summarized study, these studies reported similar muscle growth between limbs within each participant, but substantial differences in muscle growth between participants.^[18]^[19]

Overall, these results suggest that as long as someone is training hard, they’re likely achieving most of their muscle growth potential. Although manipulating resistance training variables can affect muscle gain,^[20]^[21]^[22] tweaking something like training volume is unlikely to result in sudden massive gains^[23] because muscle growth is ultimately constrained by intrinsic factors.^[24]^[25] Several intrinsic factors have been identified as potentially important determinants of muscle growth, including genetics, epigenetics, androgen receptor content, satellite cell proliferation, and ribosomal biogenesis .^[26]^[25]^[27]

Biological factors that can affect how well someone responds to resistance training

Adapted from Joanisse et al., 2020, F10000Res.

Lighter weights can achieve similar muscle gains as heavier weights when training to muscular failure Original paper