

It’s a commonly accepted tenet that resistance training adaptations follow a “strength-endurance continuum” whereby lifting heavy loads maximizes strength increases while light load training leads to optimal improvements in local muscle endurance. Conventional wisdom also postulates that at least moderately heavy loads are required for building muscle. General training guidelines proclaim that loads lighter than about 65% 1RM are insufficient to stimulate fast-twitch muscle fibers necessary for growth. The so-called “hypertrophy range” is generally considered to be 6-12 reps/set.

Recent research has challenged these established tenets. It has been proposed that if light loads are lifted to muscular failure, near-maximal recruitment of fast-twitch fibers will occur resulting in muscular adaptations similar to those obtained from training heavy.

A meta-analysis from my lab published last year in the European Journal of Sports Science found substantial increases in muscle strength and hypertrophy following low-load training. However, the magnitude of increases were not as great as that associated with using heavier loads, and a trend for superior gains was in fact shown when lifting weights >65% 1RM. I covered the specifics of this meta-analysis in a previous post.

The caveat: All previous studies employed untrained subjects, raising the possibility that results were attributed to the “newbie effect” that states those new to training build muscle from pretty much any activity — even cardio!

To achieve clarity on the topic, my lab carried out a well-controlled study on the effects of high- versus low-load training using resistance-trained individuals, which was just published in the Journal of Strength and Conditioning Research. Here’s what you need to know.

What We Did

Eighteen young men with an average of more than 3 years lifting experience were randomly assigned to a resistance training program using either moderately heavy loads (8-12RM) or light loads (25-35RM). All other aspects of the program were held constant between groups to isolate the effects of load on muscular adaptations. The program consisted of 3 sets of 7 different exercises targeting the major muscle groups (bench press, shoulder press, lat pulldown, seated pulley row, back squat, leg press, and leg extension). Training was carried out on 3 non-consecutive days-per-week (M, W, F) for 8 weeks.

Testing was conducted pre- and post-study. We used b-mode ultrasound to measure the thickness of the biceps, triceps, and quads. We assessed maximal strength via 1RM for the back squat and bench press. Finally, we measured changes in muscle endurance by having subjects perform the bench press at 50% of their 1RM to volitional failure.

What We Found

Both groups significantly increased lean mass in their biceps, triceps, and quads, but no statistically significant between-group differences were noted in any of these muscles (i.e. both groups had similar muscle growth over the course of the study). On the other hand, the heavy load group showed significantly greater strength increases in the back squat and a trend for greater increases in the bench press compared to the light load condition. Conversely, local muscle endurance was markedly greater for the low-load group.

Reconciling the Data

The primary take-home points from the study are as follows:

• Gains in muscle mass are about the same regardless of repetition range provided training is carried out to muscle failure

• Maximal strength requires the use of heavy loading

• Muscle endurance is best obtained from the use of light loads

To really understand the practical implications of the study, however, we need to look a bit deeper at the results.

The superior strength gains for heavy load training are consistent with the principle of specificity, which effectively states that training adaptations are specific to the imposed demands. No surprise here. From a mechanistic standpoint, the ability to exert maximal force has a high neural component, and the associated neural adaptations appear to be optimized through the use of heavy loads. Previous work from my lab showed that these adaptations exist even at the far left aspect of the strength-endurance continuum, as a powerlifting-type routine (3RM) was found to produce greater strength increases compared to a bodybuilding-style workout (10RM). It also makes intuitive sense that you need to train heavy to “get a feel” for using the maximal loads required to perform a 1RM.

The greater improvements seen in local muscle endurance from light-load training were expected as well. Although the topic hasn’t been well-studied, it stands to reason that low-load training is associated with adaptations specific to enhancing buffering capacity, thereby allowing for the performance of a greater number of submaximal repetitions. Again, a basic application of the principle of specifity.

On the other hand, I readily admit to being surprised by the fact that muscle growth was similar between conditions. While a number of previous studies had shown no differences in gains between light- and heavy-load training, I figured this was due to the “newbie effect.” No way could you build appreciable muscle using 30 reps per set.

Or so I thought.

I’m now a believer.

What’s particularly interesting, though, are the potential implications for how muscle growth actually manifests when training in different loading zones. A previous study from my lab showed that muscle activation was markedly greater when performing reps at 75% 1RM versus 30% 1RM. A follow up study (currently in review) found that the heavy-load superiority for activation held true when training at 80% 1RM versus 50% 1RM as well. Combined, these findings suggest that the recruitment and/or firing frequency in the high-threshold motor units associated with the largest type II fibers is suboptimal when training at low-loads. It therefore can be hypothesized that if muscle growth is indeed similar across loading zones — as found in the current study — hypertrophy from light-load training necessarily must be greater in the type I fibers. Indeed, emerging research out of Russia indicates that this is in fact that case with multiple studies showing that light loads promote greater gains in type I fibers while heavy loads increase type II fiber hypertrophy to a greater extent (Netreva et al 2007; Netreba et al 2009; Netreba et al 2013; Vinogradova et al 2013).

Bottom line: If your goal is to build as much muscle as possible, it seems appropriate to train across the spectrum of loading zones; use lighter loads to target type I fibers and heavier loads to target type IIs. In this way, you ensure maximal development of all fiber types.

An interesting point to keep in mind is that none of the subjects in my study trained with more than 15 reps/set during the course of their usual lifting routines and the majority never went above 10 reps. This raises the possibility that their endurance-oriented type I fibers were underdeveloped in relation to the strength-oriented type II fibers. If so, it’s possible that their type I fibers had a greater capacity for growth, which was realized in those who trained using light loads.

The study had some notable limitations. For one, the training period lasted only 8 weeks; whether results would have diverged over a longer time-frame is undetermined. For another, muscle thickness was measured only at the approximate mid-point of each muscle. Research has shown that muscles often hypertrophy in a non-uniform manner. Thus, it is possible that other aspects (i.e. distal or proximal) of the muscles studied might have differed in their growth response.

A final and important point to consider. While people often dismiss light-loads as being for wimps, nothing could be further from truth. Training to failure with high reps is highly demanding and the associated acidosis extremely uncomfortable. To this end, approximately half the subjects in the low-load group puked during the first week of training and several others experienced nausea and/or light-headedness. Although these issues tended to dissipate as time went by, they nevertheless can negatively affect adherence to the program. If you choose to incorporate light-loads into your program, be prepared for a grueling workout!