I recently came across some research exploring if heavy and light loads produce different regional muscle growth.
Put differently, could heavy and light loads result in different growth on parts of a muscle?
If true, this may suggest that using both heavy and light loads in your training could be effective for ensuring you experience more balanced muscle growth across a muscle or muscle group.
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As mentioned in other articles, we have two meta-analyses (one, two) indicating loads between 30% and 85% of a person’s one-rep max on an exercise can produce similar muscle growth, provided you perform repetitions to or very close to failure (the point at which no more repetitions can be performed).
Meta-analyses, for those unaware, are studies that combine the results of numerous individual studies.
But in nearly all of the studies in these meta-analyses, they measured muscle growth at only one region of a muscle, preventing us from identifying if any regional differences in muscle growth between heavy and light loads.
Likewise, we’ve previously explored the research assessing if alternating rep ranges in your routine (so using a variety of loads) could produce greater muscle growth than using only one rep range.
In the short term at least (12 weeks or less), there appeared to be no difference between the two. But all the research explored in that video assessed muscle growth at one region of a muscle, again preventing us from knowing if any regional differences in muscle growth exist between using a variety of loads in your routine or training with only one load.
The study I came across was a 1996 paper by Hisaeda et al.
11 untrained women were assigned to a hypertrophy or strength group.
Both groups trained the leg extension three times per week for 8 weeks.
The hypertrophy group performed the leg extensions with a 2-second concentric phase and a 2-second eccentric phase. Each session, they performed 5 to 6 sets of 15-20 reps to failure with 90 seconds of rest between sets.
The strength group performed the concentric and eccentric phases of the leg extension as fast as possible. Each session, they performed 8 to 9 sets of 5 reps to failure. They rested however long they needed between sets.
Cross-sectional area of rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius was each measured at 30, 50, and 70% of the thigh length.
For each muscle head, the sum of those three regions was used in the analysis.
Also, the values obtained for each head were added up to give us total quadriceps-cross-sectional area.
Total quadriceps cross-sectional area increases were similar between both groups.
However, looking at which heads actually grew for each group, the hypertrophy group experienced greater growth of the vastus lateralis, while the strength group experienced greater growth of the vastus intermedius and rectus femoris.
As mentioned, the authors of the study added up the cross-sectional area increases for each quadriceps head at 30, 50, and 70% of the quadriceps length and used that value for analysis. They did not report the data for cross-sectional area increases at each of these percentage regions on a muscle head.
That said, they did state the hypertrophy group experienced significant cross-sectional area increases at 30% of the thigh-length (so the upper regions) while the strength group experienced significant increases at 70% of the thigh length (so the lower region). However, I don’t know whether this applies to only one of the quadriceps heads or all of them, they, unfortunately, did not go into further detail.
Nevertheless, this study indicates with leg extensions, performing 15 to 20 reps (which of course requires a light load) targets the vastus lateralis more than the other quadriceps heads while performing 4-5 reps (which of course requires a heavy load) targets the rectus femoris and vastus intermedius more. Furthermore, lighter loads may target the upper region of some quadricep heads more, while heavier loads may target the lower regions more.
Now, this study is not without noteworthy limitations.
In addition to performing a different number of reps on the leg extension, both groups also used a different number of sets, rest interval lengths, and repetition tempos. These could have influenced the results.
The use of different repetition tempos is likely a noteworthy consideration. In a previous article, we demonstrated some evidence indicates different repetition tempos can produce different regional muscle growth.
Therefore, the results may have nothing to do with rep ranges or loading, rather all to do with repetition tempo.
Why Heavy and Light Weights May Produce Different Regional Hypertrophy
Is there any rational reason behind why heavy and light loads may cause different regional muscle growth?
One such reason relates to muscle fiber types.
Muscles generally consist of slow-twitch and fast-twitch fibers.
Slow-twitch fibers produce low amounts of power but are highly fatigue resistant, whereas fast-twitch fibers produce high amounts of power but are less fatigue resistant.
These characteristics bring up an interesting hypothesis.
When using heavy loads, could the body selectively recruit muscle heads or regions that are more fast-twitch.
Conversely, when using light loads, could the body selectively recruit muscle heads or regions that are more slow-twitch.
Averaging out the results from those studies, the vastus intermedius is around 53.5% fast-twitch while the rectus femoris is around 58% fast-twitch.
Could this explain why the strength group, who used heavy loads, experienced greater growth of these heads?
The vastus lateralis, on the other hand, is slightly less fast-twitch than these muscles. Based on numerous studies, it appears to be a near 50/50 split of fast and slow-twitch fibers. Could this explain why the hypertrophy group, who used lighter loads, experienced greater growth of this head?
Also, remember, the researchers broadly stated the hypertrophy group experienced greater upper region growth, while the strength group experienced greater lower region growth.
In most studies that aim to identify the fiber type composition of a muscle, they take a tissue sample from only one region of that muscle and then calculate the number of slow and fast-twitch fibers from that single region.
But, there is research indicating that when taking numerous samples from many regions of a muscle, the fiber type composition varies between them.
This might help explain why the hypertrophy and strength group experienced different growth in the upper and lower regions of the quadriceps.
As an extra note, if this hypothesis was true, it could apply to repetition tempo. Slower repetitions tempos may selectively recruit muscle heads or regions that are more slow-twitch, while faster repetition tempos may selectively recruit muscle regions or heads that are more fast-twitch.
But all of this is only a hypothesis and it does have problems with it.
Problems With the Fiber Type Hypothesis
The strength group experienced slightly greater growth of the vastus intermedius than the rectus femoris. However, the current research indicates the rectus femoris is slightly more fast-twitch than the vastus intermedius.
If this fiber type hypothesis was true, we would expect the rectus femoris to have grown more than the vastus intermedius, but this wasn’t the case.
Furthermore, the hypertrophy group experienced the greatest growth of the vastus lateralis. However, the current evidence (one, two, three) indicates the vastus medialis is more slow-twitch than the vastus lateralis.
If this fiber type hypothesis was true, we would expect the vastus medialis to have grown more than the vastus lateralis, but this wasn’t the case.
Moving on, as far as I’m aware, the study by Hisaeda et al. is the only paper to have explored differences in regional muscle growth between heavy and light load traditional training.
Now, I say traditional training for a reason. In this case, traditional training refers to performing exercises that involve both a concentric and eccentric phase. All the common exercises, such as squats, back squats, bench presses, and leg extensions fit into this criteria.
However, there is a study exploring how heavy and light isometric training impacts regional muscle growth.
12 untrained men were assigned to a 100 or 60 percent group.
Both groups performed an isometric contraction for the triceps at 90 degrees of elbow flexion with a dynamometer, three times per week for 10 weeks.
The 100 percent group contracted their triceps to 100% of their force-producing abilities for 12 sets of 6 seconds each session.
The 60 percent group contracted their triceps at 60% of their force-producing abilities for 4 sets of 30 seconds each session.
Triceps cross-sectional area was measured at 20 regions across the length of the muscle.
Increases at all regions were similar between both groups.
The graph combines the data for both groups and displays the average increase at all 20 regions across the triceps.
So, this study indicates heavy and light loads do not produce different regional muscle growth.
But there are some important considerations with this study.
Firstly, isometric contractions were used. If both groups performed dynamic exercises that involve concentric and eccentric phases, we how this would’ve impacted the results.
Also, although the researchers looked at numerous regions across the length of the triceps, they did not distinguish between the tricep heads. We don’t know if long, medial, and lateral head growth differed between the two groups.
To summarize, the Hisaeda et al. study provides us with some evidence that heavy and light loads may produce different regional muscle growth. However, the study with isometric contractions does provide us with some degree of opposing evidence.
There is an interesting hypothesis, relating to fiber type composition, behind why heavy and light loads may produce different regional growth. But as established, there are limitations to this hypothesis.
At the end of the day, a lot more research is needed in this area.
Although, as established in our alternating rep range video, in the short term (12 weeks or less), using a variety of rep ranges, meaning you combine heavy, moderate, and light loads in your training routine, appears to produce similar muscle growth to using only one rep range.
In the long term though, there is a possibility using a variety of loads is superior for muscle growth.
And as demonstrated in this video, there is a possibility that heavy and light loads produce different regional growth, meaning using a variety of loads would be effective for ensuring you experience more balanced muscle growth across a muscle or muscle group.
Therefore, using a variety of loads in your training program may be a sensible idea.
If for some reason you do not like the idea of using a variety of loads, there are still other ways to ensure more balanced muscle growth across a muscle or muscle group.
As covered in another article, using different repetition tempos could be one way.
Another more obvious way is to perform a variety of exercises for a muscle group. We’ll be covering this in more detail with future articles.