Many people take it for granted that the heavier the weights you lift, the bigger the muscles you’ll grow. When a team of researchers from McMaster University in Ontario, Canada tested this idea, however, they found that this common belief could be wrong. The new study, published April 2012 in the Journal of Applied Physiology, compared muscle tissue growth — also called muscle hypertrophy — in the leg muscles of 16 weight-lifting subjects. The authors, led by graduate student Cameron Mitchell of McMaster’s Department of Kinesiology, found that as long as the subjects lifted weights to the point of fatigue — the point when they could no longer lift the weight with proper form — the amount of weight used did not affect gains in muscle size and strength. “Loads that were quite heavy and comparatively light were equally effective at inducing muscle growth and promoting strength,” says Mitchell.
Kinesiology is the study of human physical activity and movement. Stuart Phillips, one of the study’s authors and a professor of kinesiology at McMaster, told Today’s Science that pursuing a career in the field requires “an ability to work with, and love of, people,” since kinesiologists “work with great athletes at one end of the spectrum and with older frail folks at the other.”
Exercising to Answer a Fitness Question
Kinesiologists and other researchers at McMaster had previously shown that men who exercised with heavier weights made new muscle proteins at the same rate as those lifting lighter weights, as long as the latter group lifted to the point of fatigue. If protein changes after a single session were the same, they wondered, might muscle volume gains also be the same over many training sessions? This experiment was an attempt to find out. “The analogy I use”, Phillips told Today’s Science, “is that we can measure acutely the new bricks that are added to a house, but to know whether the house got bigger we need the longer-term training study.”
To find out whether or not “the house got bigger”, the researchers had 18 healthy subjects, all young men, participate in knee extension exercises for 10 weeks. Each participant was assigned one of three workouts for one leg and a different one of the three workouts for the other leg; workouts differed in terms of amount of weight and number of sets. By measuring changes in muscle size, chemistry and performance, the researchers sought to determine whether or not the weight lifted and the number of sets completed influenced the growth of muscle tissue. Before the first session, the researchers collected tissue samples or biopsies from the subjects’ leg muscles and used magnetic resonance imaging (MRI) to scan and digitize the three-dimensional structure of the muscles.
The researchers also measured subjects’ performance on knee extension exercises, including the maximum amount of weight each participant could lift with each leg. The three possible workouts assigned, based on these measurements, were: one set of knee extensions at 80% of the maximum weight they could lift, three sets at 80% of the maximum, or three sets at a lighter 30% of maximum. To complete a set, a participant would continue doing knee extensions until he could no longer continue with proper form. The researchers called this the point of “fatigue” or “failure.” For the heavier 80% groups, this typically meant eight to twelve repetitions per set, and for the 30% group about 25 to 30 repetitions. Immediately after each workout, all the participants ate the same type of high-protein energy bar and drank approximately 300 milliliters (10 ounces) of water. After the first workout, researchers collected a second muscle sample for chemical analysis, and after the final workout, they took a third sample to look for growth in the muscle fibers. They also took a second MRI scan of the leg.
Measuring Muscle Growth
The team checked for two types of possible muscle hypertrophy: over an entire muscle, and within a small sample of muscle fibers. They examined a muscle in the thigh called the vastus lateralis, a part of the quadriceps that extends the knee. Hypertrophy, the type of muscle growth caused by exercise, generally happens when muscle fibers become thicker — as opposed to new fibers being produced in the muscle. The team expected the two types of hypertrophy measurement in the vastus lateralis to correlate, meaning that if one measure changed, the other should change correspondingly.
To measure volume changes over the entire vastus lateralis, the researchers compared the MRI scans taken before and after the 10-week program. (To conduct an MRI, the part to be scanned [in this case the leg] is first placed in a powerful magnetic field, constraining the motion of certain molecules. Then, specifically calibrated radio waves are sent into the body. By looking at the energy that comes back out, a computer can reconstruct — in three dimensions — the tissues in question.)
Researchers examined computer-generatd slices of the MRIs, with each slice being a cross section of the leg. They used specialized image analysis software to determine the area of the part of the vastus lateralis in question. They multiplied the muscle area calculated for each slice by the distance between slices to come up with an approximate muscle volume which, when added to the volume of other slices in the scan, provided a measure of the total volume of the muscle.
To measure volume changes in individual muscle fibers, the team used a different method. They compared the tissue samples they collected from the first and last of the three biopsies. They wanted to look for changes in thickness in the cross-sectional area of the muscle fibers. To collect the samples, they first administered local anesthesia to the subjects. They then used a special type of needle, called a Bergström needle, along with suction to take about 80 milligrams of tissue from each vastus lateralis.
Tissue samples from the biopsies were placed in a viscous material called OCT (optimal cutting temperature medium), frozen with the OCT into a solid block, and later sliced. The slices were stained to distinguish two kinds of fibers, called type I and type II, or slow twitch and fast twitch, respectively. With a microscope, the researchers took digital photographs of the slices. Using the same image analysis software as they had used with the MRIs, they identified individual muscle fibers and drew outlines around 55 of each of the two types. Comparing the areas outlined before and after the 10 weeks of training, the researchers could determine if the muscle fibers had gotten thicker.
Bucking Conventional Wisdom
The results of both measurements of hypertrophy supported the same conclusion: the subjects’ muscle growth was not dependent on how much weight they had lifted. As long as participants continued exercising until the point of fatigue, it did not matter whether they lifted 30% of their maximum or 80%; performing more repetitions with the lighter weight achieved the same level of growth as fewer repetitions with the heavier load.
Measures of performance on knee extension exercises also showed similar gains in strength for all three groups. There was, however, one difference among them: the maximum weight the subjects could lift. Those who trained on heavier weights (the groups training at 80% of maximum) were able to lift heavier weights after the training program than the rest of the subjects. Although the researchers did not specifically measure velocity or acceleration, there were indications that those training on lighter weights were likely able to achieve greater speeds and accelerations, but those training on heavier weights were probably able to more slowly lift more weight.
Another aspect of the experiment that yielded unclear results was the effect the number of sets had on hypertrophy. Subjects in the groups performing three sets of knee extensions showed more than twice the amount of muscle growth as those in the one-set group. However, given the limited number of people in the study, researchers could not know for sure whether those differences came from significant differences in hypertrophy or from the regular fluctuations in data that are part of every real-world experiment. Repeating the experiment with more subjects or over a longer period of time could establish whether or not the number of sets has a significant effect on hypertrophy.
An Experiment Within the Experiment: Investigating Proteins’ Role in Muscle Growth
While they were examining hypertrophy, the researchers were also curious to see if certain changes in muscle chemistry during the first exercise session would accurately predict muscle growth over the full 10-week period. They analyzed the chemistry of the muscle tissue extracted during the biopsies before and after the first session to look for changes in three proteins that are affected by exercise: Akt, mTOR and p70S6K. The researchers examined “phosphorylation” of these proteins — whether or not a phosphate ion is attached. For many proteins, phosphorylation indicates if the protein is active or not. They wanted to know if phosphorylation of these proteins was correlated with hypertrophy. The researchers used a pestle to homogenize or mash up the muscle tissue samples by hand and then conducted a series of chemical tests for phosphorylation. They found that, while phosphorylation in certain proteins correlated with some of the exercises, none of the three phosphorylation reactions examined was a consistent predictor of long-term muscle growth.
A Revised Understanding of Hypertrophy
If, as this study suggests, hypertrophy is not determined solely by the amount of weight lifted, people can increase muscle size in different ways. “A much broader range of loads including quite light loads can induce muscle growth,” explains Phillips, “provided it is lifted to the point where it is difficult to maintain good form.” If a person is training to lift heavy loads, then fewer sets with more weight could make sense, but for those training for general fitness, endurance, appearance or speed, more repetitions at a lower weight level might be just as — if not more — effective. “Many older adults can have joint problems which would prevent them training with heavy loads,” says Mitchell. “This study shows that they have the option of training with lighter and less intimidating loads and can still receive the benefits.”
Why did the researchers assign a different workout for each subject’s right and left legs? Why couldn’t the subjects perform the same workout on both legs?
Why might the researchers have chosen to exclude women from their study?
Which types of athletes would train with a heavier load, and which with a lighter load but with more repetitions? Why?
The scientists conducting this study were not able to determine conclusively whether or not the number of sets influenced muscle growth. How might you change this experiment (or design a different one) to better test the relationship between the number of sets and hypertrophy?