Why Your Brain Wants You To Lift Heavy

Barbell lifter
Lifting heavy loads makes your brain and nervous system more efficient, boosting strength levels.

Scientists at the University of Nebraska-Lincoln have given new meaning to the concepts of brain power and mind-muscle connections. Their study suggests “strongly” that the development of physical strength improves as much from training the nervous system as it does from exercising the muscles controlled by it.

In the past few years, research has shown that increased muscle mass can result as much from lifting more repetitions of lighter weight as from lower repetition, heavier weight lifting. Strength gains, however, still follow an opposite pattern. Lower repetition, higher load lifting results in greater strength gains than the opposite method.

But if strength differs even when muscle mass does not, what explains the disparity?

These answers are exactly what Nathaniel Jenkins and his band of merry researchers are pursuing. They are doing so by measuring how the brain and motor neurons – the cells responsible for sending electrical signals to the muscles – adapt to high load training vs, low load training.

Their study backs up what most strength and conditioning coaches and experts have long believed: high intensity (load) training leads to better nervous system responsiveness and higher strength levels. It trains the nervous system to more effectively deliver electric signals to muscles from the brain, increasing the force produced by those muscles.

This was true even when the comparative mass of the muscles in question is equal from one lifter to another. In other words, if two lifters are identical in muscle mass, the lifter who trains with higher loads will have higher force output (strength.)

Muscle Brain

Muscles contract when they receive electrical signals that originate in the brain’s neuron-rich motor cortex. Those signals descend from the cortex to the spinal tract, speeding through the spine while jumping to other motor neurons that then excite muscle fibers.

Evidence found by Jenkins’ team shows that the nervous system activates more of those motor neurons or fires (excites) them more frequently when the motor neurons in question are subjected to high load training. It’s this greater level of excitation that likely accounts for the greater strength gains across relatively equal growth in muscle mass.

“If you’re trying to increase strength — whether you’re Joe Shmoe, a weekend warrior, a gym rat or an athlete — training with high loads is going to result in greater strength adaptations,” said Jenkins, an assistant professor of exercise physiology at Oklahoma State University who conducted the research for his dissertation at Nebraska.

The study randomly assigned 26 men to train for six weeks on a leg-extension machine loaded with either 80 or 30 percent of 1 rep max, i.e., the maximum weight they could lift a single time. Participants were asked to lift the weight to absolute failure three times a week. While Jenkins noted roughly equal muscle growth between the two groups, there was a significantly higher strength increase – about 10 pounds – in the higher load group.

Jenkins team also supplied electric current to the nerve that stimulates the quadriceps to perform knee extension. Even at maximal effort, most people are unable to generate 100 percent of the force that muscles are capable of physiologically producing, Jenkins said.

By comparing the force production of a participant’s most forceful extension with the maximal force generated with the aid of electric current, it can be determined how much of that capacity a person has reached. This is a measure known as voluntary activation.

After adjusting for participant’s baseline scores, Jenkins and his team found that the voluntary activation of the low-load group increased from 90.07 to 90.22 percent — 0.15 percent — over a three-week span. The high-load group saw their voluntary activation jump from 90.94 to 93.29 percent, a rise of 2.35 percent.

“During a maximal contraction, it would be advantageous if we are activating — or more fully activating — more motor units,” Jenkins said. “The result of that should be greater voluntary force production — an increase in strength. That’s consistent with what we’re seeing.”

Researchers found another way to test this hypothesis. They had participants kick out at 10 percent intervals of baseline strength, from 10 to 100 percent of maximal effort. This was done at the 3 and 6 week marks of the study.

Jenkins believed that if high load training does improve muscle efficiency better than low load training, then high load lifters would also exhibit lower voluntary activation, i.e., use a smaller proportion of their strength in lifting the same relative weight.

This was reflected, generally, by the data. Voluntary activation in the low-load group did decrease slightly, from an average of about 56 percent at baseline to 54.71 percent after six weeks. But it decreased more in the high-load group, dropping from about 57 to 49.43 percent.

“If we see a decrease in voluntary activation at these sub-maximal force levels, that suggests that these guys are more efficient,” Jenkins said. “They are able to produce the same force, but they activate fewer motor units to do it.”

Electrode testing reinforced those results. Electrodes were placed on the quadriceps of participants to record the electrical signatures in the muscle during movement. The larger drop in electrical activity was noted in the high load training group. The reduction was consistent across most levels of exertion.

“From a practical standpoint, that should make the activities of daily living easier,” Jenkins said. “If I’m lifting sub-maximal loads, I should be able to do more repetitions with fewer motor units active, so maybe I fatigue a little bit slower.”

If a lifter wants to focus primarily on building mass or avoid putting extreme stress on joints, low load training is a reasonable option, Jenkins maintained. These are priorities for people recovering from injury, as well as for many older adults.

However, if the priority is building strength and power, one truth comes through strongly in this research: heavier is better.

“I don’t think anybody would argue (with the idea) that high-load training is more efficient,” Jenkins said. “It’s more time-efficient. We’re seeing greater strength adaptations. And now we’re seeing greater neural adaptations.”

Jenkins detailed his findings in the journal Frontiers in Physiology. He authored the paper with former doctoral adviser Joel Cramer, associate professor of nutrition and health sciences; Terry Housh, professor of nutrition and health sciences; Nebraska doctoral students Amelia Miramonti, Ethan Hill, Cory Smith; and doctoral graduate Kristen Cochrane-Snyman, now at California State Polytechnic University.

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Journal Reference – Nathaniel D. M. Jenkins, Amelia A. Miramonti, Ethan C. Hill, Cory M. Smith, Kristen C. Cochrane-Snyman, Terry J. Housh, Joel T. Cramer. Greater Neural Adaptations following High- vs. Low-Load Resistance Training. Frontiers in Physiology, 2017

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