Discovery: Specific Leg Muscle Can Significantly Improve Metabolic Health

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Regular physical activity is strongly recommended to avoid overweight and obesity, as well as comorbidities such as diabetes and cardiovascular disease. But according to the World Health Organization, one in four adults don’t get enough exercise. Researchers are now discovering that one of our muscles, the soleus muscle, located in the lower leg, has the ability to improve metabolic health even while sitting.

It has been proven that inactive muscle fibers require little energy; consequently, the body’s oxidative metabolism is low during many hours of a day spent in a sitting position. “During periods of inactivity, skeletal muscle accounts for only about 15% of postprandial glucose oxidation,” the researchers note. What if we could find a way to keep our metabolism high during these periods of inactivity, in other words, burn calories without moving?

The soleus muscle is a powerful muscle located on the back of the leg and extends from just below the knee to the heel. Mark Hamilton, a professor of human health and performance at the University of Houston, found that properly activated this muscle can significantly improve metabolic health. His research, published in the journal iScience, suggests that this approach will be more effective than all currently offered solutions (diet, intermittent fasting, exercise, etc.) for improving blood sugar regulation.

The muscle that can work for hours without much effort

Oxidative metabolism is the process by which the body uses oxygen to burn metabolites such as blood glucose or fats to provide the energy it needs to function; this process, of course, depends on the immediate energy needs of the muscles. This is why at rest, oxidative metabolism is reduced to a minimum.

During physical activity, glycogen stored in the muscles is one of the main carbohydrates used for energy production (it is then broken down to glucose). But a muscle biopsy showed that glycogen contributes very little to the nutrition of the soleus muscle. “When properly activated, the soleus muscle can elevate local oxidative metabolism to high levels for hours rather than minutes, and does so using a variety of fuels,” says Hamilton.

The scientist found that this calf muscle is able to use other types of fuel, including glucose circulating in the blood, and fats. The advantage is that this type of metabolism is not limited, while the muscles, which are more dependent on glycogen, can no longer function when its reserves are depleted. Thus, the soleus muscle can function for hours without effort and fatigue.

Hamilton and colleagues tested the SPU on 25 participants (including 13 women) across a wide range of BMI, age, sedentary duration, and daily steps. These people spent an average of 10.7 (± 2.1) hours per day in a sitting position. All were equipped with a “soleus pump,” an electrical stimulation device specially designed by Hamilton’s team that activates the soleus muscle in a different way than when standing or walking. The device aims to increase oxygen consumption by the muscles.

“This is the first concerted attempt to develop a specialized type of contractile activity aimed at optimizing human metabolic processes,” says the researcher. The approach is called Soleus Pushup (SPU).

Significant effect on sugar and fat metabolism

While the volunteers sit with their feet on the ground and their muscles completely relaxed, the soleus pump lifts the heel while the forefoot remains on the ground. After reaching the maximum amplitude, the stop is passively released. Contrary to appearances, movement is the opposite of walking, the researchers specify: when walking, the amount of energy expended is minimal; with a soleus pump, the muscle uses as much energy as possible for a long time. “This is a powerful physiological movement based on the unique characteristics of the soleus muscle,” the team concludes. Contractions in UPs approximately double the whole body oxygen consumption compared to that observed at rest in a seated position.

Estimated oxygen consumption per kg soleus muscle during UP contractions compared to total lower limb musculature consumption during moderate walking and high intensity treadmill exercise. © M. Hamilton et al.

The researchers monitored participants’ metabolic rate and postprandial (post-meal) carbohydrate oxidation after ingestion of 75 g of glucose. The effects are significant: Three hours after the glucose drink, the team was able to observe a 52 percent improvement in blood sugar levels (about 50 mg/dL less) and a 60 percent reduction in insulin requirements.

This approach to maintaining soleus metabolism is also effective in doubling the normal rate of fat metabolism during the fasting period between meals, thereby reducing blood fat levels. “Enhancement of negligible local energy expenditure by isolated contractions improved systemic VLDL triglyceride homeostasis.” [ndlr : des lipoprotéines de très basse densité] and glucose to a large extent,” the team reports.

Regardless of a person’s level of physical activity, excessive sitting has been shown to increase the risk of heart disease, diabetes, dementia, and more. Increasing metabolism in this way would greatly reduce the incidence of these chronic diseases. New publications are in the pipeline, focusing on how to teach people how to properly perform this unique movement without the fancy lab equipment used in this study.

M. Hamilton et al., iScience.

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