By Dr. Tasneem Pirani-Sheriff, ND
The soleus is an often-forgotten muscle; hidden behind the infamous gastrocnemius, it gets little heed. Despite taking up a mere 1% of total body weight, its power has recently proven to be mighty, particularly with regard to its metabolic effect on the body.
As health care practitioners we learn that the soleus originates on the surface of the proximal fibula as well as the soleal line of the tibia and inserts on the dorsum of the calcaneus. We become familiar with its action of plantar flexion of the foot (and of the talocrural joint) made possible because of innervation by the tibial nerve and blood supply by the posterior tibial artery. However, what has recently been uncovered about its impact on metabolic health is surprising.
Professor Marc Hamilton, who works in the area of health and human performance at the University of Houston, Texas, found that by activating this muscle in a specific way, it is able to result in a high level of oxidative metabolism using an alternate means of fuel. Not only does the “soleus pushup” or SPU, as he’s termed it, promote a sustained metabolic effect that spans hours, it also enhances blood glucose regulation. Interestingly, rather than muscle-glycogen dependence employed by the majority of our skeletal muscles, the slow-oxidative soleus muscle is able to utilize blood glucose and triglycerides from very low density lipoproteins (VLDLs) as fuel for its activity. For clarity, the soleus is identified as a slow-oxidative muscle due to the fact that it has a vast predominance of slow-twitch, slow oxidative type 1 muscle fibres. It is to note that the slow-oxidative term has nothing to do with the oxidative capacity of the soleus. Not only does the SPU double the rate of fat metabolism, Hamilton’s research suggests that the SPU even surpasses weight loss, exercise and intermittent fasting for its ability to optimize blood glucose levels.
The studies conducted by Hamilton showed that those participants who were verified to have a low VO2 max capacity during a treadmill test (essentially, those with low aerobic cardiorespiratory fitness), were able to maintain SPU contractions up to 270 minutes without cramps, joint pain or muscle soreness, without feeling local fatigue or increased effort over time. Low glycogen use of the soleus was confirmed using muscle biopsies. As well, after consuming a 75 mg load of glucose post-overnight fast, a significant difference was found between sedentary seated controls and those doing higher intensity SPU contractions (1.69 metabolic equivalents). During the 3 hour oral glucose tolerance test, where serum tests were completed every 15 minutes, those doing higher SPU contractions had up to a 52-60% decrease in both serum glucose and insulin incremental area under the curve. The average post-prandial whole-body oxidative glucose metabolism was 24 and 71 grams over 3 hours for the sedentary controls and the participants completing high intensity SPU contraction, respectively. The results were similar, despite age, BMI, sex, activity level, and cardiorespiratory performance level.
According to Statistics Canada, the majority of Canadians sit sedentary for almost 10 hours per day, often with a low metabolic rate during this time. The SPU, which can be done sitting, appears to be a game-changer in optimizing health, especially given that sedentary lifestyles are shown to increase the risk of chronic diseases, such as cardiovascular disease, obesity, diabetes, dementia as well as all-cause mortality. At rest, the several hundred skeletal muscles within our musculoskeletal system contribute to a meager 15% (approximately) of post-prandial whole-body oxidative glucose metabolism, but the soleus muscles alone can more than double the post-prandial whole-body oxidative glucose metabolism when activated in this specific manner. To note, in comparison to a larger muscle mass from multiple muscles in an extremity, a single muscle contracting in isolation has been shown to consume more local oxygen. Moreover, because of slow oxidative nature of the soleus, having more hexokinase II enzymes, GLUT-4 transporters and a reduced reliance on muscle glycogen, when activated by doing SPUs, the soleus muscles are quite fatigue-resistant, even in those with poor exercise tolerance.
The steps for completing the soleus pushup, as adapted from Professor Marc Hamilton’s instructions, are detailed below:
- Sit comfortably in a stationary seat with the tip of your toes vertically in line with your knees
- Keeping your quads relaxed, raise your heel as high as you can comfortably, moving your foot into plantar flexion, while lightly applying downward pressure into the ball of your foot (i.e. lightly pushing the balls of your feet into the floor) – avoid too much downward pressure so to prevent quad activation
- Repeat in a fluid motion at a count of 60 SPUs/min
Further information can be obtained on his website: https://www.soleusmetabolism.org. His research paper was published in the Elsevier iScience peer-reviewed journal and is available at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9404652/#mmc1 in an open access format.
Biography: Dr. Tasneem Pirani-Sheriff
Dr. Tasneem Pirani-Sheriff is a naturopathic physician with 11 years of post-second-
ary education. She has a private practice in North Burnaby where she focuses on autoimmune conditions, pain management, digestive and dermatological conditions as well as hormonal concerns. In her practice, she enjoys bridging the gap between conventional and traditional naturopathic medicine to provide the most integrative care for her patients.