Thanks, Bob, for sharing the link to the paper. I'll have a more thorough look at the suggested idea of "soleus pushups" which have gained some traction recently, but already the opening statement raises some question mark: "the entire human musculature accounts for only ∼15% of the body’s oxidative metabolism of glucose at the resting energy expenditure, despite being the body’s largest lean tissue mass." The 15% is likely an underestimation that does only reflect very limited scenarios. You need to keep in mind that muscle protein turnover, a continuous and energy-demanding process, significantly contributes to BMR, independent of glucose oxidation. Muscle is metabolically active even at rest.
But aside from this, my immediate caveats are:
Small Muscle Mass: The soleus is a relatively small muscle, especially compared to the large muscles of the thighs. Even if it's highly oxidative, its absolute contribution to whole-body glucose disposal is limited by its size. The quadriceps, hamstrings, and gluteal muscles have far greater potential to impact whole-body glucose metabolism due to their sheer mass.
Low Energy Expenditure: While "soleus push-ups" might increase energy expenditure slightly above complete rest, the increase is likely to be very small. No comparison to even light activities like walking. The metabolic demands of unloaded, small-range-of-motion contractions are minimal.
Limited Impact on Insulin Sensitivity (Systemically): While there might be some local improvements in insulin sensitivity within the soleus, it's unlikely that this will translate to a significant improvement in whole-body insulin sensitivity, especially in individuals who are already insulin resistant. Their systemic insulin resistance (driven by the body's entire muscle mass) is unlikely to benefit measurably and meaningfully.
The soleus pushups are probably not entirely useless, but we need to look carefully at the methods and the numbers. One thing is for sure: the experiments have been done on small numbers of participants only, and we need to check potential authors' bias. "Soleus pushups" are not a substitute for a comprehensive exercise program that includes both aerobic and resistance training involving larger muscle groups
