Electromyography of Sedentary Behavior: Identifying Potential for Cardiometabolic Risk Reduction.

Abstract

Muscle activation during interruptions to prolonged sedentary time is a hypothesized mechanism underlying observed cardiometabolic benefits. We examined associations of quadriceps and hamstring muscle activity patterns with cardiometabolic risk markers and how these patterns varied between different sitting-interruption countermeasures. Electromyographic (EMG) data (shorts) were gathered for 1 to 2 d from healthy adults in a free-living study ( n = 172, age 40.9 ± 12.9, BMI 23.6 ± 1.3) and a laboratory-based study ( n = 12, age 47.0 ± 7.7, BMI 30.0 ± 4.7). Patterns examined were average EMG (aEMG;%EMG MVC ); EMG activity duration (% above signal baseline 3 μV); and usual (weighted medians) EMG activity bout amplitude (%EMG MVC ) and duration (s). In the free-living study, these were regressed against risk markers (waist, fat percentage, fasting plasma glucose, total cholesterol, high-density lipid cholesterol, low-density lipid cholesterol, triglycerides); in the laboratory study, EMG patterns for the muscle groups were compared between sitting and the active countermeasures. In the free-living study, lower-extremity muscles displayed minimal overall activity, with hamstrings and quadriceps using only 2.6% and 2.0% of their capacity (%EMG MVC ), respectively, and being active for 30% and 25% of the time. Higher hamstring aEMG and EMG activity duration were beneficially associated with waist, high-density lipid cholesterol and fat percentage (duration only) and a longer quadriceps usual EMG activity bout duration was beneficially associated with fasting plasma glucose. In the laboratory study, compared with prolonged sitting, active seated or upright active-interruption countermeasures modified these EMG patterns; brief (6 min) walking and simple resistance activities (SRA) were more beneficial than was a bout of standing (30 min) with the SRAs being the only intervention that matched daily aEMG levels. Upright and physically active interruptions to sitting appear to be required to increase the typically low muscle engagement observed in free-living contexts, promoting muscle activity patterns that may help ameliorate cardiometabolic risk.