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

Introduction: 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.

Methods: Electromyographic (EMG) data (shorts) were gathered for 1 to 2 days 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 ;%EMGMVC); EMG activity duration (% above signal baseline 3 μV); and usual (weighted medians) EMG activity bout amplitude (%EMGMVC) and duration (s). In the free-living study, these were regressed against risk markers (waist; fat percentage; fasting plasma glucose [FPG];total cholesterol; HDL;LDL; triglycerides); in the laboratory study, EMG patterns for the muscle groups were compared between sitting and the active countermeasures.

Results: 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 (%EMGMVC), respectively, and being active for 30% and 25% of the time. Higher hamstring aEMG and EMG activity duration were beneficially associated with waist, HDL and fat percentage (duration only) and a longer quadriceps usual EMG activity bout duration was beneficially associated with FPG. 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.

Conclusions: 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.