Inactivity is an independent health risk : objectively measured muscle activity and inactivity during normal daily life of ordinary people

Abstract

Pesola, Arto 2011. Inactivity is an independent health risk: objectively measured muscle activity and inactivity during normal daily life of ordinary people. Master’s thesis in exercise physiology. Department of Biology of Physical Activity, University of Jyväskylä. 96 pages.

Today’s society promotes oversitting, and physical inactivity appears to be an independent health risk. The continuum from total inactivity to vigorous intensity training contains different physiological actions and objective measurements are needed to find out associations between physical activity, inactivity and health. The purpose of this study was to describe muscle activity and inactivity during normal daily life of ordinary people measured objectively with sophisticated EMG-shorts. Activity and inactivity patterns were compared between the age groups and associations between EMG results and metabolic health markers were studied.

Eighty four healthy volunteers (20-30 yrs, n=27; 30-60 yrs, n=40; 60-76 yrs, n=17) were measured in the laboratory and during 1-6 days of their normal daily life with shorts measuring muscle activity (EMG). EMG was normalized to maximum voluntary contraction (EMGMVC) and inactivity threshold was defined as an EMG level below that required during standing. Moderate and vigorous activity thresholds were defined as EMG values corresponding to 3 and 6 METs measured by indirect calorimetry during incremental treadmill walking test. The EMG results were compared between the age groups by One-way ANOVA with Tukey post-hoc test. Complete blood count, anthropometrics and blood pressure were measured in the morning in fasting condition and compared with the EMG results in a cross-sectional manner by bivariate correlation analyses (Pearson correlation coefficient, r) and forced-entry regression models.

Activity burst amplitude was 5.8±3.4% of EMGMVC and was higher in the elderly (60-76 yrs) compared to the younger (p<.001). The subjects had 12600±4000 muscle activity bursts and the average duration was 1.4±1,4s. Regardless of age, thigh muscles were inactive for 67.5±11.9% of the waking hours. Corresponding times for low, moderate and vigorous activity were 16.7±9.9%, 8.5±3.4% and 7.3±4.2%, respectively. The elderly had more moderate activity compared to the younger (p<.01). The longest inactivity periods lasted for 13.9±7.3 minutes and were longer in the elderly compared to the younger (p<.01). The longest continuous inactivity period was positively associated with triglycerides and systolic and diastolic blood pressure independent of gender (P<.01), gender and age (P<.05), gender and waist circumference (P<.01) and gender and VO2max (P<.05).

Only a minor part of muscle capacity is used in a normal daily life and the most of the time thigh muscles are totally inactive. The elderly had the same total inactivity time, but they had longer continuous inactivity periods compared to the younger. However, the elderly used a higher percentage of their muscle capacity during the day. The longest continuous inactivity period was positively and independently associated with elevated risk for metabolic diseases. Breaking prolonged periods of sitting would be a valuable and accessible way to easily promote metabolic health.