Fatigue has been associated with reductions in daily activity of older people. Summary measures of daily physical activity provide limited understanding of how fatigue affects physical activity behavior. This study examined the hour-by-hour energy expenditure estimated from accelerometry data to provide insight into physical activity behaviors of older people experiencing fatigue. Fatigued participants were matched to 'not fatigued' participants by age, sex, and BMI. Each group consisted of 86 people with a mean age 73.8 years (SD 2.0), BMI 26.5 kg⋅m-2 (SD 3.9) and 61% female. The phase-space plot, constructed to express rate of change of average vertical axis counts per hour as a time series, showed fatigued participants deviated from the not fatigued participants during the morning period, when hour-by-hour activity was increasing. Older people who feel fatigued have a different morning activity pattern, which appears to lead to the lower overall levels of physical activity.
Keywords: aging; asthenia; bioenergetics; circadian rhythm; physical activity behavior.