Accelerometer-measured absolute versus relative physical activity intensity: cross-sectional associations with cardiometabolic health in midlife

Abstract

Background: Observational studies investigating the association between accelerometer-measured physical activity and health all use absolute measures of physical activity intensity. However, intervention studies suggest that the physical activity intensity required to improve health is relative to individual fitness. The aim of this study was to investigate the associations between accelerometer-measured absolute and relative physical activity intensity and cardiometabolic health, and what implications these associations may have on the interpretation of health-associated physical activity.

Methods: A sample of the cross-sectional Swedish CArdioPulmonary bioImage Study (SCAPIS) consisting of 4,234 men and women aged 55-64 years was studied. Physical activity intensity was measured by accelerometry and expressed as absolute (e.g., metabolic equivalents of task) or relative (percentage of maximal oxygen consumption). Fitness was estimated by the submaximal Ekblom-Bak test. A composite ('metabolic syndrome') score combined measures of waist circumference, systolic blood pressure, high-density lipoprotein, triglycerides, and glycated hemoglobin. Associations of absolute and relative physical activity intensity with the health indicators (i.e., fitness and metabolic syndrome score) were studied by partial least squares regression. Analyses were stratified by fitness level.

Results: Both absolute and relative physical activity intensity associated with the health indicators. However, the strongest associations for absolute intensity varied depending on fitness levels, whereas the associations for relative intensity were more synchronized across fitness groups. The dose-response relationship between moderate-to-vigorous intensity and the health indicators was stronger for relative than for absolute intensity. The absolute and relative moderate-to-vigorous intensity cut-offs intersected at the 5th fitness percentile, indicating that the absolute intensity cut-off is too low for 95% of individuals in this sample. While 99% of individuals fulfilled the general physical activity recommendations based on absolute intensity measures, only 21% fulfilled the recommendations based on relative intensity measures. In relation to a "sufficient" fitness level, 9% fulfilled the recommendations.

Conclusions: Accelerometer-measured relative physical activity intensity represents the intensity related to health benefits regardless of fitness level. Traditional absolute moderate intensity accelerometer cut-offs are too low for most individuals and should be adapted to the fitness level in the sample studied. Absolute and relative physical activity intensity cannot be used interchangeably.

Keywords: Cardiometabolic risk factors; Cardiovascular disease; Cardiovascular fitness; Recommendations.

Fig. 1

Fitness-group means of standardized PA intensity spectrum represented as absolute (left) and relative mean group fitness (right). Values are calculated in relation to total sample mean and standard deviation where 0 represents the mean and 1 the standard deviation for each of the PA intensity spectrum variables. Dashed lines indicate where less than one third of the individuals had any movement. Shaded areas represent 95% confidence intervals. Since no established relative cut off for sedentary is available, the absolute cut off at 1.5 METs was used as reference for relative sedentary time based on the average fitness in the sample. SD standard deviation, SED sedentary, LPA light PA, MPA moderate PA, VPA vigorous PA, VVPA very-vigorous PA

Fig. 2

The absolute (left) and relative (right) PA intensity patterns associated with metabolic syndrome score (top) and fitness (bottom). The selectivity ratio represents the influence of each PA intensity level in the association with the outcome. The thick lines represent the main statistically significant part based on 95% confidence intervals, and the dashed lines indicate where less than one third of the individuals had any movement. Since no established relative cut off for sedentary is available, the absolute cut off at 1.5 METs was used as reference for relative sedentary time based on the average fitness in the sample. SED sedentary, LPA light PA, MPA moderate PA, VPA vigorous PA, VVPA very-vigorous PA

Fig. 3

Dose–response relationship from time spent at absolute or relative moderate-to-vigorous physical activity (MVPA) as independent variable and metabolic syndrome (left) and fitness (right) as dependent variable in each of the three fitness groups. Coefficients from regression models are expressed as number of standard deviations difference in the health variable from one minute increase in time spent at MVPA per day. Error bars indicate 95% confidence intervals

Fig. 4

Translation between absolute and relative PA intensity levels. Individual fitness level on the x-axis and different measures of absolute intensity on the y-axis. Background colors with solid borders represent relative intensity. Dashed lines denote absolute intensity. The black dotted line represents the distribution of fitness in the study sample. The intersects between the dashed and solid lines represent the fitness level where the absolute and relative PA intensity is congruent. In individuals with a higher fitness than this, time spent at different intensities will be overestimated, which is emphasized by the colored arrows. For example, a relatively unfit individual with a maximal oxygen consumption of 23 mL/min/kg (x-axis) will have a relative MPA accelerometer cut point at 201 mg (y-axis) which corresponds to a locomotion speed of 3.2 km/h (slow walking) and a MET of 3.0 (at the intersection of the green dashed and solid lines). For comparison, an individual with a higher maximal oxygen consumption of 35 mL/min/kg will have a relative MPA accelerometer cut point at 388 mg which corresponds to a locomotion speed of 4.8 km/h (brisk walking) and a MET of 4.6. If the cut point of the unfit individual (MET of 3.0) is applied to the individual with higher fitness, the time spent in MPA is overestimated. LPA, light PA, MPA moderate PA, VPA vigorous PA, VVPA very-vigorous PA