4 The Short Physical Performance Battery does not correlate with daily life gait quality and quantity | 67 quality measure is also only weakly associated with daily life gait speed in 254 community-dwelling participants of 18 years or older (220). They suggested that the four meter gait speed and daily life gait speed represent related but different constructs, i.e., performance and physical activity, respectively (220). Similarly as suggested in the study of Van Ancum et al., it is possible that we found weak correlations between the SPPB and daily life gait quality and quantity because they represent different constructs (220). The SPPB is assessed in a standardized and optimal clinical environment. This, in combination with the design of the test, makes it a measure for capacity, as also suggested by a study of Lamb et al.: what an individual maximally can do (184, 221). Daily life gait quality and quantity, as assessed over a week using inertial sensors, represents performance: what an individual actually does, in their own environment (184). The weak correlations between the SPPB and gait quality and quantity imply that the older adults in this study are not functioning close to their capacity in daily life and it may suggest that training one’s capacity may not automatically lead to an improvement in daily performance (222). If daily life gait quality and quantity are indeed measures for performance, it is expected that correlations between optimal performance in daily life gait quality and the SPPB should be stronger compared to the correlations between the SPPB and median scores of sensor-based daily life gait quality. Additional explorative analyses in which the 90th or 10th percentile of the gait quality was correlated with the SPPB showed correlations which were comparable to the analysis using the 50th percentile data. A possible explanation may be that the 90th percentile of gait quality and quantity measures in daily life still not equals capacity, as measured by the SPPB in a clinical environment. Another explanation for finding weak correlations between the SPPB and daily life gait quality and quantity may be the large interindividual variation in daily life gait quality and quantity in combination with the small interindividual variation in SPPB scores in our study population (223). In addition, it is important to note that the SPPB assesses more than just gait; only one of the three tests pertains to gait performance (183). While muscle strength, needed to rise from a chair and balance performance are requirements for gait function, but possibly, these aspects had less influence on daily life gait performance and hence limited the association of the total SPPB score (224). Moreover, the SPPB, designed as a composite measure, includes a 4-meter walk along with balance and sit-to-stand components, providing a measure of functional status in a controlled setting. In contrast, our wearable sensor data focused on continuous monitoring of gait quality and quantity during daily life, excluding very short walking bouts which may not provide meaningful data on gait dynamics. The SPPB provides insights into functional status during structured tasks, while wearable sensors offer insights into real-world mobility and activity patterns.
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