Supplementary Material for: Chair-rising power as digital biomarker: Validation against jumping power and chair rising-time in adults aged 32 to 92 years of age

Introduction The Chair-rising Test (CRT) is being widely used to assess lower body power. The test provides valuable information about functional capacity and other health outcomes. However, most centers use timing-based outcomes, which may compromise its suitability in younger people and fitter geriatric patients, and which may also introduce confounding effects of body height. We therefore aimed to compare the traditional use of timing-based outcome with digitally assessed measurements of neuromuscular power. Methods Data were collected from a longitudinal population-based study that examined changes of muscle and bone health. CRT and jumping mechanography were performed on a ground reaction force plate. In 346 people (age: 32 to 92 years), chair-rising rate (fCRT) was manually assessed, and peak chair-rising power (PCRT) and jumping power (PJMG) were computed. Statistical analyses targeted breakpoints in the relationships between fCRT, PCRT and PJMG. Effects of age, body height and sex were assessed with linear and partial regression analyses. Results Breakpoints were found at (fCRT = 0.778 Hz, PJMG= 35.2 Watt/kg, P<0.001) and at (fCRT = 0.669 Hz, PCRT = 9.9Watt/kg, P<0.001). Slow chair-risers, defined by fCRT < 0.669 Hz, were older than fast chair-risers (P < 0.001), albeit with largely overlapping age range (fast chair-risers: 32-90 years, slow chair-risers: 32-92 years). Body height was correlated with fCRT (P < 0.001) and PCRT (P = 0.009), but not with PJMG (P = 0.59). Conclusion Timing-based chair-rising test does not unequivocally reflect neuromuscular power. Its association with chair-rising power holds only in people who take more than 75 seconds for 5 stand-ups. For jumping power, the cutoff is at 6.4 seconds. Slow and fast chair-risers cannot be easily discerned by age. Bias by body height can substantially obscure age-effects in timing-based CRT assessments. We conclude that chair-rising power represents a more universally applicable biomarker and is less influenced by body height compared to timing-based chair-rising assessments.