Table 1 Operational definition of count- and model-based physical frailty phenotype

Slowness

Habitual gait speed was measured based on time taken to walk 10m at comfortable pace. Based on existing guidelines, slowness was defined by a gait speed of <1.0 m/s.

Gait speed was modeled continuously and nonlinearly using thin plate regression splines.

Weakness

Maximum handgrip strength measured using a handheld dynamometer. Weakness was defined using the AWGS 2019 cutoffs: handgrip strength <28 kg for men and <18 kg for women

Handgrip strength was modeled continuously and nonlinearly using thin plate regression splines. Instead of using gender-specific cut-points, gender was included in the model

Shrinking

Consistent with previous work, shrinking was defined by a body mass index (BMI) of <18.5kg/m²

Both body weight and height were modeled continuously and nonlinearly using thin plate regression splines

Low physical activity level

To facilitate comparability between studies that used different physical activity scales, physical activity level was measured by the total walking time per week based on the product of the self-reported frequency of walking per week (0-7days) and duration of walking per day (mins/day). Low physical activity level was defined by a total waking time < 2 hours or 120mins/week

Total walking time (hours/week) was cubic root transformed and modeled nonlinearly using thin plate regression splines.

Exhaustion

Exhaustion was measured by 2 items of the Centre for Epidemiological Studies-Depression Scale (CES-D): (Q1) I felt that everything I did was an effort and (Q2) I could not get going. Exhaustion was defined by answering at least “a moderate amount of the time” to either question

Both CES-D items were modeled as monotonic predictors.

Scoring

Each criterion yields a dichotomous score of 0 or 1. Count-based PFP was the sum of criteria and it classified patients as robust (0), prefrail (1–2), and frail (3–5).

PFP component criteria and gender were included in a Bayesian model which generated a continuous FI measure, from which the 3 frailty categories could be derived: robust (≤0.10), pre-frail (>0.10–0.21), and frail (FI>0.21)