How Should Youth Handgrip Strength be Normalized? New Insights Using 3-D Allometry with "Generalizable"...

Handgrip strength (HGS) is an important marker of health. Using allometric scaling, we previously identified that adult HGS should be normalized by a cross-sectional or surface area measure of body size, although it is unclear whether scaling youth HGS by the same body size dimension is appropriate. We therefore aimed to (1) identify the optimal body size dimension(s) to normalize youth HGS for differences in body size and (2) generate norm-referenced values for HGS using the identified body size dimension(s).

Data were from the National Health and Nutrition Examination Survey (NHANES), a representative sample of the US non-institutionalized civilian population. Exclusions resulted in a final sample of 4816 youth (51.2% male) aged 6-19 years. Handgrip strength was measured using electronic hand dynamometry. Body size dimensions included body mass, height, and waist circumference. Allometry was used to identify the most appropriate dimension(s) associated with HGS. Population-weighted, sex-stratified generalized additive models for location, scale, and shape were used to develop norms by sex and age. Norms were tabulated as percentile values (3rd to 97th) and visualized as smoothed percentile curves.

Predicting HGS using all three body size dimensions (three-dimensional) resulted in collinearity predominantly owing to the presence of waist circumference, prohibiting the use of all three body size dimensions as normalizers. However, collinearity was not an issue when two of the three dimensions (body mass and height) were adopted. Allometry identified a "generalizable" normalizing ratio as HGS_n =  $HGS/({\mathrm{HT}}^2\ast M^{0.333})$ . If only a single body size dimension were available, then HGS should be normalized using height² (i.e., $HGS/{\mathrm{HT}}^2$ ) because height was identified as the strongest single body size dimension associated with HGS. Sex- and age-specific norms for $HGS/({\mathrm{HT}}^2\ast M^{0.333})$ declined from age 6-8 years and progressively increased thereafter.

Allometrically scaling HGS by $({\mathrm{HT}}^2\ast M^{0.333})$ helps normalize strength for body size in population-based youth research.