**3. Results**

First, we tested whether there were differences in grip strength between males and females and, for each sex, between both hands to test the effect of hand dominance asymmetry using ANOVAs. We investigated potential differences in both absolute grip strength

and relative grip strength, in which hand area was used as a proxy for size. The results of the ANOVA showed that males were significantly stronger than females for both absolute grip strength (F(1, 1303) = 1782.72, *p* < 0.0001, η2 = 0.58) (Table 2 and Figure 3) and relative grip strength (F(1, 1303) = 820.36, *p* < 0.0001, η2 = 0.39), and both males and females were significantly stronger with their dominant hand compared with their non-dominant hand (grip strength, F(1, 1303) = 16.16, *p* < 0.0001, η2 = 0.005; relative grip strength, F(1, 1303) = 16.88, *p* < 0.0001, η2 = 0.007), indicating dominant hand asymmetry. When looking at the strength of this asymmetry, we found a mean difference in grip strength between the two hands of 5.5% for males, ranging between 1.9% (35–39 years old) and 11.4% (55–59 years old), and 4.2% for females, ranging between 0.4% (40–44 years old) and 8.8% (60 years and older) (Table S1). As a result, we searched for possible differences in the strength of hand dominance asymmetry across ages, and we found no significant interaction between age category and grip strength difference between dominant and nondominant hands for either males (F(9, 494) = 0.134, *p* > 0.05) or females (F(9, 772) = 0.207, *p* > 0.05) (Table S1). We tested for the homogeneity of variance between males and females, and for both hands, males showed significantly more variation than females across all age categories (F(3, 1302) = 41.822, *p* < 0.0001, η2 = 0.09).

**Table 2.** Summary statistics for grip strength (Ibs) of the dominant and non-dominant hand according to the sex and the age categories.


Males (M), Females (F), Standard deviation of the mean (SD).

**Figure 3.** Grip strength (lbs) performance in males (grey) and females (white) for the dominant hand (**A**) and the non-dominant hand (**B**). The boxplots show medians (solid line) and interquartile ranges of grip strength according to age groups. The dotted lines indicate variability outside the upper and lower quartiles, except for "outliers" (dots).

We then tested differences in handedness, as an indicator of DA, in absolute grip strength (sexes pooled). There were no significant differences in grip strength between right- and lefthanded participants for either hand (F(1, 1302) = 0.180, *p* > 0.05), although left-handed individuals had, on average, a higher hand dominance asymmetry with a stronger difference in grip strength between their dominant and non-dominant hands (males = 8.2%, females = 6.4%) compared with right-handed individuals (males = 5.2%, females = 4.0%). No significant interaction was found between age category, handedness and grip strength difference between dominant and non-dominant hands (F(9, 1266) = 0.028, *p* > 0.05) (Table S1).

We tested for the effect of the predictor variables (age, occupation, hand shape, hand preference, playing music, and playing sport) on grip strength using Fitting linear models. Results revealed that male grip strength of the dominant hand showed a trend towards being affected by hand shape (Figure 4) but not significantly so (F(1, 249) = 3.562, *p* = 0.06). However, for the non-dominant hand, male grip strength was significantly affected by hand shape (F(1, 249) = 9.489, *p* < 0.01, η2 = 0.034), such that males with wider hands were stronger than males with longer hands (Figure 4). Grip strength for the non-dominant hand was also significantly affected by occupation (F(2, 249) = 5.278, *p* < 0.01, η2 = 0.038), such that males doing forceful manual labour were significantly stronger than males doing

an office job (post-hoc analyses, *p* < 0.05), and males who practiced sports (F(1, 249) = 4.125, *p*< 0.05, η2 = 0.015) were stronger than males who did not (Figure 4).

**Figure 4.** Predictor effect plots for the fitting linear models in males for the dominant hand (**A**) and non-dominant hand (**B**). On the age and hand shape graphics, the blue shaded area is a pointwise confidence band for the fitted values at a level of confidence of 95%. The rug plots at the bottom of the graphs shows the location of the age values and the ratio W/L values. On the other graphics, the pink bars represent the confidence intervals at a level of 95%.

For females, linear modelling revealed that grip strength of the dominant hand was significantly affected by hand shape (F(1, 388) = 4.733, *p* < 0.05, η2 = 0.017), with females with wider hands being stronger than females with longer hands, and by age (F(1, 388) = 5.369, *p* < 0.05, η2 = 0.013), such that younger females (~<30 age) were significantly stronger than older females (~>50 age; Figure 5). For the non-dominant hand, female grip strength was also significantly affected by hand shape (F(1, 388) = 5.891, *p* < 0.02, η2 = 0.014) and by age (F(1, 388) = 4.463, *p* < 0.05, η2 = 0.011), following the same pattern as the dominant hand.

However, females practicing hand sports also had a significantly stronger non-dominant hand (F(1, 388) = 4.858, *p* < 0.05, η2 = 0.012) than females who did not (Figure 5). Given the effect of age on female grip strength, we tested which factors potentially interacted with age. Linear modelling revealed that for both hands, a significant interaction was found for grip strength between age (continuous) and hand shape (dominant hand, F(1, 388) = 4.123, *p* < 0.05, η2 = 0.010; non-dominant hand, F(1, 388) = 6.092, *p* < 0.05, η2 = 0.015). Moreover, while younger females (~<30 age) showed a similar grip strength regardless of differences in hand shape, older females (~>50 age) with wider hands were stronger than older females with longer hands (Figure S2).

**Figure 5.** Predictor effect plots for the fitting linear models in females for the dominant hand (**A**) and non-dominant hand (**B**). On the age and hand shape graphics, the blue shaded area is a pointwise confidence band for the fitted values at a level of confidence of 95%. The rug plots at the bottom of the graphs shows the location of the age values and the ratio W/L values. On the other graphics, the pink bars represent the confidence intervals at a level of 95%.
