Association between the morphology of forearm flexor muscles and the number of penetrating muscular nerve branches: An anatomical cadaveric study

BACKGROUND: Terminal innervation of forearm muscles is anatomically variable, but the extent to which gross morphology explains differences in the number of penetrating muscular branches remains unclear. This study evaluated associations between forearm flexor muscle morphology and the number of muscular nerve branches independently penetrating the epimysium. METHODS: This observational analytical cadaveric study included 16 fresh-frozen upper limbs from 16 donors. Eight anterior forearm muscles were examined, yielding 118 matched muscle-donor observations. Preparation weight, muscle belly length and width, and independently penetrating terminal nerve branches were recorded. Associations were assessed using Spearman correlations and multivariable Poisson regression adjusted for muscle identity, with donor-clustered and generalized estimating equation sensitivity analyses. RESULTS: Penetrating branch counts were positively correlated with preparation weight (ρ=0.718), muscle belly length (ρ=0.554), and width (ρ=0.471; all P<0.001). In the conventional multivariable Poisson model, preparation weight was associated with branch count (incidence rate ratio per 1-standard-deviation increase, 1.207; 95% confidence interval, 1.033-1.410; P=0.018). However, this association was not statistically significant after donor-clustered adjustment (incidence rate ratio, 1.206; 95% confidence interval, 0.953-1.526; P=0.120) or in generalized estimating equation analysis (incidence rate ratio, 1.197; 95% confidence interval, 0.974-1.472; P=0.087). CONCLUSION: Gross muscle morphology was associated with the number of terminal nerve branches penetrating the epimysium in unadjusted analyses. The weight association observed in the conventional multivariable model was not robust to accounting for within-donor clustering. These findings are exploratory and do not establish causal relationships, motor endplate distribution, or clinical prediction of nerve entry points.