The normal growth of the biceps brachii muscle in human fetuses.

BACKGROUND The biceps brachii muscle, the strongest flexor and supinator at the elbow joint, and an accessory flexor of the glenohumeral joint is characterized by the two heads, long and short. OBJECTIVES The purpose of this study was to examine the linear parameters (length and width) of the biceps brachii in human fetuses and to provide their mathematical growth models. MATERIAL AND METHODS Using methods of anatomical dissection, digital analysis (Multiscan v.14.02), and statistics (Student's t-test and regression analysis) the authors bilaterally measured the length (mm) and width (mm) of the biceps brachii muscle in 30 fetuses of both sexes (14♂,16♀), aged 17-30 weeks. RESULTS Neither sex nor laterality differences were found. All the parameters studied increased proportionally with age. Both the mean length (5.68 mm) and widths, measured at its mid-length (0.60 mm) and at the widest level (0.65 mm) of the long head's belly, were found to be statistically shorter (5.93 mm, 0.65 mm and 0.72 mm, respectively) when compared to its short head's belly. For these parameters, the following linear models were generated in relation to the long head's belly: y = -0.801 + 0.276 x Age (R2 = 0.591), y = -0.254 + 0.036 x Age (R2 = 0.201) and y = -0.238 + 0.038 x Age (R2 = 0.226), and in relation to the short head's belly: y = -0.134 + 0.258 x Age (R2 = 0.551), y = -0.227 + 0.038 x Age (R2 = 0.241) and y = -0.316 + 0.044 x Age (R2 = 0.333). The tendon length turned out to be the only significantly greater value for the long head (1.89 mm vs. 1.09 mm). The following linear models y = 1.024 + 0.037 x Age (R2 = 0.084) for the long and y = 0.177 + 0.039 x Age (R2 = 0.157) for the short heads were computed. CONCLUSIONS Neither sex differences nor laterality differences are observed in morphometric parameters of the biceps brachii muscle. The long head's belly is shorter and thinner than that of the short head's belly. The long head's tendon is longer than that of the short head. The developmental dynamics of the biceps brachii muscle follow proportionately.