Growth dynamics of the triceps brachii muscle in the human fetus.

Background: The triceps brachii muscle, the strongest extensor of the elbow joint, is characterized by the three heads: long, lateral and medial.
Objectives: In the present study we aimed to examine the linear parameters (length, width) of the fetal triceps brachii muscle and to provide their growth dynamics.
Material and Methods: Using anatomical dissection, digital image analysis (Multiscan v.14.02), and statistics (Student's t-test, regression analysis) we measured in mm the length and width of the triceps brachii in 30 fetuses of both sexes (12♂,18♀) aged 12-29 weeks.
Results: Neither sex nor laterality differences were found. All the parameters studied increased proportionately with age. The linear functions were computed as follows: y = 6.797 + 2.079 x Age (r = 0.886) for length of the long head's belly, y = - 0.041 + 0.215 × Age (r = 0.786) for width of the long head's belly, y = 1.889 + 0.174 × Age (r = 0.796) for length of the long head's proximal tendon, y = 0.158 + 0.052 × Age (r = 0.864) for width of the long head's proximal tendon, y = 5.270 + 1.809 × Age (r = 0.855) for length of the lateral head's belly, y = 0.348 + 0.284 × A ge (r = 0.829) for width of the lateral head's belly, y = 0.942 + 1.837 × Age (r = 0.839) for length of the medial head's belly, y = 0.314 + 0.234 × Age (r = 0.852) for width of the medial head's belly, y = - 3.191 + 0.984 × Age (r = 0.929) for lenght of the common tendon, and y = - 0.478 + 0.133 × Age (r = 0.933) for width of the common tendon.
Conclusions: Neither male-female nor right-left differences are observed in morphometric parameters of the triceps brachii muscle. The long head's belly is the thinnest, while the lateral head's belly is the widest one. The long head is the longest and the medial head is the shortest one. The developmental dynamics of the triceps brachii muscle follow proportionately.