Background: Neuromuscular partitioning of a muscle is often related to muscle architecture. Detailed muscle architecture studies can be used to correlate innervation patterns. Knowledge of either innervation patterns or muscle architecture alone does not provide a comprehensive view of neuromuscular partitioning within a muscle. Although architectural parameters of the lateral (LG) and medial (MG) heads of gastrocnemius have been studied from small samples of muscle fiber bundles, the location of measured fiber bundles within the muscle volume has not been well documented. Detailed innervation has not been documented.

Purpose: To study the detailed muscle architecture and innervation of the medial and lateral heads gastrocnemius.

Methods: The lateral and medial heads of a cadaveric gastrocnemius specimen were serially dissected in situ. Fiber bundles (a total of 750 in both MG and LG) were pinned, digitized with a Microscribe 3D-X digitizer, and the data compiled with a B-spline modeling program (DANCE). 3-D fiber bundles were constructed and fiber length and angles of pennation were measured. Innervation is currently being studied using the same technology.

Results: On average, MG has shorter fiber bundles and larger pennation angles than LG. Within MG, the distal third has longer fibers and smaller angles of pennation than the proximal third. Distinct architectural regions are evident in LG; angle of pennation is relatively smaller in the lateral and deep medioproximal regions in comparison to the remainder of the medial side. Innervation to each of these architecturally distinct regions is currently being documented. Also the investigators in vivo, ultrasound studies of dystrophic muscle have noted that the MG is affected before the LG.

Conclusion: A detailed documentation of the muscle architecture and innervation pattern of the MG and LG may provide a better understanding of normal and pathological function of the muscle.