Terminal innervation ratios and fiber type grouping in normal porcine skeletal muscle.

Terminal innervation ratios were determined in normal porcine semiendinosus to test the hypothesis that intramuscular collateral branching of subterminal axons gives rise to fiber type grouping. Innervation ratios were near 1.00 in both the lateral pale portion which exhibits type II predominance and in the medial red portion which exhibits extensive grouping of type I fibers. Type grouping observed in porcine skeletal muscle is not the result of multi-fiber innervation by subterminal axons, but, rather, may be the manifestation of a unique motor unit topography.