Muscle Architecture of the Rat Hindlimb

Abstract

The rat is a commonly-used experimental model, however, the architectural properties of its muscular system are poorly documented. PURPOSE: To define the architectural properties of the entire rat hindlimb and to compare the design characteristics among muscle groups. METHODS: Thirty-one muscles from each of six paired hindlimbs were dissected from adult male rats (323 g ± 15 g). Measurements of muscle mass, length, fiber length, sarcomere length, and pennation angle were measured directly and used to calculate normalized fiber length and physiological cross-sectional area (PCSA). Contralateral legs were used to quantify myosin heavy chain (MHC) isoform percentages. Muscles groups were classified according to their joint action. Average fiber length, total muscle group PCSA, and average MHC percentage were used to make among group comparisons. Mann-Whitney U-tests were used for statistical comparisons and reported values are mean ± SE. RESULTS: At the hip joint, extensors had significantly longer fiber lengths than flexors (33.0 mm ±2.2 mm vs. 17.6 mm ±2.2 mm) and larger total PCSAs (1.61 cm2 ± 0.05 cm2 vs. 1.38 cm2 ± 0.05 cm2). Flexors had a greater percentage of type IIa (10 % ± 1 % vs. 8 % ± 1 %) and IIx (25 % ± 2 % vs. 19 % ± 1 %) fibers while extensors had a greater percentage of type IIb (72 % ± 2 % vs. 63 %± 2 %) fibers. At the knee joint, flexors had longer fiber lengths than extensors (35.0 mm ±1.8 mm vs. 14.9 mm ±0.8 mm) and smaller total PCSAs (1.39 cm2 ± 0.04 cm2 vs. 1.69 cm2 ± 0.07 cm2). Knee extensors also had greater type I (9 % ± 2 % vs. 2 % ± 1 %), IIa (14 % ± 1 % vs. 8 % ± 1 %), and IIx (25 % ± 1 % vs. 18 % ± 1 %) percentages while flexors had a greater type IIb (72 % ± 2 % vs. 53 % ± 3 %) percentage. At the ankle joint, plantarflexors had a trend towards longer fiber lengths (16.3 mm ±0.8 mm vs. 13.9 mm ±0.6 mm, P = 0.064) and significantly greater total PCSA (1.45 cm2 ±0.11 cm2vs. 0.49 cm2 ±0.03 cm2). Plantarflexors also had a significantly greater type I percentage (24 % ± 67 % vs. 1 % ± 1%) while dorsiflexors had a significantly greater type IIb percentage (60 % ± 3 % vs. 42 % ± 5 %). CONCLUSIONS: Consistent with other their antigravity function, total extensor muscle group PCSA was larger compared to the total flexor muscle group PCSA at each joint. However, muscle fiber length differences were less distinct, with hip extensors, knee flexors, and ankle plantarflexors, tending to have longer fibers. MHC percentages were also less distinct, with hip extensors, knee flexors, and ankle dorsiflexors having the greater percentage of type IIb fibers. These data will form the foundation for understanding muscle group functional differences in the rat hindlimb.