Key result
Varying knee joint angle to induce epimuscular myofascial force transmission did not change mean sarcomere length but significantly altered the length distribution of sarcomeres within passive TA.
Epimuscular myofascial force transmission alters the length distribution of sarcomeres within passive tibialis anterior muscles in rats, without changing mean sarcomere length.
Rat model supplies first direct measures of epimuscular effects on sarcomere lengths; leaves open translation to human muscle mechanics.
Results from imaging studies and finite element models suggest epimuscular myofascial effects on sarcomere lengths in series within muscle fibers. However, experimental evidence is lacking. We evaluated epimuscular myofascial effects on (1) muscle belly, fiber, and mean sarcomere length and (2) sarcomere length distribution within passive fibers of the rat tibialis anterior (TA) and soleus (SO) muscles. Hindlimbs (n = 24) were positioned in predefined knee (55°, 90°, 125°, 160°) and ankle (either 90° or 125°) angles, and fixed in a formaldehyde solution. Varying knee joint angle causes changes in muscle-tendon unit length of SO and TA's synergists, but not of SO and TA. Whole fibers were taken from SO and TA and photographed along their length. Mean sarcomere length was assessed for the entire fiber and for the proximal, intermediate, and distal thirds (fiber segments) separately. Mean sarcomere length of the fiber was not affected by knee angle, neither for SO (mean: 2.44 ± 0.03 μm and 2.19 ± 0.05 μm for ankle angles of 90° and 125°, respectively) nor for TA (mean: 2.33 ± 0.05 μm and 2.51 ± 0.07 μm for ankle angle set to 90° and 125°, respectively). Only for TA, a significant interaction between knee angle and fiber segment was found, indicating changes in the distribution of lengths of in-series sarcomeres. Thus, while epimuscular myofascial force transmission did not cause mean sarcomere length changes within passive SO and TA, it did alter the length distribution of sarcomeres within passive TA.
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Tijs et al. (2015) studied this question. Varying knee joint angle was evaluated on Mean sarcomere length and sarcomere length distribution within passive fibers. Varying knee joint angle to induce epimuscular myofascial force transmission did not change mean sarcomere length but significantly altered the length distribution of sarcomeres within passive TA.
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