Abstract
Force transmission between rat ankle plantar-flexors has been found for physiological muscle lengths and relative positions, but only with all muscles maximally activated. The aims of this study were to assess intermuscular mechanical interactions between ankle plantar-flexors during (i) fully passive conditions, (ii) excitation of soleus (SO), (iii) excitation of lateral gastrocnemius (LG), and (iv) during co-activation of SO, and LG (SO&LG). We assessed effects of proximal lengthening of LG and plantaris (PL) muscles (i.e., simulating knee extension) on forces exerted at the distal SO tendon (FSO) and on the force difference between the proximal and distal LG+PL tendons (ΔFLG+PL) of the rat. LG+PL lengthening increased FSO to a larger extent (p = 0.017) during LG excitation (0.0026 N/mm) than during fully passive conditions (0.0009 N/mm). Changes in FSO in response to LG+PL lengthening were lower (p = 0.002) during SO only excitation (0.0056 N/mm) than during SO&LG excitation (0.0101 N/mm). LG+PL lengthening changed ΔFLG+PL to a larger extent (p = 0.007) during SO excitation (0.0211 N/mm) than during fully passive conditions (0.0157 N/mm). In contrast, changes in ΔFLG+PL in response to LG+PL lengthening during LG excitation (0.0331 N/mm) were similar (p = 0.161) to that during SO&LG excitation (0.0370 N/mm). In all conditions, changes of FSO were lower than those of ΔFLG+PL. This indicates that muscle forces were transmitted not only between LG+PL and SO, but also between LG+PL and other surrounding structures. In addition, epimuscular myofascial force transmission between rat ankle plantar-flexors was enhanced by muscle activation. However, the magnitude of this interaction was limited.
Highlights
Numerous animal studies have reported clear evidence that muscle force can be transmitted to the skeleton via epimuscular myofascial connections (Huijing, 2009; Maas and Sandercock, 2010)
During SO&lateral gastrocnemius (LG) excitation, FSO increased from 1.38 ± 0.13 N at LREF−3 mm to 1.44 ± 0.12 N at LREF+3 mm (p = 0.001), which is a change of 4.3% relative to the force exerted at LREF. These results indicate that effects of LG+PL muscle-tendon unit (MTU) length changes on FSO were enhanced by synergistic muscle activation
Because passive forces are small at low LG+PL MTU lengths (e.g., LREF−3 mm: FLG+PL_PROX: 0.002 ± 0.001 N, FLG+PL_DIST: 0.019 ± 0.004 N), these results indicate that active force generated by SO muscle fibers was exerted at the distal LG+PL tendon
Summary
Numerous animal studies have reported clear evidence that muscle force can be transmitted to the skeleton via epimuscular myofascial connections (Huijing, 2009; Maas and Sandercock, 2010). Proximal lengthening of active lateral gastrocnemius (LG) and plantaris (PL) muscles (i.e., simulating knee extension) increased the force exerted at the distal tendon of the soleus (SO) muscle significantly (by 12%). This indicates that myofascial linkages can be mechanically relevant within physiological MTU lengths and relative positions. The MTU length of bi- and poly-articular muscles was changed at their origin, by imposing changes in knee joint angle, while the MTU length of the synergistic mono-articular ankle muscles was kept constant by testing at a constant ankle angle In those studies, the ankle moment exerted on excitation of the monoarticular muscle was not significantly affected when the length and relative position of passive synergistic muscles were changed by varying knee angle. Epimuscular myofascial force transmission per se could not be excluded
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