title: Synergistic Co-activation Increases the Extent of Mechanical Interaction between Rat Ankle Plantar-Flexors.
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 FSOto a larger extent (p= 0.017) during LG excitation (0.0026 N/mm) than during fully passive conditions (0.0009 N/mm). Changes in FSOin 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+PLto 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+PLin 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 FSOwere 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.
authors.affiliation: Department of Human Movement Sciences, Faculty of Behavioral and Movement Sciences, MOVE Research Institute Amsterdam, Vrije Universiteit AmsterdamAmsterdam, Netherlands; Department of Organismic and Evolutionary Biology, Harvard UniversityCambridge, MA, USA.
authors.forename: Chris
authors.initials: C
authors.lastname: Tijs
authors.affiliation: Department of Human Movement Sciences, Faculty of Behavioral and Movement Sciences, MOVE Research Institute Amsterdam, Vrije Universiteit Amsterdam Amsterdam, Netherlands.
authors.forename: Jaap H
authors.initials: JH
authors.lastname: van Dieën
authors.affiliation: Department of Human Movement Sciences, Faculty of Behavioral and Movement Sciences, MOVE Research Institute Amsterdam, Vrije Universiteit Amsterdam Amsterdam, Netherlands.
authors.forename: Guus C
authors.initials: GC
authors.lastname: Baan
authors.affiliation: Department of Human Movement Sciences, Faculty of Behavioral and Movement Sciences, MOVE Research Institute Amsterdam, Vrije Universiteit Amsterdam Amsterdam, Netherlands.
authors.forename: Huub
authors.initials: H
authors.lastname: Maas
keywords: Achilles tendon; co-activation; connective tissue; gastrocnemius; plantaris; soleus