P.A.J.B.M. Huijing

Effects of inter- and extramuscular myofascial force transmission on adjacent synergistic muscles: assessment by experiments and finite-element modeling

The effects of inter- and extramuscular myofascial force transmission on muscle length force characteristics were studied in rat. Connective tissues at the bellies of the experimental synergistic muscles of the anterior crural compartment were left intact. Extensor digitorium longus (EDL) muscle was lengthened distally whereas tibialis anterior (TA) and extensor hallucis longus (EHL) were kept at constant muscle-tendon complex length. Substantial differences were found in EDL force measured at the proximal and distal tendons (maximally 46% of the proximal force). EDL with intact inter- as well as extramuscular connections had an increased length range between active slack and optimum length compared to EDL with extramuscular connections exclusively: optimum muscle length was shifted by more than 2 mm. Distal EDL lengthening caused the distal force exerted by TA+EHL complex to decrease (approximately 17% of the initial force). This indicates increased intermuscular myofascial force transmission from TA+EHL muscle complex to EDL muscle. Finite-element modeling showed that: (1) Inter- and extramuscular myofascial force transmission leads to a substantial distribution of the lengths of the sarcomeres arranged in series within muscle fibers. Distribution of stress within the muscle fibers showed that the muscle fiber cannot be considered as a unit exerting equal forces at both ends. (2) Increased heterogeneity of mean fiber sarcomere lengths (i.e., a parallel distribution of length of sarcomeres among different muscle fibers) is found, particularly at high muscle lengths. This also explains the shift in muscle optimum length to higher lengths. It is concluded that inter- and extramuscular myofascial force transmission has substantial effects on muscle length-force characteristics.

Muscular force transmission necessitates a multilevel integrative approach to the analysis of function of skeletal muscle

HUIJING, P. A. Muscular force transmission necessitates a multilevel integrative approach to the analysis of function of skeletal muscle. Exerc. Sport Sci. Rev., Vol. 31, No. 4, pp. 167–175, 2003. Muscular force is transmitted not only to tendon but also to other structures. Connections to extramuscular connective tissue of a compartment and to other muscles are stiff enough to transmit force. The concept of myofascial force transmission is reviewed and some functional consequences considered. An approach for analysis is suggested.

The relative position of EDL muscle affects the length of sarcomeres within muscle fibers: experimental results and finite-element modeling.

BACKGROUNDnEffects of extramuscular connective tissues on muscle force (experimentally measured) and lengths of sarcomeres (modeled) were investigated in rat. It was hypothesized that changes of muscle-relative position affect the distribution of lengths of sarcomeres within muscle fibers.nnnMETHOD OF APPROACHnThe position of extensor digitorum longus muscle (EDL) relative to intact extramuscular connective tissues of the anterior crural compartment was manipulated without changing its muscle-tendon complex length.nnnRESULTSnSignificant effects of EDL muscle relative position on proximal and distal EDL forces were found, indicating changes of extramuscular myofascial force transmission. EDL isometric force exerted at its proximal and distal tendons differed significantly. Finite-element modeling showed that the distribution of lengths of sarcomeres is altered by changes of muscle-relative position.nnnCONCLUSIONSnIt is concluded that forces exerted on a muscle via extramuscular myofascial pathways augment distributions of lengths of sarcomeres within that muscle.

IMPLICATIONS OF MUSCLE RELATIVE POSITION AS A CO-DETERMINANT OF ISOMETRIC MUSCLE FORCE: A REVIEW AND SOME EXPERIMENTAL RESULTS

Force is transmitted from muscle fiber to bone via several pathways: (1) via the tendons (i.e. myotendinous force transmission), (2) via intermuscular connective tissue to adjacent muscles (i.e. intermuscular myofascial force transmission), (3) via structures other than muscles (i.e. extramuscular myofascial force transmission). In vivo, the position of a muscle relative to adjacent muscles changes due to differences in moment arm between synergists as well as due to the fact that some muscles span only one joint and other muscles more than one joint. The position of a muscle relative to non-muscular structures within a compartment is altered with each change of the length of the muscle. The aim of this article is to describe recent experimental results, as well as some new experimental data, that have elucidated the role of muscle relative position on force transmission from muscle. Furthermore, relevant literature is discussed, taking into consideration these new insights of muscle functioning. It is concluded that the position of a muscle relative to surrounding tissues is a major co-determinant of isometric muscle force. For muscles operating within their in vivo context of connective tissue, such position effects should be taken into account.

Low-frequency fatigue is fibre type related and most pronounced after eccentric activity in rat medial gastrocnemius muscle.

Effects of fibre type composition and type of contraction on low-frequency fatigue (LFF) were investigated in isolated rat medial gastrocnemius (GM) muscle. Fast oxidative or fast glycolytic GM muscle parts of anaesthetised male Wistar rats (n=18) were activated selectively by maximal electrical stimulation of the nerve after selective cutting of sub-branches. LFF was induced by a series of 40 isometric, concentric or eccentric contractions. Post exercise (55xa0min), the force–frequency curves differed significantly from the pre-exercise curves. Decreased forces were exerted mainly at the lower frequencies. This effect was significantly greater for glycolytic than oxidative muscle parts and following eccentric compared to isometric and concentric exercise. Seventy minutes following eccentric exercise, the relative values of the 60:200xa0Hz force ratios for the oxidative compared to the glycolytic parts were 65.6±2.2% and 43.6±4.6% (mean±SE) of the pre-fatigue values (=100%), respectively. In conclusion, for conditions of identical activation, eccentric exercise led to significantly more LFF than isometric and concentric exercise. In addition, and independent of the exercise type, fast glycolytic muscle parts were more susceptible to LFF than fast oxidative muscle parts.

Rat medial gastrocnemius muscles produce maximal power at a length lower than the isometric optimum length

The interaction of relative muscle length and force-velocity characteristics was investigated in the fully activated rat medial gastrocnemius muscle in situ. Average maximal isometric force (as a percentage of the of the maximal isometric force at Lo,iso) at relative lengths measured below isometric optimum (Lo,iso) was 96% at Lo,iso−2xa0mm, 88% at Lo,iso−4xa0mm and 58% at Lo,iso−6xa0mm. Force-velocity curves were obtained at the four relative muscle lengths. There were no significant differences in maximal shortening velocity (~280xa0mm.s-1) between the different muscle lengths. The highest power output (P<0.05) was found at Lo,iso−2xa0mm (mean±SEM 435±19xa0mW). Peak power values at Lo,iso (390±10xa0mW) and Lo,iso−4xa0mm (395±12xa0mW) were not significantly different, whereas peak power was lowest (P<0.05) at Lo,iso−6xa0mm. There was a significant (P<0.01) shift of ~1.5xa0mm in optimum muscle length for force generation during shortening contractions compared with isometric contractions. Shortening velocity had only a minor influence on optimum muscle length for force generation. It is concluded that fully activated muscles produce their maximal power at a length lower than Lo,iso. The difference in optimum length between isometric and dynamic contractions may be related to length-dependent variations in sarcomere length in series during shortening.

Force/velocity curves of fast oxidative and fast glycolytic parts of rat medial gastrocnemius muscle vary for concentric but not eccentric activity

The purpose of this study was to compare the force exerted by the rat medial gastrocnemius (GM) muscle with either fast oxidative or fast glycolytic parts active during concentric and eccentric contractions at different velocities. The proximal end of the GM contains mainly fast oxidative fibres and the distal end predominantly fast glycolytic fibres. Different parts of GM were activated by selective stimulation of nerve branches. Fast oxidative or fast glycolytic muscle parts of anaesthetised male Wistar rats were activated maximally. After assessment of concentric force/velocity (F/v) relations (n=11), some of the muscles were subjected to a fatiguing series of isometric contractions (n=5). Fast oxidative muscle parts showed a significantly lower mean (±SD) maximal power output (Pmax 0.12±0.06xa0W) and fatigability than fast glycolytic muscle parts (Pmax 0.20±0.06xa0W). The remaining muscles performed eccentric contractions. The eccentric F/v curves were not significantly different for fast oxidative and fast glycolytic muscle parts (n=6). Maximum eccentric force relative to the maximum isometric force (157±3% and 153±6% respectively, P=0.99) was reached at a velocity of 60xa0mm s−1. It is concluded that eccentric F/v relations of rat GM with either fast oxidative or fast glycolytic parts active are very similar despite the differences in the concentric F/v relations.