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Effects of lengthening of the whole group of anterior crural muscles (tibialis anterior and extensor hallucis longus muscles (TA+EHL) and extensor digitorum longus (EDL)) on myofascial interaction between synergistic EDL and TA+EHL muscles, and on myofascial force transmission between anterior crural and antagonistic peroneal muscles, were investigated. All muscles were either passive or maximally active. Peroneal muscles were kept at a constant muscle tendon complex length. Either EDL or all anterior crural muscles were lengthened so that effects of lengthening of TA+EHL could be analyzed. For both lengthening conditions, a significant difference in proximally and distally measured EDL passive and active forces, indicative of epimuscular myofascial force transmission, was present. However, added lengthening of TA+EHL significantly affected the magnitude of the active and passive load exerted on EDL. For the active condition, the direction of the epimuscular load on EDL was affected; at all muscle lengths a proximally directed load was exerted on EDL, which decreased at higher muscle lengths. Lengthening of anterior crural muscles caused a 26% decrease in peroneal active force. Extramuscular myofascial connections are thought to be the major contributor to the EDL proximo-distal active force difference. For antagonistic peroneal complex, the added distal lengthening of a synergistic muscle increases the effects of extramuscular myofascial force transmission.

The purpose of the present study was to test the hypothesis that myofascial force transmission may not be limited by compartmental boundaries of a muscle group to synergists. Muscles of the anterior tibial compartment in rat hindlimb as well as of the neighbouring peroneal compartment (antagonistic muscles) were excited maximally. Length-force data, based on proximal lengthening, of EDL, as well as distal lengthening of the tibial muscles (TA+EHL) and the peroneal muscle group (PER) were collected independently, while keeping the other two muscle groups at a constant muscle-tendon complex length. Simultaneously measured, distal and proximal EDL active forces were found to differ significantly throughout the experiment. The magnitude of this difference and its sign was affected after proximal lengthening of EDL itself, but also of the tibial muscle complex and of the peroneal muscle complex. Proximal lengthening of EDL predominantly affected its synergistic muscles within the anterior crural compartment (force decrease <4%). Lengthening of either TA or PER caused a decrease in distal EDL isometric force (by 5-6% of initial force). It is concluded also that mechanisms for mechanical intermuscular interaction extend beyond the limits of muscle compartments in the rat hindlimb. Even antagonistic muscles should not be considered fully independent units of muscular function. Particular, strong mechanical interaction was found between antagonistic tibial anterior muscle and peroneal muscle complexes: Lengthening of the peroneal complex caused tibial complex force to decrease by approximately 25%, whereas for the reverse a 30% force decrease was found.

Details and concepts of intramuscular, extramuscular and intermuscular myofascial force transmission are reviewed. Some new experimental data are added regarding myofascial force transmission between antagonistic muscles across the interosseal membrane of the lower hind limb of the rat. Combined with other result presented in this issue, it can be concluded that myofascial force transmission occurs between all muscles within a limb segment. This means that force generated within sarcomeres of an antagonistic muscle may be exerted at the tendon of target muscle or its synergists. Some, in vivo, but initial indications for intersegmental myofascial force transmission are discussed. The concept of myofascial force transmission as an additional load on the muscle proved to be fruitful in the analysis of its muscular effects. In spastic paresis and for healthy muscles distal myofascial loads are often encountered, but cannot fully explain the movement limitations in spastic paresis. Therefore, the concept of simultaneous and opposing myofascial loads is analyzed and used to formulate a hypothesis for explaining the movement limitation: Myofascially transmitted antagonistic force is borne by the spastic muscle, but subsequently transmitted again to distal tendons of synergistic muscles.

The purpose of this study was to investigate the short-term history effect of a decreasing frequency train on force and the influence of joint angle on such effect in human dorsiflexor muscles. Six able-bodied and three spinal cord injured (SCI) subjects took part in the study. Their isometric left dorsiflexor muscles were stimulated with two-second bursts at three ankle joint positions and movements at the ankle were measured. Trains with constant stimulation frequencies (CSF: 50, 25, 20, 16, 12, 8 Hz) and with decreasing stimulation frequencies (DSF1,2) were used. Each DSF tetanus consisted of four 0.5 second bursts of different frequencies (DSF1: 50, 25, 16, 8 Hz; DSF2: 50, 20, 12, 8 Hz). To evaluate the effect of preceding higher stimulation frequencies (DSF), the average moment at corresponding time intervals in the DSF and CSF trials were compared for 25, 20, 16, 12, 8 Hz. Preceding higher stimulation frequencies caused increase of the moment elicited by a given frequency. This was true for all the subjects at dorsiflexed positions, but the effect is highly dependent on joint ankle. At plantar flexed positions moment enhancement was seen only in SCI subjects. We conclude that effects of joint angle as well as individual muscle properties should be taken into account when optimizing muscle force by means of frequency modulation.

The purpose of this paper is to review exemplary aspects of different views of skeletal muscle characteristics. A classical view of muscle characteristics plays a very important role in modelling of muscles and movement. However, it often also pervades concepts on which our understanding of muscle function is based. In this view length effects, velocity effects and effects of degrees of activation and recruitment are distinguished and, often implicitly, assumed to be independent effects. It will be illustrated that using the classical approach many valuable things may be learned about muscle function and adaptation. At the same time we should realize that such a classical approach is too limited for use in generating knowledge about properties of muscles during daily use. The use of scaling of force to estimate muscular properties during submaximal activity on the basis of properties during maximal activation is shown to be very inadequate. An alternative view is described and particular examples are provided of changes in length-force characteristics as a consequence of submaximal activation, previous length change, as well as the effect of short-term histories of these variables. In addition, effects of inhomogeneities of muscle in morphology as well as physiological properties are considered. It is concluded that length-velocity-force characteristics are not unique properties of a muscle, and that these characteristics are not only strongly influenced by actual effects of recruitment, firing frequency, shortening performed and actual velocity of shortening but also by the short time history of these factors. Therefore, length, velocity and activation cannot be considered as independent determinants of muscle functioning. It is also shown that we are confronted with many indications of physiological individuality regarding these phenomena.

Effects of stimulation frequency history on length-force characteristics were determined for rat medial gastrocnemius muscle (GM). The peripheral nerve was stimulated supramaximally according to two stimulation protocols. First, a complete set of length-force data were obtained by stimulating the nerve with a decreasing stimulation frequency (DSF) staircase composed of five successive pulse trains (200 ms each) of 100-, 50-, 40-, 30- and 15-Hz stimulation. Then length-force data were obtained using constant stimulation frequency (CSF; during the isometric contraction inter-stimulus interval was constant). The acquisition order of a complete set of length-force data was: first 15-Hz, then 30-, 40-, 50- and finally 100-Hz stimulation. For all DSF conditions, both optimum muscle length as well as active slack length were shifted significantly (P < 0.05) to lower muscle length with respect to CSF. Muscle length range between active slack and optimum length for all DSF conditions was increased significantly with respect to CSF. DSF thus caused a marked shift of the length-force relationship to lower muscle length compared to the CSF-dependent length-force relationship. As a result of this shift, muscle force enhancement (potentiation) was non-linearly related to muscle length; force enhancement decreased exponentially with increasing muscle length. In addition, DSF-dependent length-force characteristics are not scaled and shifted versions of those for CSF. Possible factors affecting these length-force characteristics are higher intracellular calcium concentration, myosin light chain phosphorylation, fatigue during sustained contractions, interaction between aponeurosis and fibre length, distribution of fibre mean sarcomere length with respect to muscle length, and muscle length changes during unfused tetanic contractions. It is concluded that length-force characteristics of rat GM are dependent on both short-term stimulation frequency history as well as stimulation frequency per se.

Length-force characteristics of maximally and submaximally stimulated rat medial gastrocnemius muscle were studied in relation to muscle geometry. Maximal stimulation was obtained by stimulating the nerve innervating the muscle with supramaximal current. Submaximal stimulation was obtained by using 600-Hz blocking stimuli in addition to the 100-Hz excitation stimuli. Geometry of the active muscle was recorded by photography. Length-force characteristics during progressively developing blocks showed muscle optimum lengths occurring at increasing lengths. A significant correlation (r = -0.9) was found between decrease of optimum force as a consequence of the block and the length shift for optimum length. For the length range in which optimum lengths were found muscle geometry could not be distinguished on the basis of stimulation protocol used. At shorter lengths, however, differences of geometry were encountered. It is concluded that any differences of fiber length and/or shortening velocity could not explain the shifts of optimum lengths of up to 14%. Two major factors for explaining the stimulation level-dependent length-force characteristics were indicated, but could not be distinguished unequivocally as yet: (a) The occurrence of a distribution of fiber optimum lengths with respect to muscle optimum length in such a way that fibers of large motor units reach their optimum at shorter muscle lengths and fibers of motor units of decreasing size reach their optimum length at progressively higher muscle lengths. (b) If the blocking technique involved slowing of firing of motor units before derecruitment, a dependence of optimum length on firing frequency could have played a major role in the stimulation level-dependent shifts of this length. Possible functional significance of these phenomena for in vivo performance of movement is discussed., (Copyright © 1992. Published by Elsevier Ltd.)