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The aim of this study was to investigate effects of albumin and insulin separately as well as in combination on mature muscle fibres during long-term culture. Single muscle fibres were dissected from m. iliofibularis of Xenopus laevis and attached to a force transducer in a culture chamber. Fibres were cultured in a serum-free medium at slack length (mean sarcomere length 2.3 mum) for 8 to 22 days. The medium was supplemented with (final concentrations): (1) bovine insulin (6 nmol/L or 200-600 nmol/L), (2) 0.2% bovine albumin or (3) 0.2% bovine albumin in combination with insulin (120 nmol/L). In culture medium with insulin, 50% of the muscle fibres became in-excitable within 7-12 days, whereas the other 50% were stable. Caffeine contractures of in-excitable muscle fibres produced 80.4 +/- 2.4% of initial peak tetanic force, indicating impaired excitation-contraction (E-C) coupling in in-excitable fibres. In the presence of albumin, all cultured muscle fibres were stable for at least 10 days. Muscle fibres cultured in medium with insulin or albumin exclusively did not hypertrophy or change the number of sarcomeres in series. In contrast, muscle fibres cultured with both albumin and insulin showed an increase in tetanic force and fibre cross-sectional area of 19.6 +/- 2.8% and 32.5 +/- 4.9%, respectively, (means +/- SEM.; P = 0.007) after 16.3 +/- 1.7 days, whereas the number of sarcomeres in series remained unchanged. We conclude that albumin prevents muscle fibre damage and preserves E-C coupling in culture. Furthermore, albumin is important in regulating muscle fibre adaptation by a synergistic action with growth factors like insulin.

Effects of extramuscular myofascial force transmission on the acute effects of aponeurotomy were studied using finite element modeling and implications of such effects on surgery were discussed. Aponeurotomized EDL muscle of the rat was modeled in two conditions: (1) fully isolated (2) with intact extramuscular connections. The specific goal was to assess the alterations in muscle length-force characteristics in relation to sarcomere length distributions and to investigate how the mechanical mechanism of the intervention is affected if the muscle is not isolated. Major effects of extramuscular myofascial force transmission were shown on muscle length-force characteristics. In contrast to the identical proximal and distal forces of the aponeurotomized isolated muscle, substantial proximo-distal force differences were shown for aponeurotomized muscle with extramuscular connections (for all muscle lengths F (dist) > F (prox) after distal muscle lengthening). Proximal optimal length did not change whereas distal optimal length was lower (by 0.5 mm). The optimal forces of the aponeurotomized muscle with extramuscular connections exerted at both proximal and distal tendons were lower than that of isolated muscle (by 15 and 7%, respectively). The length of the gap separating the two cut ends of the intervened aponeurosis decreases substantially due to extramuscular myofascial force transmission. The amplitude of the difference in gap length was muscle length dependent (maximally 11.6% of the gap length of the extramuscularly connected muscle). Extramuscular myofascial force transmission has substantial effects on distributions of lengths of sarcomeres within the muscle fiber populations distal and proximal to the location of intervention: (a) Within the distal population, the substantial sarcomere shortening at the proximal ends of muscle fibers due to the intervention remained unaffected however, extramuscular myofascial force transmission caused a more pronounced serial distribution towards the distal ends of muscle fibers. (b) In contrast, extramuscular myofascial force transmission limits the serial distribution of sarcomere lengths shown for the aponeurotomized isolated muscle in the proximal population. Fiber stress distributions showed that extramuscular myofascial force transmission causes most sarcomeres within the aponeurotomized muscle to attain lengths favorable for higher force exertion. It is concluded that acute effects of aponeurotomy on muscular mechanics are affected greatly by extramuscular myofascial force transmission. Such effects have important implications for the outcome of surgery performed to improve impeded function since muscle in vivo is not isolated both anatomically and mechanically.

Finite element modeling of aponeurotomized rat extensor digitorium longus muscle was performed to investigate the acute effects of proximal aponeurotomy. The specific goal was to assess the changes in lengths of sarcomeres within aponeurotomized muscle and to explain how the intervention leads to alterations in muscle length-force characteristics. Major changes in muscle length-active force characteristics were shown for the aponeurotomized muscle modeled with (1) only a discontinuity in the proximal aponeurosis and (2) with additional discontinuities of the muscles' extracellular matrix (i.e., when both myotendinous and myofascial force transmission mechanisms are interfered with). After muscle lengthening, two cut ends of the aponeurosis were separated by a gap. After intervention (1), only active slack length increased (by approximately 0.9 mm) and limited reductions in muscle active force were found (e.g., muscle optimum force decreased by only 1%) After intervention (2) active slack increased further (by 1.2 mm) and optimum length as well (by 2.0 mm) shifted and the range between these lengths increased. In addition, muscle active force was reduced substantially (e.g., muscle optimum force decreased by 21%). The modeled tearing of the intramuscular connective tissue divides the muscle into a proximal and a distal population of muscle fibers. The altered force transmission was shown to lead to major sarcomere length distributions [not encountered in the intact muscle and after intervention (1)], with contrasting effects for the two muscle fiber populations: (a) Within the distal population (i.e. fibers with no myotendinous connection to the muscles' origin), sarcomeres were much shorter than within the proximal population (fibers with intact myotendinous junction at both ends). (b) Within the distal population, from proximal ends of muscle fibers to distal ends, the serial distribution of sarcomere lengths ranged from the lowest length to high lengths. In contrast within the proximal population, the direction of the distribution was reversed. Such differences in distribution of sarcomere lengths between the proximal and distal fiber populations explain the shifts in muscle active slack and optimal lengths. Muscle force reduction after intervention (2) is explained primarily by the short sarcomeres within the distal population. However, fiber stress distributions showed contribution of the majority of the sarcomeres to muscle force: myofascial force transmission prevents the sarcomeres from shortening to nonphysiological lengths. It is concluded that interfering with the intramuscular myofascial force transmission due to rupturing of the intramuscular connective tissue leads to a complex distribution of sarcomere lengths within the aponeurotomized muscle and this determines the acute effects of the intervention on muscle length-force characteristics rather than the intervention with the myotendinous force transmission after which the intervention was named. These results suggest that during surgery, but also postoperatively, major attention should be focused on the length and activity of aponeurotomized muscle, as changes in connective tissue tear depth will affect the acute effects of the intervention.

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 (55 min), 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 60:200 Hz 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.

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 (P(max) 0.12+/-0.06 W) and fatigability than fast glycolytic muscle parts (P(max) 0.20+/-0.06 W). 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 60 mm 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.

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 L(o,iso)) at relative lengths measured below isometric optimum (L(o,iso)) was 96% at L(o,iso)-2 mm, 88% at L(o,iso)-4 mm and 58% at L(o,iso)-6 mm. Force-velocity curves were obtained at the four relative muscle lengths. There were no significant differences in maximal shortening velocity (approximately 280 mm x s(-1)) between the different muscle lengths. The highest power output (P<0.05) was found at L(o,iso)-2 mm (mean+/-SEM 435+/-19 mW). Peak power values at L(o,iso) (390+/-10 mW) and L(o,iso)-4 mm (395+/-12 mW) were not significantly different, whereas peak power was lowest (P<0.05) at L(o,iso)-6 mm. There was a significant (P<0.01) shift of approximately 1.5 mm 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 L(o,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.

The functions and geometrical characteristics of a pennate and a parallel-fibered muscle in rats are quantified and compared to each other. The pennate medial gastrocnemius and the more parallel fibered semimembranosus are investigated in fourteen male Wistar rats. The length force diagram, twitch time characteristics and muscle architecture are quantified. In the parallel fibered semimembranosus the length of the muscle fibres is about 70% of the muscle optimum length (the length at which the muscle performs maximal active force), while in the pennate fibered gastrocnemius the length of the muscle fibres is 36% of the muscle optimum length. The active length force diagrams normalized with respect to the muscle optimum length show considerable differences between semimembranosus and gastrocnemius: The normalized active length force diagram runs from about 71%-129% of the muscle optimum length for the semimembranosus and from about 82-118% for the gastrocnemius. The latter muscle also has a steeper normalized passive length force curve and produces more active tetanic force per gram muscle (877 g/g for gastrocnemius versus 379 g/g for semimembranosus). No differences between the semimembranosus and gastrocnemius are found with respect to the passive tension at twitch optimum length (38 g/cm2 versus 32 g/cm2), the maximal active tension (1.27 kg/cm2 versus 1.18 kg/cm2) and the twitch contraction time at twitch optimum length (43.8 ms versus 48.4 ms). It is concluded that several functional characteristics show a linear relationship with the index of architecture, a measure for the muscle architecture, while others (especially time and tension characteristics) are independent of muscle architecture.

The length dependence of twitch time characteristics is quantified for several skeletal muscles of the rat: lateral gastrocnemius, medial gastrocnemius, plantaris, soleus and semimembranosus. It is shown that muscle architecture influences the length dependent behaviour of twitch time characteristics of muscles. Twitch contraction time is less susceptible to length changes of the muscles than the twitch relaxation time. With the exception of the relaxation time in the second part of the relaxation, the twitch time characteristics behave different with respect to their dependence on fibre length, in fact muscles that differ in architecture. It is concluded that twitch time characteristics are dependent on the actual muscle length and therefore should be determined at a well defined length.

For rat medial gastrocnemius muscle economy (i.e. the ratio of time integral of force and total energy-rich phosphate consumption) was calculated. Muscles in situ at 35 degrees C were stimulated to perform either one continuous or several repetitive isometric contractions at one muscle length in the range from 70% to 130% of optimum muscle length for force generation. Whereas during one continuous contraction economy increased, no differences in economy were found between 6, 12 or 18 successive contractions. Economy during intermittent exercise was always lower than during continuous exercise. The difference in economy is a result of different rates of metabolism, whereas no difference was found for force generation. Economy was highest at optimum muscle length for force generation and decreased at muscle lengths smaller as well as greater than optimum muscle length. Force-dependent energy consumption was calculated by subtracting the force-independent part (obtained by extrapolation) from total energy consumption. The calculated force produced per mumol force-dependent energy-rich phosphate consumption was similar in muscles stretched beyond optimum length. In contrast, a decreasing amount of force per mumol force-dependent energy-rich phosphate consumption was observed at lengths smaller than optimum length.

The dynamic passive response of the left gastrocnemius medialis muscle of thirty male Wistar rats was studied as a function of muscle dimensions and absolute and relative amount of connective tissue. Values of the absolute active and passive length-force curves (active force, passive force, active working range) correlated well (coefficients of correlation in a range of 0.62-0.92) with morphological variables (such as muscle optimum length, mean muscle fibre optimum length, physiological cross section, muscle weight and amount of intramuscular connective tissue). To eliminate dimensional effects the active and passive length-force curves were normalized taking maximal active twitch force and muscle optimum length as reference values (100%). The width of the normalized active length-force curve (relative active working range) was correlated negatively with muscle weight, muscle optimum length and physiological cross section. Relative amount of connective tissue and passive tension at optimum length (both independent of muscle dimensions) were positively correlated, indicating that passive muscles are stiffer when relative amount of intramuscular connective tissue is higher. Sinusoidal movements with several amplitudes and frequencies of movement were imposed on the passive gastrocnemius medialis muscle over a range of muscle lengths. In accordance with the approximately exponential increase of static passive muscle force with length, muscle length has a large influence on the shape and magnitude of the hysteresis diagrams resulting from sinusoidal movements: the value of all variables selected increases approximately exponentially with muscle length with the exception of the value of loss tangent, a factor indicating the amount of energy dissipated during each cycle relative to the amount of energy stored and released elastically. Velocity of movement has only minor influence on variables of the hysteresis diagrams as is shown by changing the frequency of movement. As loss tangent and relative amount of connective tissue did not vary with muscle dimensions in the muscles studied, it is likely that material properties of the components causing passive resistance were similar in these muscles.