Extramuscular myofascial force transmission alters substantially the acute effects of surgical aponeurotomy: assessment by finite element modeling

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 aponeurotomy: altered intramuscular myofascial force transmission yields complex sarcomere length distributions determining acute effects

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.     

Acute effects of intramuscular aponeurotomy on rat gastrocnemius medialis: force transmission, muscle force and sarcomere length

Acute effects of intramuscular aponeurotomy on muscle force and geometry as a function to muscle length were studied in rat m. gastrocnemius medialis (GM). Acutely after aponeurotomy, activation of the muscle at increasing lengths (acute trajectory) showed a spontaneous and progressive but patial tearing of the connective tissue interface between the fibres inserting directly proximally and distally to the location of the section. After this the muscle consisted morphologically of a stable proximal and a distal part (post-aponeurotomy). Post-aponeurotomy mean active sarcomere length within fibres of the proximal part was shown to be unaffected. In contrast, mean sarcomere length within the distal part was reduced substantially after aponeurotomy. However active sarcomeres in the distal part were still attaining higher lengths with increasing muscle lengths (p<0.005), indicating myofascial force transmission through the intact part of the connective tissue interface of the muscle parts. Post-aponeurotomy optimum muscle force was reduced substantially to less than 45% of pre-aponeurotomy values. During the acute trajectory the muscle yielded approximately 20% higher forces than post-aponeurotomy, indicating that myofascial force transmission was related to the area of connective tissue interface. It is concluded that after aponeurotomy of the proximal aponeurosis of rat GM, fibres without direct myotendinous connection to the origin of the muscle are still able to contribute to muscle force. As the magnitude of reduction in muscle force can only be explained partially by the spontaneous rupture of the connective tissue interface between proximal and distal muscle part, other factors causing a decrease of muscle force are present. Clinical implication of acute effects of intramuscular aponeurotomy are discussed.     

Mechanical principles of effects of botulinum toxin on muscle length–force characteristics: An assessment by finite element modeling

Recent experiments involving muscle force measurements over a range of muscle lengths show that effects of botulinum toxin (BTX) are complex e.g., force reduction varies as a function of muscle length. We hypothesized that altered conditions of sarcomeres within active parts of partially paralyzed muscle is responsible for this effect. Using finite element modeling, the aim was to test this hypothesis and to study principles of how partial activation as a consequence of BTX affects muscle mechanics. In order to model the paralyzing effect of BTX, only 50% of the fascicles (most proximal, or middle, or most distal) of the modeled muscle were activated. For all muscle lengths, a vast majority of sarcomeres of these BTX-cases were at higher lengths than identical sarcomeres of the BTX-free muscle. Due to such “longer sarcomere effect”, activated muscle parts show an enhanced potential of active force exertion (up to 14.5%). Therefore, a muscle force reduction originating exclusively from the paralyzed muscle fiber populations, is compromised by the changes of active sarcomeres leading to a smaller net force reduction. Moreover, such “compromise to force reduction” varies as a function of muscle length and is a key determinant of muscle length dependence of force reduction caused by BTX. Due to longer sarcomere effect, muscle optimum length tends to shift to a lower muscle length. Muscle fiber–extracellular matrix interactions occurring via their mutual connections along full peripheral fiber lengths (i.e., myofascial force transmission) are central to these effects. Our results may help improving our understanding of mechanisms of how the toxin secondarily affects the muscle mechanically.     

Interaction between series compliance and sarcomere kinetics determines internal sarcomere shortening during fixed-end contraction

The interaction between contractile force and in-series compliance was investigated for the intact skeletal muscle-tendon unit (MTU) of Rana pipiens semitendinosus muscles during fixed-end contraction. It was hypothesized that internal sarcomere shortening is a function of the length-force characteristics of contractile and series elastic components. The MTUs (n=18) were dissected, and, while submerged in Ringer's solution, muscles were activated at nine muscle lengths (-2 to +6 mm relative to optimal length in 1 mm intervals), while measuring muscle force and sarcomere length (SL) by laser diffraction. The MTU was clamped either at the bone (n=6), or at the proximal and distal ends of the aponeuroses (n=6). Muscle fibers were also trimmed along with aponeuroses down to 5-20 fibers and identical measurements were performed (n=6). The magnitude of shortening decreased as MTU length increased. The magnitude of shortening ranged from -0.08 to 0.3 microm, and there was no significant difference between delta SL as a function of clamp location. When aponeuroses were trimmed, sarcomere shortening was not observed at L(0) and longer. These results suggest that the aponeurosis is the major contributor to in-series compliance. Results also support our hypothesis but there also appear to be other factors affecting internal sarcomere shortening. The functional consequence of internal sarcomere shortening as a function of sarcomere length was to skew the muscle length-tension relationship to longer sarcomere lengths.     

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.     

Sarcomere length dependence of the force-velocity relation in single frog muscle fibers.

The force-velocity relation of single frog fibers was measured at sarcomere lengths of 2.15, 2.65, and 3.15 microns. Sarcomere length was obtained on-line with a system that measures the distance between two markers attached to the surface of the fiber, approximately 800 microns apart. Maximal shortening velocity, determined by extrapolating the Hill equation, was similar at the three sarcomere lengths: 6.5, 6.0, and 5.7 microns/s at sarcomere lengths of 2.15, 2.65, and 3.15 microns, respectively. For loads not close to zero the shortening velocity decreased with increasing sarcomere length. This was the case when force was expressed as a percentage of the maximal force at optimal fiber length or as a percentage of the sarcomere-isometric force at the respective sarcomere lengths. The force-velocity relation was discontinuous around zero velocity: load clamps above the level that kept sarcomeres isometric resulted in stretch that was much slower than when the load was decreased below isometric by a similar amount. We fitted the force-velocity relation for slow shortening (less than 600 nm/s) and for slow stretch (less than 200 nm/s) with linear regression lines. At a sarcomere length of 2.15 microns the slopes of these lines was 8.6 times higher for shortening than for stretch. At 2.65 and 3.15 microns the values were 21.8 and 14.1, respectively. At a sarcomere length of 2.15 microm, the velocity of stretch abruptly increased at loads that were 160-170% of the sarcomere isometric load, i.e., the muscle yielded. However, at a sarcomere length of 2.65 and 3.15 microm yield was absent at such loads. Even the highest loads tested (260%) resulted in only slow stretch.(ABSTRACT TRUNCATED AT 250 WORDS)     

Changes in thick filament length in Limulus striated muscle

Here we describe the change in thick filament length in striated muscle of Limulus, the horseshoe crab. Long thick filaments (4.0 microns) are isolated from living, unstimulated Limulus striated muscle while those isolated from either electrically or K+-stimulated fibers are significantly shorter (3.1 microns) (P less than 0.001). Filaments isolated from muscle glycerinated at long sarcomere lengths are long (4.4 microns) while those isolated from muscle glycerinated at short sarcomere lengths are short (2.9 microns) and the difference is significant (P less than 0.001). Thin filaments are 2.4 microns in length. The shortening of thick filaments is related to the wide range of sarcomere lengths exhibited by Limulus telson striated muscle.     

Overextended sarcomeres regain filament overlap following stretch

Sarcomere overextension has been widely implicated in stretch-induced muscle injury. Yet, sarcomere overextensions are typically inferred based on indirect evidence obtained in muscle and fibre preparations, where individual sarcomeres cannot be observed during dynamic contractions. Therefore, it remains unclear whether sarcomere overextensions are permanent following injury-inducing stretch-shortening cycles, and thus, if they can explain stretch-induced force loss. We tested the hypothesis that overextended sarcomeres can regain filament overlap in isolated myofibrils from rabbit psoas muscles. Maximally activated myofibrils (n=13) were stretched from an average sarcomere length of 2.6±0.04μm by 0.9μm sarcomere(-1) at a speed of 0.1μm sarcomere(-1)s(-1) and immediately returned to the starting lengths at the same speed (sarcomere strain=34.1±2.3%). Myofibrils were then allowed to contract isometrically at the starting lengths (2.6μm) for ～30s before relaxing. Force and individual sarcomere lengths were measured continuously. Out of the 182 sarcomeres, 35 sarcomeres were overextended at the peak of stretch, out of which 26 regained filament overlap in the shortening phase while 9 (～5%) remained overextended. About 35% of the sarcomeres with initial lengths on the descending limb of the force-length relationship and ～2% of the sarcomeres with shorter initial lengths were overextended. These findings provide first ever direct evidence that overextended sarcomeres can regain filament overlap in the shortening phase following stretch, and that the likelihood of overextension is higher for sarcomeres residing initially on the descending limb.     

Residual force enhancement in myofibrils and sarcomeres

Residual force enhancement has been observed following active stretch of skeletal muscles and single fibres. However, there has been intense debate whether force enhancement is a sarcomeric property, or is associated with sarcomere length instability and the associated development of non-uniformities. Here, we studied force enhancement for the first time in isolated myofibrils (n=18) that, owing to the strict in series arrangement, allowed for evaluation of this property in individual sarcomeres (n=79). We found consistent force enhancement following stretch in all myofibrils and each sarcomere, and forces in the enhanced state typically exceeded the isometric forces on the plateau of the force-length relationship. Measurements were made on the plateau and the descending limb of the force-length relationship and revealed gross sarcomere length non-uniformities prior to and following active myofibril stretching, but in contrast to previous accounts, revealed that sarcomere lengths were perfectly stable under these experimental conditions. We conclude that force enhancement is a sarcomeric property that does not depend on sarcomere length instability, that force enhancement varies greatly for different sarcomeres within the same myofibril and that sarcomeres with vastly different amounts of actin-myosin overlap produce the same isometric steady-state forces. This last finding was not explained by differences in the amount of contractile proteins within sarcomeres, vastly different passive properties of individual sarcomeres or (half-) sarcomere length instabilities, suggesting that the basic mechanical properties of muscles, such as force enhancement, force depression and creep, which have traditionally been associated with sarcomere instabilities and the corresponding dynamic redistribution of sarcomere lengths, are not caused by such instabilities, but rather seem to be inherent properties of the mechanisms of contraction.     

History-dependent properties of skeletal muscle myofibrils contracting along the ascending limb of the force–length relationship

There is a history dependence of skeletal muscle contraction: stretching activated muscles induces a long-lasting force enhancement, while shortening activated muscles induces a long-lasting force depression. These history-dependent properties cannot be explained by the current model of muscle contraction, and its mechanism is unknown. The purposes of this study were (i) to evaluate if force enhancement and force depression are present at short lengths (the ascending limb of the force–length (FL) relationship), (ii) to evaluate if the history-dependent properties are associated with sarcomere length (SL) non-uniformity and (iii) to determine the effects of cross-bridge (de)activation on force depression. Rabbit psoas myofibrils were isolated and attached between two microneedles for force measurements. Images of the myofibrils were projected onto a linear photodiode array for measurements of SL. Myofibrils were activated by either Ca²⁺ or MgADP; the latter induces cross-bridge attachment to actin independently of Ca²⁺. Activated myofibrils were subjected to three stretches or shortenings (approx. 4% SL at approx. 0.07 µm s⁻¹ sarcomere⁻¹) along the ascending limb of the FL relationship separated by periods (approx. 5 s) of isometric contraction. Force after stretch was higher than force after shortening at similar SLs. The differences in force could not be explained by SL non-uniformity. The FL relationship produced by Ca²⁺- and MgADP-activated myofibrils were similar in stretch experiments, but after shortening MgADP activation produced forces that were higher than Ca²⁺ activation. Since MgADP induces the formation of strongly bound cross-bridges, this result suggests that force depression following shortening is associated with cross-bridge deactivation.     

Sarcomere shortening in pressure overload hypertrophy

Sarcomere shortening during contraction was measured by using laser diffraction, in thin, rabbit right ventricular (RV) trabeculae from normal hearts (N) (n = 5) and from hearts subjected to RV pressure overload by pulmonary banding (H) (n = 5). Banding resulted in substantial RV hypertrophy after 2 wk. Hypertrophied preparations had the same resting muscle length (H = 3.15 +/- 0.29 mm) and resting sarcomere lengths (H = 2.16 +/- 0.005 micron) as the normal preparations (3.10 +/- 0.37 mm, 2.16 +/- 0.008 micron, respectively). Total tension at the peak of isometric twitches was the same as normal in the hypertrophied muscles (N = 8.06 +/- 1.20, H = 8.51 +/- 1.95 g/mm2). However, the amount of auxotonic sarcomere shortening was much less than normal in the hypertrophied preparations (N = 0.39 +/- 0.028, H = 0.19 +/- 0.034 micron; P less than 0.001). In isotonic contractions in which the ratio of muscle shortening to resting muscle length was the same in both the normal and hypertrophied muscles (ratio of 0.05 in both groups), the extent of sarcomere shortening relative to resting sarcomere length was less in the hypertrophied muscles than in the normal preparations (N = 0.14 +/- 0.01), H = 0.07 +/- 0.01; P less than 0.01). Series elasticity was the same as normal in the hypertrophied muscle P less than 0.05). Less auxotonic sarcomere shortening for a given level of isometric tension development and less isotonic sarcomere shortening per unit muscle shortening indicate that there is less than normal work per sarcomere during contraction in hypertrophied myocardium. These findings may have important implications for intracellular compensatory adaptation in pressure overload cardiac hypertrophy.     

Sarcomere length organization as a design for cooperative function amongst all lumbar spine muscles

The functional design of spine muscles in part dictates their role in moving, loading, and stabilizing the lumbar spine. There have been numerous studies that have examined the isolated properties of these individual muscles. Understanding how these muscles interact and work together, necessary for the prediction of muscle function, spine loading, and stability, is lacking. The objective of this study was to measure sarcomere lengths of lumbar muscles in a neutral cadaveric position and predict the sarcomere operating ranges of these muscles throughout full ranges of spine movements. Sarcomere lengths of seven lumbar muscles in each of seven cadaveric donors were measured using laser diffraction. Using published anatomical coordinate data, superior muscle attachment sites were rotated about each intervertebral joint and the total change in muscle length was used to predict sarcomere length operating ranges. The extensor muscles had short sarcomere lengths in a neutral spine posture and there were no statistically significant differences between extensor muscles. The quadratus lumborum was the only muscle with sarcomere lengths that were optimal for force production in a neutral spine position, and the psoas muscles had the longest lengths in this position. During modeled flexion the extensor, quadratus lumborum, and intertransversarii muscles lengthened so that all muscles operated in the approximate same location on the descending limb of the force-length relationship. The intrinsic properties of lumbar muscles are designed to complement each other. The extensor muscles are all designed to produce maximum force in a mid-flexed posture, and all muscles are designed to operate at similar locations of the force-length relationship at full spine flexion.     

Compartmental fasciotomy and isolating a muscle from neighboring muscles interfere with myofascial force transmission within the rat anterior crural compartment

Muscles within the anterior crural compartment (extensor digitorum longus, EDL; tibialis anterior, TA; and extensor hallucis longus, EHL) and within the peroneal compartment were excited simultaneously and maximally. All muscles were kept at constant length with the exception of EDL, for which muscle length was changed by moving its proximal tendon. Active and passive force was measured at proximal as well as distal EDL tendons and at the combined distal tendons of TA and EHL (TA+EHL). In the initial experimental condition, a difference (F(proximal) > F(distal)) in EDL force, amounting to 0-14% of proximal force, was confirmed for most EDL lengths. This is interpreted as a clear proof of extramuscular myofascial force transmission, as no significant EDL length effects could be shown on TA+EHL force. Repeated measurements were confirmed to cause marked changes of both proximal and distal length-force characteristics, such as a shift of the whole ascending limb of the active curve, including optimum length, to higher lengths without decreasing optimum force, and decreasing active force at low lengths (by approximately 57%). Repeated measurements also lowered proximal and distal EDL passive force (by up to 35%). The proximo-distal difference in passive as well as active EDL force was decreased, but persisted. At most lengths, this difference for active force amounted to a constant fraction (14%) of proximal force. TA+EHL force was not affected significantly. Subsequently, acute effects of experimental surgical alterations were studied: The first manipulation was full lateral fasciotomy of the anterior crural compartment that caused a further decrease in active force at the proximal EDL but not at the distal EDL tendon. Passive forces showed no further significant changes. The proximo-distal EDL active force difference decreased to 0-5% of proximal force. After fasciotomy, TA+EHL force increased by 30%. This was interpreted as evidence of increased intramuscular and decreased extramuscular myofascial force transmission. The second manipulation was full isolation of EDL from TA+EHL, but not from extramuscular connective tissues, which caused a further decrease of the EDL proximo-distal force differences, indicating a stiffening effect of the presence of TA+EHL on the extramuscular matrix. For EDL active force the difference was no longer significantly different from zero. In contrast, for EDL passive force the proximo-distal force difference persisted. It is concluded that extramuscular myofascial force transmission is an important feature of the anterior crural compartment. The magnitude of this force transmission requires that it be considered in analysis of muscular function.     

Polarized light observations on striated muscle contraction in a mite

Contraction of individual sarcomeres within the living mite Tarsonemus sp. was observed by polarized light microscopy. In unflattened animals the usual range of contraction was such that the minimum sarcomere length approximated the length of the A region, and the maximum sarcomere length was about twice the length of the A region. The central sarcomeres of the dorsal metapodosomal muscles were observed in detail. The A band length increased slightly with increasing sarcomere length since the regression of I region length on sarcomere length had an average slope of 0.91. When the A band length in a sarcomere which was shortening was compared with the length when the same sarcomere lengthened, no significant difference was seen. The A band of each sarcomere seemed to act as a not too rigid limit to further shortening; this agreed with the reversible shortening of a muscle in which the A band had been experimentally shortened. An H region was visible at long sarcomere lengths and was not visible at short sarcomere lengths, even when the muscle was actively shortening. The rate of change of H region length with sarcomere length suggested that I filament length may increase as sarcomere length increases. Despite this effect and the small increase in A length with sarcomere length, the results are considered to be consistent with a model in which shortening occurs by the relative movement of A and I filaments, with little or no change in length of either set of filaments. Sarcomere shortening was clearly associated with an increase in the retardation of the A region.     

Substantial effects of epimuscular myofascial force transmission on muscular mechanics have major implications on spastic muscle and remedial surgery

The specific aim of this paper is to review the effects of epimuscular myofascial force transmission on muscular mechanics and present some new results on finite element modeling of non-isolated aponeurotomized muscle in order to discuss the dependency of mechanics of spastic muscle, as well as surgery for restoration of function on such force transmission.

The etiology of the effects of spasticity on muscular mechanics is not fully understood. Clinically, such effects feature typically a limited joint range of motion, which at the muscle level must originate from altered muscle length–force characteristics, in particular a limited muscle length range of force exertion. In studies performed to understand what is different in spastic muscle and what the effects of remedial surgery are, muscle is considered as being independent of its surroundings. Conceivably, this is because the classical approach in muscle mechanics is built on experimenting with dissected muscles. Certainly, such approach allowed improving our understanding of fundamental muscle physiology yet it yielded implicitly a narrow point of view of considering muscle length–force characteristics as a fixed property of the muscle itself.

However, within its context of its intact connective tissue surroundings (the in vivo condition) muscle is not an isolated and independent entity. Instead, collagenous linkages between epimysia of adjacent muscles provide direct intermuscular connections, and structures such as the neurovascular tracts provide indirect intermuscular connections. Moreover, compartmental boundaries (e.g., intermuscular septa, interosseal membranes, periost and compartmental fascia) are continuous with neurovascular tracts and connect muscular and non-muscular tissues at several locations additional to the tendon origins and insertions. Epimuscular myofascial force transmission occurring via this integral system of connections has major effects on muscular mechanics including substantial proximo-distal force differences, sizable changes in the determinants of muscle length–force characteristics (e.g. a condition dependent shift in muscle optimum length to a different length or variable muscle optimal force) explained by major serial and parallel distributions of sarcomere lengths. Therefore, due to epimuscular myofascial force transmission, muscle length–force characteristics are variable and muscle length range of force exertion cannot be considered as a fixed property of the muscle.

The findings reviewed presently show that acutely, the mechanical mechanisms manipulated in remedial surgery are dominated by epimuscular myofascial force transmission. Conceivably, this is also true for the mechanism of adaptation during and after recovery from surgery. Moreover, stiffened epimuscular connections and therefore a stiffened integral system of intra- and epimuscular myofascial force transmission are indicated to affect the properties of spastic muscle. We suggest that important advancements in our present understanding of such properties, variability in the outcome of surgery and considerable recurrence of the impeded function after recovery cannot be made without taking into account the effects of epimuscular myofascial force transmission.     

Elastic Energy Storage and Radial Forces in the Myofilament Lattice Depend on Sarcomere Length

We most often consider muscle as a motor generating force in the direction of shortening, but less often consider its roles as a spring or a brake. Here we develop a fully three-dimensional spatially explicit model of muscle to isolate the locations of forces and energies that are difficult to separate experimentally. We show the strain energy in the thick and thin filaments is less than one third the strain energy in attached cross-bridges. This result suggests the cross-bridges act as springs, storing energy within muscle in addition to generating the force which powers muscle. Comparing model estimates of energy consumed to elastic energy stored, we show that the ratio of these two properties changes with sarcomere length. The model predicts storage of a greater fraction of energy at short sarcomere lengths, suggesting a mechanism by which muscle function shifts as force production declines, from motor to spring. Additionally, we investigate the force that muscle produces in the radial or transverse direction, orthogonal to the direction of shortening. We confirm prior experimental estimates that place radial forces on the same order of magnitude as axial forces, although we find that radial forces and axial forces vary differently with changes in sarcomere length.     

Force enhancement following an active stretch in skeletal muscle

When skeletal muscle is stretched during a tetanic contraction, the resulting force is greater than the purely isometric force obtained at the corresponding final length. Several mechanisms have been proposed to explain this phenomenon, but the most accepted mechanism is the sarcomere length non-uniformity theory. This theory is associated with the notion of instability of sarcomeres on the descending limb of the force–length relationship. However, recent evidence suggests that this theory cannot account solely for the stretch-induced force enhancement. Some of this evidence is presented in this paper, and a new mechanism for force enhancement is proposed: one that is associated with the engagement of a passive force during stretch. We speculate that this passive force enhancement may be caused by titin, a protein associated with passive force production at long sarcomere lengths.     

Architectural specialization of the intrinsic thoracic limb musculature of the American badger (Taxidea taxus)

Evaluation of the relationships between muscle structure and digging function in fossorial species is limited. Badgers and other fossorial specialists are expected to have massive forelimb muscles with long fascicles capable of substantial shortening for high power and applying high out‐force to the substrate. To explore this hypothesis, we quantified muscle architecture in the thoracic limb of the American badger (Taxidea taxus) and estimated the force, power, and joint torque of its intrinsic musculature in relation to the use of scratch‐digging behavior. Architectural properties measured were muscle mass, belly length, fascicle length, pennation angle, and physiological cross‐sectional area. Badgers possess hypertrophied shoulder flexors/humeral retractors, elbow extensors, and digital flexors. The triceps brachii is particularly massive and has long fascicles with little pennation, muscle architecture consistent with substantial shortening capability, and high power. A unique feature of badgers is that, in addition to elbow joint extension, two biarticular heads (long and medial) of the triceps are capable of applying high torques to the shoulder joint to facilitate retraction of the forelimb throughout the power stroke. The massive and complex digital flexors show relatively greater pennation and shorter fascicle lengths than the triceps brachii, as well as compartmentalization of muscle heads to accentuate both force production and range of shortening during flexion of the carpus and digits. Muscles of most functional groups exhibit some degree of specialization for high force production and are important for stabilizing the shoulder, elbow, and carpal joints against high limb forces generated during powerful digging motions. Overall, our findings support the hypothesis and indicate that forelimb muscle architecture is consistent with specializations for scratch‐digging. Quantified muscle properties in the American badger serve as a comparator to evaluate the range of diversity in muscle structure and contractile function that exists in mammals specialized for fossorial habits. J. Morphol. 2013. © 2012 Wiley Periodicals, Inc.     

Critical sarcomere extension required to recruit a decaying component of extra force during stretch in tetanic contractions of frog skeletal muscle fibers

29 single frog skeletal muscle fibers were stretched during fused tetanic contractions. The force increase during stretch exhibited a breakpoint at a critical length change (average: 16.6 nm per one-half sarcomere) that was independent of velocity of stretch and of sarcomere length between 1.8 and 2.8 microns. After stretch there was an early decaying force component with a force-extension curve similar to that during stretch, which disappeared over approximately 2 s. This component was removed by a small, quick release, leaving a longer- lasting component. The critical amplitude of release required to produce this result was found by clamping the fiber to a load at which there was zero velocity of shortening. This amplitude increased with time up to the angle in the force record during stretch, was constant for the remainder of the stretch, and decreased with time after the end of stretch; it was consistently less than the critical amplitude of stretch required to reach the breakpoint of force enhancement during stretch but was also independent of sarcomere length. The force drop accompanying the critical release showed a small increase up to an optimum magnitude at 2.4--2.7 microns sarcomere length, with a decrease at longer lengths.