Longitudinal impedance of single frog muscle fibers

The longitudinal impedance of single skeletal muscle fibers has been measured from1 to 10,000 Hz in an oil gap apparatus which forces current to flow longitudinally down the fiber. The impedance observed is purely resistive in some fibers from the semitendinosus muscle and in two fibers from the sartorius muscle. In other fibers from the semitendinosus muscle a small phase shift is observed. The mean value of the maximum phase shift observed from all fibers is 1.07 degrees. The artifacts associated with the apparatus and method are examined theoretically and it is shown that one of the likely artifacts could account for the small phase observed. It is concluded that the longitudinal impedance of skeletal muscle fibers is essentially resistive and that little, if any, longitudinal current crosses the membranes of the sarcoplasmic reticulum.     

The effect of calcium activation of skinned fiber bundles on the structure of Limulus thick filaments

Here we present evidence that strongly suggests that the well-documented phenomenon of A-band shortening in Limulus telson muscle is activation dependent and reflects fragmentation of thick filaments at their ends. Calcium activation of detergent-skinned fiber bundles of Limulus telson muscle results in large decreases in A-band (from 5.1 to 3.3 microns) and thick filament (from 4.1 to 3.3 microns) lengths and the release of filament end fragments. In activated fibers, maintained stretched beyond overlap of thick and thin filaments, these end fragments are translocated to varying depths within the I-bands. Here they are closely associated with fine filamentous structures that also span the gap between A- and I-bands and attach to the distal one-third of the thick filaments. End-fragments are rarely, if ever, present in similarly stretched and skinned, but unstimulated fibers, although fine "gap filaments" persist. Negatively stained thick filaments, separated from skinned, calcium-activated, fiber bundles, allowed to shorten freely, are significantly shorter than those obtained from unstimulated fibers, but are identical to the latter with respect to both the surface helical array of myosin heads and diameters. Many end-fragments are present on grids containing thick filaments from activated fibers; few, if any, on those from unstimulated fibers. SDS-PAGE shows no evidence of proteolysis due to activation and demonstrates the presence of polypeptides with very high molecular weights in the preparations. We suggest that thick filament shortening is a direct result of activation in Limulus telson muscle and that it occurs largely by breakage within a defined distal region of each polar half of the filament. It is possible that at least some of the fine "gap filaments" are composed of a titin-like protein. They may move the activation-produced, fragmented ends of thick filaments to which they attach, into the I-bands by elastic recoil, in highly stretched fibers.     

Stretch and radial compression studies on relaxed skinned muscle fibers of the frog.

The influence of stretch and radial compression on the width of mechanically skinned fibers from the semitendinosus muscle of the frog (R. pipiens) was examined in relaxing solutions with high-power light microscopy. Fibers were skinned under mineral oil. We find that, after correcting for water uptake in the oil, fiber width increased by an average of 28% upon transfer from oil to relaxing medium, with some tendency for greater swelling at longer sarcomere lengths. Subsequently, fibers were compressed by addition of the long-chain polymer polyvinylpyrrolidone (PVP-40, number average molecular weight 40,000) to relaxing solutions. Sarcomere length does not appear to be affected by addition of PVP. At any PVP concentration, the inverse square of the fiber width increased smoothly and linearly with increasing stretch for sarcomere lengths between 2.10 and 4.60 micrometer. At any fixed sarcomere length, fiber width decreased linearly with the logarithm of the osmotic compressive pressure exerted by PVP (2-10% concentration). From this logarithmic relation we estimate that the swelling pressure of the intact fiber is 3.40 x 10(3) N/m2, between that of a 2 and a 3% PVP solution. The pressure giving rise to fiber swelling is not due to dilation of the sarcoplasmic reticulum (SR), since the experimental results above were not significantly different after treatment with 0.5% BRIJ-58, a nonionic detergent that disrupts the SR. Swelling may be due simply to elastic structures within the fiber that are constrained in the intact cell. Values of bulk moduli of fibers, calculated from the compression experiments, and preliminary measurements of Young's modulus from stretch experiments, are quantitatively consistent with the idea that skinned fibers behave as nonisotropic elastic bodies.     

The effect of sarcomere non-uniformity on the sarcomere length-tension relationship of skinned fibers

It has proved difficult to activate skinned muscle fibers to produce high tension (3 kg/cm² level) without loss of clear striations. A new method was developed which permits high tension production in skinned muscle fibers while retaining clear striations. Clear striations allow reliable measurement of the sarcomere lengths during contraction by microscopy and diffractometry. The method is to increase the Ca⁺⁺ concentration of the bathing solution very gradually over a time period of 5 to 10 minutes. Once the skinned fiber is conditioned by this slow activation, subsequent contractions can be elicited by ordinary quick activations without loss of striations. When the experiments are carried out with careful controls for the uniformity of the sarcomere length distribution along the entire length of the fiber, contractions are highly repeatable. Using the new method and stringent quality control of fibers, the sarcomere length-isometric tension relationship of skinned rabbit soleus fibers was obtained. The results differ from those previously obtained by conventional activation methods in that tension increases with sarcomere length not only at low (pCa = 5.8), but also at high (pCa = 5.2), calcium concentration.     

Pax7 reveals a greater frequency and concentration of satellite cells at the ends of growing skeletal muscle fibers.

The main sites of longitudinal growth in skeletal muscle are the ends of the fibers. This study tests the hypothesis that satellite cells (SCs) are at a greater frequency (#SC nuclei/all nuclei within basal laminae) and concentration (closer together) within growing fiber ends of posthatch chicken pectoralis. SCs were localized by their Pax7 expression, and fiber ends were identified by their retention of neonatal myosin heavy chains and small cross-sectional profiles. Whereas SC frequency decreased from about 20% at 9 days posthatch to <5% at 115 days, fiber ends retained a frequency of approximately 16%. Calculated mean area of sarcolemma per SC revealed higher concentrations of SCs at fiber ends. There was also a strong inverse correlation between SC frequency and fiber profile cross-sectional size throughout development. This study suggests that SCs at fiber ends play a key role in the longitudinal growth of muscle fibers, and that fiber profile size may impact SC distribution.     

Radial forces within muscle fibers in rigor

Considering the widely accepted cross-bridge model of muscle contraction (Huxley. 1969. Science [Wash. D. C.]. 164:1356-1366), one would expect that attachment of angled cross-bridges would give rise to radial as well as longitudinal forces in the muscle fiber. These forces would tend, in most instances, to draw the myofilaments together and to cause the fiber to decrease in width. Using optical techniques, we have observed significant changes in the width of mechanically skinned frog muscle fibers when the fibers are put into rigor by deleting ATP from the bathing medium. Using a high molecular weight polymer polyvinylpyrrolidone (PVP-40; number average mol. wt. (Mn) = 40,000) in the bathing solution, we were able to estimate the magnitude of the radial forces by shrinking the relaxed fiber to the width observed with rigor induction. With rigor, fiber widths decreased up to approximately 10%, with shrinking being greater at shorter sarcomere spacing and at lower PVP concentrations. At higher PVP concentrations, some fibers actually swelled slightly. Radial pressures seen with rigor in 2 and 4% PVP ranged up to 8.9 x 10(3) N/m2. Upon rigor induction, fibers exerted a longitudinal force of approximately 1 x 10(5) N/m2 that was inhibited by high PVP concentrations (greater than or equal to 13%). In very high PVP concentrations (greater than or equal to 20%), fibers exerted an anomalous force, independent of ATP, which ranged up to 6 x 10(4) N/m2 at 60% PVP. Assuming that all the radial force is the result of cross- bridge attachment, we calculated that rigor cross-bridges exert a radial force of 0.2 x 1.2 x 10(-9) N per thick filament in sarcomeres near rest length. This force is of roughly the same order of magnitude as the longitudinal force per thick filament in rigor contraction or in maximal (calcium-activated) contraction of skinned fibers in ATP- containing solutions. Inasmuch as widths of fibers stretched well beyond overlap of thick and thin filaments decreased with rigor, other radially directed forces may be operating in parallel with cross-bridge forces.     

Histochemical and molecular determination of fiber types in chemically skinned single equine skeletal muscle fibers

Until now, there has been no reliable method for histochemical determination of fiber types of single skinned muscle fibers. The major problem arises from the fact that most histochemical techniques use cross-sections of a large group of fibers and compare a given fiber with those surrounding it. This is not possible with a single skinned fiber which has been separated from a bundle to be used for mechanical analysis. A further problem is that the skinning procedure itself may alter the staining pattern. We have developed a procedure by which multiple cross-sections of single skinned fibers can be exposed to various histochemical reactions and the staining patterns compared on the same slide to those of frozen muscle and skinned bundles. By this procedure, three fiber types were distinguished by both Ca2+-ATPase and SDH reactions. The fiber typings determined from both enzyme systems correlated well with each other. Although we were able to differentiate only between slow and fast fibers by SDS-PAGE, these results corroborated the histochemical classification. This procedure will clearly be useful in skinned single muscle fiber mechanics experiments performed to determine functional differences among fiber types.     

Inferences on force transmission from muscle fiber architecture of the canine diaphragm

Functional properties of the diaphragm are mediated by muscle structure. Modeling of force transmission necessitates a precise knowledge of muscle fiber architecture. Because the diaphragm experiences loads both along and transverse to the long axes of its muscle fibers in vivo, the mechanism of force transmission may be more complex than in other skeletal muscles that are loaded uniaxially along the muscle fibers. Using a combination of fiber microdissections and histological and morphological methods, we determined regional muscle fiber architecture and measured the shape of the cell membrane of single fibers isolated from diaphragm muscles from 11 mongrel dogs. We found that muscle fibers were either spanning fibers (SPF), running uninterrupted between central tendon (CT) and chest wall (CW), or were non-spanning fibers (NSF) that ended within the muscle fascicle. NSF accounted for the majority of fibers in the midcostal, dorsal costal, and lateral crural regions but were only 25-41% of fibers in the sternal region. In the midcostal and dorsal costal regions, only approximately 1% of the NSF terminated within the fascicle at both ends; the lateral crural region contained no such fibers. We measured fiber length, tapered length, fiber diameters along fiber length, and the taper angle for 271 fibers. The lateral crural region had the longest mean length of SPF, which is equivalent to the mean muscle length, followed by the costal and sternal regions. For the midcostal and crural regions, the percentage of tapered length of NSF was 45.9 +/- 5.3 and 40.6 +/- 7.5, respectively. The taper angle was approximately 0.15 degrees for both, and, therefore, the shear component of force was approximately 380 times greater than the tensile component. When the diaphragm is submaximally activated, as during normal breathing and maximal inspiratory efforts, muscle forces could be transmitted to the cell membrane and to the extracellular intramuscular connective tissue by shear linkage, presumably via structural transmembrane proteins.     

Polarization of fluorescence from single skinned glycerinated rabbit psoas fibers in rigor and relaxation

Single skinned glycerinated muscle fibers were labelled with the fluorescent dye N-(iodoacetylamino)-1-naphthylamine-5-sulfonic acid (1,5-IAEDANS). The heavy chain of myosin (EC 3.6.1.3) was labelled predominantly when the reaction was carried out in relaxation at 0 °C. Mechanical properties of skinned fibers were little affected by labelling with the fluorophore. Rigor tension developed upon transferring native or labelled skinned fibers from relaxing to rigor solutions lacking Ca²⁺ was very small but could be enhanced by progressively increasing Ca²⁺ concentration; the rigor tension decreased with increasing sarcomere length.Polarization of fluorescence of skinned fibers reacted with 1,5-IAEDANS was measured along the line of excitation as well as at 90° to it. The mean values of parallel and perpendicular components of polarization of labelled fibers measured at 0° were close to the values obtained for native fibers irrigated with 1,5-IAEDANS-labelled heavy meromyosin, fiber “ghosts” irrigated with labelled heavy meromyosin, and oriented bundles of myofibrils reacted with the same fluorophore. Skinned fibers stretched above the rest length and then irrigated with 1,5-IAEDANS-labelled heavy meromyosin gave rise to polarized fluorescence close to the values theoretically predicted for an assembly of helically arranged fluorophores. Using 90° detection system a satisfactory fit to the theory could be obtained from single fibers labelled with 1,5-IAEDANS and measured in rigor. The angle between the fiber axis and the direction of the emission dipole of 1,5-IAEDANS attached to subfragment-1 was estimated to be near 40°.     

Local longitudinal muscle shortening of the human esophagus from high-frequency ultrasonography

We analyzed local longitudinal shortening by combining concurrent ultrasonography and manometry with basic principles of mechanics. We applied the law of mass conservation to quantify local axial shortening of the esophageal wall from ultrasonically measured cross-sectional area concurrently with measured intraluminal pressure, from which correlations between local contraction of longitudinal and circular muscle are inferred. Two clear phases of local longitudinal shortening were observed during bolus transport. During luminal filling by bolus fluid, the muscle layer distends and the muscle thickness decreases in the absence of circular or longitudinal muscle contraction. This is followed by local contraction, first in longitudinal muscle, then in circular muscle. Maximal longitudinal shortening occurs nearly coincidently with peak intraluminal pressure. Longitudinal muscle contraction begins before and ends after circular muscle contraction. Larger longitudinal shortening is correlated with higher pressure amplitude, suggesting that circumferential contractile forces are enhanced by longitudinal muscle shortening. We conclude that a peristaltic wave of longitudinal muscle contraction envelops the wave of circular muscle contraction as it passes through the middle esophagus, with peak longitudinal contraction aligned with peak circular muscular contraction. Our results suggest that the coordination of the two waves may be a physiological response to the mechanical influence of longitudinal shortening, which increases contractile force while reducing average muscle fiber tension by increasing circular muscle fiber density locally near the bolus tail.     

Approximating the isometric force-calcium relation of intact frog muscle using skinned fibers.

In previous papers we used estimates of the composition of frog muscle and calculations involving the likely fixed charge density in myofibrils to propose bathing solutions for skinned fibers, which best mimic the normal intracellular milieu of intact muscle fibers. We tested predictions of this calculation using measurements of the potential across the boundary of skinned frog muscle fibers bathed in this solution. The average potential was -3.1 mV, close to that predicted from a simple Donnan equilibrium. The contribution of ATP hydrolysis to a diffusion potential was probably small because addition of 1 mM vanadate to the solution decreased the fiber actomyosin ATPase rate (measured by high-performance liquid chromatography) by at least 73% but had little effect on the measured potential. Using these solutions, we obtained force-pCa curves from mechanically skinned fibers at three different temperatures, allowing the solution pH to change with temperature in the same fashion as the intracellular pH of intact fibers varies with temperature. The bath concentration of Ca2+ required for half-maximal activation of isometric force was 1.45 microM (22 degrees C, pH 7.18), 2.58 microM (16 degrees C, pH 7.25), and 3.36 microM (5 degrees C, pH 7.59). The [Ca2+] at the threshold of activation at 16 degrees C was approximately 1 microM, in good agreement with estimates of threshold [Ca2+] in intact frog muscle fibers.     

Dynamic shape of tapered skeletal muscle fibers

The muscle fibers of the feline biceps femoris have tapered ends, across which tension is transmitted to the endomysium. The angle of taper of 11 ends, measured on scanning electron micrographs, varied between 0.16 degrees and 1.18 degrees. The muscle fibers are highly variable in cross-sectional shape. The shape of the fibers has been quantified as the ratio (form factor [FF]) of the measured perimeter to the calculated circumference of a circle having an area equal to that contained by the fiber perimeter. The FF for 173 terminal portions of fibers varied between 1.06 and 1.85 and was found to have a highly significant negative correlation with sarcomere length. The slope of the regression line suggests that the fibers maintain both volume and surface area as they change length. These studies suggest that isovolumic muscle fibers maintain a constant surface area by changing shape as they change length.     

Lattice shrinkage with increasing resting tension in stretched, single skinned fibers of frog muscle.

The 1,0 lattice spacing d1,0 in chemically and mechanically skinned single fibers of frog muscle was measured at various sarcomere lengths, L, in the range from L = 2.1 to 6.0 microns by an x-ray diffraction method. In chemically skinned fibers, d1,0 decreased with a similar slope to that of mechanically skinned fibers up to L congruent to 3 microns, but beyond this point d1,0 steeply decreased with further stretching. This steep decrease in d1,0 could be ascribed mainly to an increase in the compressing force associated with the longitudinal extension of a remnant of the sarcolemma. In mechanically skinned fibers, the gradual decrease in d1,0 continued beyond filament overlap (L greater than or equal to 3.5 microns) and was highly proportional to a resting tension. This decrease in d1,0 at L greater than or equal to 3.5 microns could be ascribed to an increase in the force exerted by lateral elastic components, which is proportional to the longitudinal resting tension. A conceptual model is proposed of a network structure of elastic components in a sarcomere.     

Doxorubicin induces calcium release from terminal cisternae of skeletal muscle. A study on isolated sarcoplasmic reticulum and chemically skinned fibers

In this study, we investigated the effect of the anticancer drug doxorubicin on Ca2+ fluxes of isolated highly purified sarcoplasmic reticulum fractions (longitudinal tubules and terminal cisternae (Saito, A., Seiler, S., Chu, A., and Fleischer, S. (1984) J. Cell Biol. 99, 875-885] and of chemically skinned skeletal muscle fibers of the rabbit. In terminal cisternae, doxorubicin inhibits Ca2+ uptake (IC50 at 0.5 microM) and increases 2.6-fold Ca2+-dependent ATPase rate (half-maximal activation at 3 microM) and unidirectional Ca2+ efflux (8-fold stimulation at 25 microM). On the contrary, doxorubicin is without effect on longitudinal tubules. In skinned muscle fibers, doxorubicin induces rapid and transient Ca2+ release, as measured by tension development (half-maximal stimulation at 6 microM), which is completely and reversibly inhibited by ruthenium red, a known inhibitor of Ca2+ release from isolated terminal cisternae. Doxorubicin has no effect on the sarcoplasmic reticulum Ca2+ pump and on the contractile apparatus of skinned muscle fibers. It is concluded that doxorubicin activates Ca2+ release from sarcoplasmic reticulum and opens a Ca2+ efflux pathway (Ca2+ channel) selectively localized in terminal cisternae. Doxorubicin might interact with Ca2+ channels involved in physiological Ca2+ release.     

Optical depolarization changes on the diffraction pattern in the transition of skinned muscle fibers from relaxed to rigor state

Light diffraction spectra from single or small bundles of skinned striated muscle fibers show large changes in polarization properties when muscles are placed into rigor. The technique of combining optical diffraction and ellipsometry measurements has previously been shown by Yeh and Pinsky to be a sensitive probe of periodic anisotropic regions of the fiber. In the present work, using this method, the observed spectrum shows marked decrease in the measured phase angle, delta, as the fiber approaches the rigor state. The degree of phase angle change is a function of sarcomere length: Maximum overlap of approximately 2.3 microns gives the most change in delta a delta delta R-R approximately 35 degrees decrease for a bundle of three fibers. At a sarcomere length of 2.9 microns this delta delta R-R value is only 10 degrees. At a nonoverlapping length of approximately 3.8 microns, delta does not vary at all upon the removal of ATP. The rigor state was confirmed by stiffness measurements made after small-amplitude (0.75%), quick length changes. Upon re-relaxation, the stiffness of the skinned fiber decreased to the value of the resting state (4 mM ATP) and the phase angle delta returned to its original value. A model based on either anisotropic subunit-2 (S-2) movements or other cross-bridge-related structural anisotropy (form birefringence) changes during the relaxed-rigor transition is suggested.     

Quantitative electrophoretic analysis of myosin heavy chains in single muscle fibers.

To better understand the molecular basis of the large variation in mechanical properties of different fiber types, there has been an intense effort to relate the mechanical and energetic properties measured in skinned single fibers to those of their constituent cross bridges. There is a significant technical obstacle, however, in estimating the number of cross bridges in a single fiber. In this study, we have developed a procedure for extraction and quantification of myosin heavy chains (MHCs) that permits the routine and direct measurement of the myosin content in single muscle fibers. To validate this method, we also compared MHC concentration measured in single fibers with the MHC concentration in whole fast-twitch (psoas and gracilis) and slow-twitch (soleus) muscles of rabbit. We found that the MHC concentration in intact psoas (184 microM) was larger than that in soleus (144 microM), as would be expected from their differing mitochondrial content and volume of myofibrils. We obtained excellent agreement between MHC concentration measured at the single fiber level with that measured at the whole muscle level. This not only verifies the efficacy of our procedure but also shows that the difference in concentration at the whole muscle level simply reflects the concentration differences in the constituent fiber types. This new procedure should be of considerable help in future attempts to determine kinetic differences in cross bridges from different fiber types.     

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.     

Sarcomeric domain organization within single skinned rabbit psoas fibers and its effects on laser light diffraction patterns.

Total intensity and fine structure of first-order laser light diffraction maxima from single skinned rabbit psoas fibers were studied. Total intensity of the diffraction maxima was measured as a function of the incidence angle (omega-scan). In the most homogeneous fibers, most of the intensity in the diffraction maxima is confined to a rather narrow range of incidence angles. Fibers with less homogeneous striation patterns, apparently composed of several regions of distinct sarcomere length and tilt of striation (domains), give rise to several narrow intensity peaks in their omega-scans. Left and right first-order diffraction lines produce omega-scans of almost identical shape, composed of one or more intensity peaks, with each pair of corresponding peaks separated by about the same angle. The data indicated that in single skinned rabbit psoas fibers, light diffraction is dominated by Bragg diffraction and that the peaks within omega-scans can be directly correlated with domains within the illuminated fiber segment. In the most homogeneous fiber segments the diameter of domains, estimated from the width of the corresponding maxima in the omega-scans, could almost be as large as the fiber diameter. On average, from the number of peaks in the omega-scans two to three domains with an average length of approximately 250-350 microns can be identified in a fiber cross-section. Therefore, on average only a small number of domains (8 per mm) are found within skinned rabbit psoas fiber segments. In contrast, the number of substructural lines within the diffraction maxima is large even for microscopically homogeneous fibers. Substructural lines appear to be present only when several domains are illuminated simultaneously. Separation and width of these substructural lines are approximately inversely proportional to the length of the illuminated region of the fiber. These data suggest that the substructural lines are due to interference between domains, illuminated simultaneously by a light source with a high degree of spatial coherence (laser). The relevance of these findings for measurements of sarcomere length by laser light diffraction is discussed.     

Transmission of forces within mammalian skeletal muscles

Most models of in vivo musculoskeletal function fail to take into account the diversity of force trajectories defined by muscle fiber architecture. It has been shown for many muscles, across species, that muscle fibers commonly end within muscle fascicles without reaching a myotendinous junction, and that many of these fibers show a progressive decline in cross-sectional area along the length of the muscle. The significance of these anatomical observations is that the tapering would seem to preclude forces generated at the largest cross-sectional area of the fibers being transmitted to the sarcomeres toward the ends of the tapered fiber. If all of the forces are transmitted via the sarcomeres arranged in series, those few sarcomeres at the smaller ends of the fibers must tolerate the stress exerted by the more numerous sarcomeres arranged in parallel at the portions of the fiber with larger cross-sectional areas. A logical alternative would be for forces to be transmitted laterally along the length of a fiber to the cell membrane and the extracellular matrix. Such a structural arrangement would permit an alternative force transmission vector and minimize the necessity for a precise level of force to be generated along the entire length of a fiber. There are cytoarchitectural and biochemical data demonstrating the presence of a subcellular network which is appropriately located to transmit forces from the active intracellular contractile elements to the extracellular intramuscular connective tissues. However, to fully comprehend how forces are transmitted from individual cross bridges to the tendon, it will be necessary to understand the interactions of all of the components of the muscle tendon complex from the molecular to the multicellular level. It is insufficient to know the physiology of the individual components in a restricted experimental paradigm and assume that these conditions account for the functional characteristics in vivo. Thus, the challenge is to understand how the sarcomeres and all of the associated structures transmit the forces of the whole muscle to its attachments.