Magnitude Of Shortening Research Articles

Measuring mechanical function in the failing heart

A common pathology in heart failure is a detrimental change in the mechanics of both contraction and filling. In familial hypertrophic cardiomyopathy, a genetic disease characterized by left ventricular hypertrophy and myofiber disarray, left ventricular diastolic dysfunction is common and contributes to congestive heart failure. In dilated cardiomyopathy, a common correlate to reduced wall thickening and increased chamber volume is an asynchronous activation of the left ventricle due to left bundle branch block. Local measures of the timing and magnitude of myocardial shortening and relaxation can be obtained with magnetic resonance (MR) tissue tagging, MR cine phase contrast, or MR cine displacement encoding. In familial hypertrophic cardiomyopathy, these methods have been shown to quantify the restrictive filling of the ventricle. Characterizing the regions of the failing heart which are activated late has allowed investigators to measure the change in protein expression in those regions compared to normal myocardium. Also, these MR imaging methods have led to a better quantification of the asynchronous activation in dilated cardiomyopathy, which can be used to predict response to resynchronization therapy with pacing.

High Stiffness of Human Digital Flexor Tendons Is Suited for Precise Finger Positional Control

The objective of this study was to define the biomechanical properties of the human digital flexor tendons and to compare these biomechanical properties to other muscle-tendon units in the forearm. Mechanical measurements were performed on fresh-frozen tendons under physiological load and temperature conditions. Loads were determined by first measuring the physiological cross-sectional area of each digital belly of the flexor digitorum superficialis (FDS) and flexor digitorum profundus (FDP) and estimating maximum tension (P(o)) of that specific muscle head. Loading each tendon to the appropriate P(o) resulted in no significant difference in tendon strain among any of the tendons within each muscle (P > 0.05; digits 2-5) or between muscle types (FDP vs. FDS). The one exception to this finding was that a significantly higher strain at Po was observed in the FDP tendon to the small finger (P < 0.05). Average absolute strains observed for the FDP and FDS tendons (1.20 +/- 0.38%, mean +/- SD; n = 39) were significantly lower than those observed previously in a study of the prime movers of the wrist. The measured strain of approximately 1.5% was less than half of that predicted to occur in muscles of this architectural design. Modeling sarcomere shortening magnitudes during FDP or FDS contraction yielded a value of only 0.10 microm, which would have a negligible effect on the force generating capacity of these muscles. Thus the high stiffness of the digital flexor tendons suits them well for fine positional control and would render their muscle spindles quite sensitive to length perturbations at the fingertips.

Effect of positional changes of anatomic structures on upper airway dilating muscle shortening during electro- and chemostimulation

Positional changes of anatomic structures surrounding the upper airway are known to affect pharyngeal mechanics and collapsibility. We hypothesized that these alterations also affect the ability of the upper airway dilator muscles to enlarge the pharynx by altering their ability to shorten when activated. Using sonomicrometry, we evaluated in seven anesthetized dogs the effects of changes in tracheal and head position on the length of the genioglossus (GG) and the geniohyoid (GH) and the effects of these positional changes on the magnitude of shortening of the two muscles in response to electro- (ES) and chemostimulation (CS). Caudal traction of the trachea lengthened the GG and GH in all dogs, whereas cranial displacement of the trachea and flexion of the head to a vertical position shortened the muscles. Compared with the magnitude of ES-induced shortening in the neutral position, ES-induced shortening of the GG was 144.7 +/- 14.6, 49.3 +/- 4.3, and 33.5 +/- 11.6% during caudal and cranial displacement of the trachea and during head flexion, respectively. Similar effects of the positional changes were found for the GH, as well as for both muscles during respiratory stimulation with P(CO2) of 90 Torr at the end of CO(2) rebreathing, although inspiratory muscle shortening during CS reached only one-quarter to one-third of the magnitude observed during ES. We conclude that positional alterations of anatomic structures in the neck have a dramatic effect on the magnitude of shortening of the activated GG and GH, which may reduce substantially their ability to protect pharyngeal patency.

Effects of Intravenous Magnesium in a Prolonged QT Interval Model of Polymorphic Ventricular Tachycardia Focus on Transmural Ventricular Repolarization

This study was performed to clarify the antiarrhythmic effects of magnesium sulfate (Mg(++)) in a prolonged QT interval canine model of polymorphic ventricular tachyarrhythmia (VTA). In six experiments in a canine model of prolonged QT by anthopleurin-A, Mg(++) was administered in boluses of 0.2 mL/kg during repetitive episodes of self-terminating polymorphic VTA or frequent premature ventricular complexes (PVCs). The distribution of ventricular repolarization across the left ventricular(LV) wall and dispersion of transmural repolarization were analyzed before, and 30 and 120 seconds after Mg(++) administration, during ventricular pacing at 100 bpm. Transmural unipolar electrograms were recorded from multipolar needle electrodes, and local activation-recovery intervals (ARI) were measured. Mg(++) rapidly eliminated self-terminating polymorphic VTA and all isolated PVCs. During ventricular pacing at 100 bpm, Mg(++) caused modest shortening of ARI at all recording sites. Since the magnitude of ARI shortening was greater at mid-myocardial sites than at other ventricular sites, mean transmural ARI dispersion decreased from 80 +/- 22 to 45 +/- 18 ms within 30 seconds after Mg(++) injection. However, this effect was transient, and, at 120 seconds after Mg(++) administration, ARI had increased all sites and transmural ARI dispersion lengthened to 65 +/- 18 ms. Besides suppression of triggered premature activity, homogenization of transmural ventricular repolarization was associated with the antiarrhythmic effects of intravenous Mg(++) in this model. Since these effects were transient, a continuous intravenous infusion of Mg(++) is preferred to prevent recurrences of VTA.

Quantifying different modes of the late Cenozoic shortening in the central Andes

Abstract Since the late Miocene, the two main segments of the central Andean high plateau, Altiplano and Puna, demonstrate different styles and magnitudes of tectonic shortening. Through numerical simulation of thermomechanical processes, we show that different shortening modes—pure and simple shear accompanied by thin-or thick-skinned tectonics—might be controlled by strength of the foreland uppermost crust and by temperature of the foreland lithosphere. Mechanical weakening and failure of the thick Paleozoic sediments overlying the cold lithosphere in the Altiplano foreland at 13–9 Ma explains the transition from pure to simple shear shortening accompanied by broad thin-skinned thrusting, started before the major uplift of the plateau. However, the high strength of the uppermost crust combined with a relatively warm lithosphere results in the thick-skinned shortening typical for the foreland of the Puna. Failure of Paleozoic sediments in the Altiplano foreland significantly reduces the force required to shorten the lithosphere, which may be the reason for the increased bulk shortening rate in the late Miocene.

Effects of halothane, isoflurane, sevoflurane and desflurane on contraction of ventricular myocytes from streptozotocin-induced diabetic rats.

Various clinically used volatile general anaesthetics (e.g. sevoflurane, halothane, isoflurane and desflurane) have been shown to have significant negative inotropic effects on normal ventricular muscle. However, little is known about their effects in ventricular tissue from diabetic animals. Streptozotocin (STZ)-induced diabetes is known to induce changes in the amplitude and time course of shortening and one report suggests that the inotropic effects of anaesthetics are ameliorated in papillary muscles from diabetic animals. The aim of these studies was to investigate this further in electrically stimulated (1 Hz) ventricular myocytes. Cells were superfused with either normal Tyrode (NT) solution or NT containing anaesthetic (1 mM) for a period of 2 min (at 30-32 degrees C). Myocytes from STZ rats were shown to have a significantly longer time to peak shortening (p > 0.001, n = 50) and the amplitude of shortening tended to be greater but this was not significant (p = 0.13, n = 50). Halothane, isoflurane, desflurane and sevoflurane significantly (p < 0.05) reduced the magnitude of shortening of control cells by 72.5 +/- 3.2%, 46.5 +/- 9.7%, 28.9 +/- 4.3% and 22.8 +/- 5.6%, respectively (n > 11 per group) but their steady-state negative inotropic effect was found to be no different in cells from STZ-treated rats (73.0 +/- 4.8%, 40.7 +/- 4.7%, 25.0 +/- 5.2% and 19.8 +/- 5.2%, respectively, n > 10 per group). Therefore, we conclude that the inotropic effects of volatile anaesthetics were not altered by STZ treatment.

Theory of luminescent emission in nanocrystal ZnS:Mn with an extra electron

We consider the effect of an extra electron injected into a doped quantum dot $ZnS:Mn^{2+}$. The Coulomb interaction and the exchange interaction between the extra electron and the states of the Mn ion will mix the wavefunctions, split the impurity energy levels, break the previous selection rules and change the transition probabilities. Using this model of an extra electron in the doped quantum dot, we calculated the energy and the wavefunctions, the luminescence probability and the transition lifetime and compare with the experiments. Our calculation shows that two orders of magnitudes of lifetime shortening can occur in the transition $^4T_1-^6A_1$ when an extra electron is present.

Effects of shortening on stretch-induced force enhancement in single skeletal muscle fibers

The main purpose of this study was to evaluate the effects of shortening on the stretch-induced force enhancement in single muscle fibers, and indirectly test the hypothesis that force enhancement may be associated with the engagement of a passive element upon activation. Fibers were placed on the descending limb of the force–length relationship, and stretch and shortening contractions were performed. Fibers underwent two sets of shortening–stretch cycles. First, fibers were shortened by a fixed amplitude and speed (10% fiber length, and at 40% fiber length/s), and then were stretched (10% fiber length, and at 40% fiber length/s) immediately following shortening, or 500 or 1000 ms following the shortening. Second, fibers were shortened by varying amounts (5%, 10% and 15% fiber length) and at a constant speed (40% fiber length/s) immediately preceding a given fiber stretch (10% fiber length, and at 40% fiber length/s). When stretching was immediately preceded by shortening, force enhancement was decreased proportionally with the shortening magnitude. When intervals were introduced between shortening and stretch, the effects of shortening on the stretch-induced force enhancement became less prominent. We concluded that, in contrast to published suggestions, shortening affects the stretch-induced force enhancement in an amplitude-dependent manner in single fibers, as it does in whole muscles, but this effect is diminished by increasing the time period between the shortening and stretch phases.

Building the Northern Tien Shan: Integrated Thermal, Structural, and Topographic Constraints

Paired apatite fission track and U‐Th/He dates provide the first Late Cenozoic cooling ages for the northern Tien Shan. These data clearly argue for pulsed deformation since the Late Miocene, with early (10–11 Ma) and late (0–3 Ma) intervals of rapid exhumation separated by an extended interval of much slower rates. By integrating these bedrock cooling rates with shortening estimates derived from a balanced section, detrital cooling ages, and geomorphological estimates of conditions before deformation, we reconstruct a four‐stage history of range growth and exhumation. Following ∼100 m.yr. of tectonic quiescence, abruptly accelerated rock uplift, exhumation, and cooling in the Kyrgyz Range commenced at ∼11 Ma with rates exceeding ∼1 km/m.yr. During the subsequent 7 m.yr., deformation and cooling rates decreased three‐ to sixfold before accelerating by comparable amounts during the past 3 m.yr. Since mid‐Miocene times, the surface elevation of the Kyrgyz Range has increased ∼2 km, consistent with the reconstructed magnitude of crustal shortening (∼11 km) and thickening (∼12 km) across the range. The highly pulsed deformation rates indicate that the locus of deformation probably shifted repeatedly within the Tien Shan from the Miocene to present. Even at their most rapid, Cenozoic shortening rates in the Kyrgyz Range were equivalent to only 10%–20% of the modern geodetic convergence rate across the entire Tien Shan. This requires several ranges within the Tien Shan to have deformed simultaneously since the Middle Miocene, a situation analogous to the distributed shortening seen today.

Potassium-induced Slow Motility is Partially Calcium-dependent in Isolated Outer Hair Cells

Low flow rate (0.6 μl/min) administration of high concentration potassium solutions (12.5, 25 and 37.5 mM) was tested for evoking slow-motility length changes in isolated, apical turn, guinea pig outer hair cells (OHCs) (length 65–80 μm; n=38). Control OHCs (n=16) showed a flow rate-dependent, reversible, longitudinal shortening of 0.5–3 μm during perfusion with normal saline. Potassium, an effective depolarizing agent for OHCs, induced a concentration-dependent cell shortening of 0.5–13 μm. These cell shape changes were reversible. The magnitude of shortening was significantly (p<0.01) decreased in a calcium-free incubation medium (n=8). The velocity of the shortening was 300 nm/s in the first 10 s after application of 37.5 mM K+ in a normal incubation medium and decreased to 100 nm/s during the next 10 s. Corresponding velocities in calcium-free solutions were 100 and 50 nm/s, respectively. K+-induced shortening velocities were not significantly different from control values after 30 s. It appears that K+-induced OHC shortening is sensitive to the calcium content of the incubation medium during the first 10 s. Higher flow rate (1.5 μl/min) administration of K+ makes the velocity and magnitude of slow motility of OHCs insensitive to the absence of calcium. These results highlight the fact that one of the critical technical points in fluid perfusion experiments with isolated OHCs is selecting a safe low flow rate of <0.6 μl/min. At this perfusion rate, K+-induced OHC shortening is composed of both calcium-sensitive and -insensitive components.

Mechanics of sculpture formation in Magadiceramus? rangatira rangatira (Inoceramidae, Bivalvia) from the Upper Cretaceous of New Zealand

The surface sculpture of the inoceramid bivalve Magadiceramus? rangatira rangatira consists of commarginal ribs and curious, transverse wrinkles. The wrinkles typically are at a high angle or orthogonal to the shell margin (‘antimarginal’) and thus differ from purely radial structures. They show features of distribution and morphology that reveal them to be products of margin-parallel compression of the shell-secreting mantle and its adjacent, flexible, uncalcified periostracum. The interaction of wrinkles with commarginal ribs indicates that the ribs also formed as folds of the mantle margin. During growth, commarginal folding caused withdrawal of the entire mantle margin towards the umbo, with a consequent reduction in perimeter length. Measurement of specimens indicates that fabrication of the commarginal ribs resulted in the magnitude of commarginal shortening that is required for the formation of transverse wrinkles. We infer that early in ontogeny, at the first development of these sculptures, the wrinkles resulted entirely from mantle contraction and resultant commarginal shortening. With subsequent growth, total wrinkling included a component of ‘pre-wrinkling’ inherited from the preceding growth stage; the contribution of pre-wrinkling to total wrinkling increased with shell size. The proposed mechanical model is two-phase. First, the transversely corrugated (pre-wrinkled) mantle and periostracum advanced and secreted a slightly concave growth increment. Secondly, the mantle subsequently contracted to create a commarginal rib and increase the number and amplitude of transverse wrinkles. This model is consistent with a homogeneous mantle lacking any differentiated and specialized rib-constructing segments.

Initial plate geometry, shortening variations, and evolution of the Bolivian orocline

Comparisons of newly published cross sections across the Bolivian Andes with existing cross sections through Argentina emphasize significant along-strike changes in crustal shortening. A sharp decrease in the magnitude of crustal shortening from ∼530 km to ∼150 km (north to south) occurs at ∼23°S. A 20–40 m.y. difference in the ages at which deformation was initiated accompanies the abrupt decrease in the magnitude of shortening. Extending the western margin of South America to account for 530 km of shortening in the Bolivian Andes and 150 km of shortening in Argentina produces a central Andean salient that is perpendicular to the Nazca plate shortening direction from 60 to 26 Ma. During this same time interval, the Chilean coast south of 23°S was in an orientation sufficiently oblique to oceanic convergence to allow for predominantly strike-slip offset and backarc extension. Deformation within the Andean mountain chain may be a function of plate convergence where the oblique nature of convergence south of ∼23°S inhibited mountain building, whereas north of ∼23°S, normal convergence to a central Andean salient facilitated contractional deformation. The magnitude of deformation north of ∼23°S is a consequence of both plate-convergence direction, providing a longer period of contractional deformation (from ca. 70 Ma to the present), and a thick Phanerozoic sedimentary package that permitted large magnitudes of thin-skinned deformation. Significant along-strike changes in the shape of the South American margin—allowing for convergence to change from compression to extension along the strike of the orogen—may help explain the dramatic differences in timing, amount, and style of deformation in the Andes.

Deformation history of the Roanoke recess, Appalachians, USA

The evolution of the Roanoke recess is controversial because agreement is lacking about the timing and continuity of deformation increments; because it may or may not have behaved like an adjacent salient and recess; because while many salients have been investigated, few recesses have; and because the role of blind thrusts is unclear. To gain insight into these controversies, we estimated shortening directions, shortening magnitudes and relative timing in the recess from map-scale folds, outcrop-scale stylolites, and grain-scale solution structures and twins. The key results from combining these new data with previous work are as follows. (1) The deformation history around the Roanoke recess is most simply described as temporally continuous with deformation migrating northeastward while the primary shortening direction rotated counterclockwise from N-directed to almost W-directed. (2) Consequently, the Roanoke recess did not deform in the symmetrical manner of the adjacent Pennsylvania salient nor with the two-step history of the Kingston recess further to the north in the Appalachians. (3) The timing and magnitude of shortening increments are sufficient to necessitate a blind thrust under the present surface rocks of the inner recess throughout the deformation history.

Effects of cyclic changes in muscle length on force production in in-situ cat soleus

Muscle shortening and stretch are associated with force depression and force enhancement, respectively. Previously, we have investigated the effect of combined dynamic contractions (i.e. a single shortening-stretch and stretch-shortening cycle) on force production (Herzog and Leonard, 2000). In order to investigate the relationship between force depression and force enhancement systematically, we studied the effects of a single as well as multiple stretch–shortening and shortening-stretch cycles on the ascending limb of the force–length relationship. Furthermore, by systematically varying the speed and magnitude of stretch preceding shortening and the speed and magnitude of shortening preceding stretch, we investigated the influence of these varying contractile conditions on force depression and force enhancement, respectively. Test contractions were performed on cat soleus ( n=6) by electrical stimulation using four conceptually different protocols containing a single or repeated stretch-shortening and shortening-stretch cycles. The results of this study showed that: (1) force depression was not influenced by stretch preceding shortening independent of the speed and amount of stretch; (2) force enhancement was influenced in a dose-dependent manner by the amount of shortening preceding stretch but was not affected by the speed of shortening; (3) repeated stretch-shortening (shortening-stretch) cycles showed cumulative effects; (4) the number of shortening steps over a given distance did not influence the amount of force depression. The findings of this study support the idea that the mechanism of force depression associated with muscle shortening is different from that of force enhancement associated with muscle stretch. Furthermore, they support and extend our previous findings that stretch-shortening and shortening-stretch cycles are not commutative.

Kinematic linkage between internal zone extension and shortening in more external units in the NW Alps

In the Alps a major extensional shear zone unroofed eclogites in its footwall from 30 kbar to c. 10 kbar, implying of the order of 60 km of relative vertical movement in the period 45–36 Ma. The horizontal component of divergence was of the order of 100 km. We use the foreland basin sedimentary record to prove that the extension was contemporaneous with thrusting in units structurally beneath the eclogites (the Briançonnais and/or External Zones). This thrust-related convergence during that time period was of the order of 44–56 km and therefore of the same order of magnitude as internal zone divergence. Extension may have been driven by internal density contrasts. Buoyant continental crust underneath the mafic eclogites, or dense subducted slab offset from the orogen, can induce extension. Any dynamic model must account for the comparable magnitudes of extension and shortening in adjacent parts of the orogen: relative plate motions in the period 45–36 Ma played a subsidiary role.

Northeast-trending folds in the western Skeena Fold Belt, northern Canadian Cordillera: a record of Early Cretaceous sinistral plate convergence

The western portion of the Skeena Fold Belt, northern Canadian Cordillera, contains northeast-trending folds that are highly oblique to northwest-trending folds in the eastern portion of the fold belt, and to most Mesozoic contractional structures in the northern Cordillera. The northeast-trending folds locally interfere with the northwest-trending folds, and one region includes transected folds. Geometric relationships within and between the two fold sets are not easily reconciled by notions of the northeast-trending folds resulting from vertical axis rotation of blocks, influence of basement features, or lateral variations in magnitude of shortening. The northeast-trending folds are inferred to result from sinistral plate convergence early in the history of the fold belt (Early Cretaceous). Northeast-trending folds in the Skeena Fold Belt are the most conspicuous elements of a seldom-studied group of similarly oriented contractional structures, which collectively define a belt at least 1700 km long, within and bordering the Coast Belt. The extent of Early Cretaceous structures potentially related to sinistral convergence supports them having originated in response to the relative plate motion rather than local controls (e.g. indentors). This agrees with relative plate motion studies based on ocean floor reconstructions, which suggest a mid-Cretaceous change from sinistral to dextral convergence.

Functional desensitization to isoproterenol without reducing cAMP production in canine failing cardiocytes.

To corroborate alterations in the functional responses to beta-adrenergic receptor (beta-AR) stimulation with changes in beta-AR signaling in failing cardiomyocytes, contractile and L-type Ca(2+) current responses to isoproterenol along with stimulated cAMP generation were compared among cardiomyocytes isolated from canines with tachycardia-induced heart failure or healthy hearts. The magnitude of shortening of failing cardiomyocytes was significantly depressed (by 22 +/- 4.4%) under basal conditions, and the maximal response to isoproterenol was significantly reduced (by 45 +/- 18%). Similar results were obtained when the responses in the rate of contraction and rate of relaxation to isoproterenol were considered. The L-type Ca(2+) current amplitude measured in failing cardiomyocytes under basal conditions was unchanged, but the responses to isoproterenol were significantly reduced compared with healthy cells. Isoproterenol-stimulated cAMP generation was similar in sarcolemmal membranes derived from the homogenates of failing (45 +/- 6.8) and healthy cardiomyocytes (52 +/- 8.5 pmol cAMP. mg protein(-1). min(-1)). However, stimulated cAMP generation was found to be significantly reduced when the membranes were derived from the homogenates of whole tissue (failing: 67 +/- 8.1 vs. healthy: 140 +/- 27.8 pmol cAMP. mg protein(-1). min(-1)). Total beta-AR density was not reduced in membranes derived from either whole tissue or isolated cardiomyocyte homogenates, but the beta(1)/beta(2) ratio was significantly reduced in the former (failing: 45/55 vs. healthy: 72/28) without being altered in the latter (failing: 72/28, healthy: 77/23). We thus conclude that, in tachycardia-induced heart failure, reduction in the functional responses of isolated cardiomyocytes to beta-AR stimulation may be attributed to alterations in the excitation-contraction machinery rather than to limitation of cAMP generation.

Positive inotropic effects of calcium sensitizers on normal and failing cardiac myocytes.

Calcium sensitizers increase myocardial contractile function without affecting Ca2+ homeostasis, which might be beneficial in the treatment of patients with heart failure. However, it remains uncertain whether Ca sensitizers induce quantitatively similar inotropic responses in control and failing hearts. To compare their effects in normal versus failing hearts at the cellular level, shortening mechanics and intracellular calcium ([Ca2+]i) transient were simultaneously measured in the left ventricular myocytes isolated from normal dogs (n = 8) and dogs with rapid pacing-induced heart failure (n = 7). CGP 48506 and EMD 57033 exerted a positive inotropic effect in a dose (0.1-3 microM)-dependent manner in both normal and heart failure myocytes. The percent increase of cell shortening magnitude was comparable between the two groups. CGP 48506 and EMD 57033 did not affect the diastolic cell length and resting [Ca2+]i level. They did not affect the duration of [Ca2+]i transient dynamics. Thus Ca2+ sensitizers exerted comparable positive inotropic effects without affecting the rest cell length and rest [Ca2+]i in normal and heart failure myocytes.

The effects of change in lead stimulus modality on the modulation of acoustic blink startle

In two experiments we investigated the effect of generalized orienting induced by changing the modality of the lead stimulus on the modulation of blink reflexes elicited by acoustic stimuli. In Experiment 1 (n = 32), participants were presented with acoustic or visual change stimuli after habituation training with tactile lead stimuli. In Experiment 2 (n = 64), modality of the lead stimulus (acoustic vs. visual) was crossed with experimental condition (change vs. no change). Lead stimulus change resulted in increased electrodermal orienting in both experiments. Blink latency shortening and blink magnitude facilitation increased from habituation to change trials regardless of whether the change stimulus was presented in the same or in a different modality as the reflex-eliciting stimulus. These results are not consistent with modality-specific accounts of attentional startle modulation.

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 μm , and there was no significant difference between ΔSL 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.