Sarcomere Length Measurements Research Articles

Technical note: Sampling methodology for relating sarcomere length, collagen concentration, and the extent of postmortem proteolysis to beef and pork longissimus tenderness.

The objective of this study was to determine the effect of sampling methodology on the relationship between longissimus tenderness and measures of biochemical meat traits. Sampling methodology included measurements of sarcomere length, collagen concentration, and postmortem desmin proteolysis on raw samples and measurements of these same traits on the same cooked meat used for shear force measurement. Twenty crossbred steers and 20 crossbred barrows were used for these studies. The beef longissimus thoracis were vacuum-packaged, stored at 2 degrees C until 14 d postmortem, then frozen and stored at -30 degrees C. The pork longissimus thoracis et lumborum were vacuum-packaged, stored at 2 degrees C until 7 d postmortem, then frozen and stored at -30 degrees C. Trained sensory panel tenderness rating ranged from 3.1 to 7.6 for beef and 4.1 to 7.4 for pork. The coefficient of variation was lower for sarcomere length than for all other traits. Simple correlation coefficients between measurements on raw and cooked samples were 0.58 (beef) and 0.11 (pork) for sarcomere length, 0.66 (beef) and 0.59 (pork) for collagen, and 0.74 (beef) and 0.76 (pork) for desmin degradation. Simple correlation coefficients between biochemical traits and measures of tenderness (Warner-Bratzler shear force and trained sensory tenderness rating) were higher or not different for cooked compared to raw samples. Correlation coefficients between biochemical traits and tenderness rating were 0.38 (raw) and 0.22 (cooked) for sarcomere length, -0.12 (raw) and -0.45 (cooked) for collagen, and 0.48 (raw) and 0.80 (cooked) for desmin degradation in beef longissimus and 0.14 (raw) and 0.15 (cooked) for sarcomere length, -0.38 (raw) and -0.33 (cooked) for collagen, and 0.53 (raw) and 0.67 (cooked) for desmin degradation in pork longissimus. The coefficients of determination for explaining variation in tenderness rating using sarcomere length, collagen concentration, and desmin degradation for raw and cooked samples were 0.43 and 0.73 (beef) and 0.48 and 0.57 (pork), respectively. This study indicates that measurements of biochemical traits on the same cooked meat as used for shear force determination account for more of the variation in measures of tenderness than biochemical measurements made on a separate raw sample.

Sarcomere length-dependence of activity-dependent twitch potentiation in mouse skeletal muscle

BackgroundIt has been reported that potentiation of a skeletal muscle twitch response is proportional to muscle length with a negative slope during staircase, and a positive slope during posttetanic potentiation. This study was done to directly compare staircase and posttetanic responses with measurement of sarcomere length to compare their length-dependence.MethodsMouse extensor digitorum longus (EDL) muscles were dissected to small bundles of fibers, which permit measurement of sarcomere length (SL), by laser diffraction. In vitro fixed-end contractions of EDL fiber bundles were elicited at 22°C and 35°C at sarcomere lengths ranging from 2.35 μm to 3.85 μm. Twitch contractions were assessed before and after 1.5 s of 75 Hz stimulation at 22°C or during 10 s of 10 Hz stimulation at 22°C or 35°C.ResultsStaircase potentiation was greater at 35°C than 22°C, and the relative magnitude of the twitch contraction (Pt*/Pt) was proportional to sarcomere length with a negative slope, over the range 2.3 μm – 3.7 μm. Linear regression yielded the following: Pt*/Pt = -0.59·SL+3.27 (r2 = 0.74); Pt*/Pt = -0.39·SL+2.34 (r2 = 0.48); and Pt*/Pt = -0.50·SL+2.45 (r2 = 0.80) for staircase at 35°C, and 22°C and posttetanic response respectively. Posttetanic depression rather than potentiation was present at long SL. This indicates that there may be two processes operating in these muscles to modulate the force: one that enhances and a second that depresses the force. Either or both of these processes may have a length-dependence of its mechanism.ConclusionThere is no evidence that posttetanic potentiation is fundamentally different from staircase in these muscles.

Sarcomeres Are Added in Series to Emphysematous Rat Diaphragm After Lung Volume Reduction Surgery

The diaphragm adapts to its shortened state in experimental emphysema primarily by losing sarcomeres in series, thus reducing its optimal operating length. One would expect improved diaphragmatic function after lung volume reduction surgery (LVRS) only if the muscle can readapt to its elevated, lengthened postoperative position by either adding back sarcomeres or lengthening sarcomeres. We used a model of elastase-induced emphysema in rats to test the hypothesis that sarcomere addition occurs following LVRS. A cohort of emphysematous rats was created by the intratracheal instillation of elastase. Five months after the instillation, one group of rats underwent measurement of in situ costal diaphragm length via laparotomy, the determination of optimal muscle fiber operating length (Lo) on stimulated diaphragm strips in vitro, and the measurement of sarcomere length by electron microscopy on strips fixed at Lo. Another group of rats underwent LVRS or sham sternotomy 5 months after the instillation, and 5 months following the operation these animals underwent the same series of diaphragmatic studies. Lo was significantly greater in rats that underwent LVRS than those that underwent sternotomy (mean [+/- SE] Lo after LVRS, 2.50 +/- 0.08 cm; mean Lo after sternotomy, 2.27 +/- 0.06 cm; p = 0.013). There was no significant difference in sarcomere lengths between the two groups (2.95 +/- 0.04 vs 3.04 +/- 0.04 microm, respectively; p = 0.10). Using Lo as the length basis, the mean sarcomere number was calculated to be 8,712 +/- 192 in animals that had undergone LVRS and 7,144 +/- 249 in animals that had undergone sternotomy (p < 0.001). Sarcomere length is not significantly altered but sarcomeres are added in series following LVRS in this experimental model of emphysema/LVRS. It is likely that this sarcomere addition is a prerequisite to the improvement in inspiratory muscle function that has been observed following LVRS in humans.

High-throughput assessment of calcium sensitivity in skinned cardiac myocytes.

Isolated permeabilized cardiac myocytes have been used in the study of myofilament calcium sensitivity through measurement of the isometric force-pCa curve. Determining this force-pCa relationship in skinned myocytes is relatively expensive and carries a high degree of variability. We therefore attempted to establish an alternative high-throughput method to measure calcium sensitivity in cardiac myocytes. With the use of commercially available software that allows for precise measurement of sarcomere spacing, we measured sarcomere length changes in unloaded skinned cardiac myocytes over a range of calcium concentrations. With the use of this technique, we were able to accurately detect acute increases or decreases in myofilament calcium sensitivity after exposure to 10 mM caffeine or 5 mM 2,3-butanedione monoxime, respectively. This technique allows for the simple and rapid determination of myofilament calcium sensitivity in cardiac myocytes in a reproducible and inexpensive manner and could be used for high-throughput screening of pharmacological agents and/or transgenic mouse models for changes in myofilament calcium sensitivity.

Inhibition of protease activity. Part 1. The effect on tenderness and indicators of proteolysis in ovine muscle

To examine the effect of particular enzyme groups on tenderness specific cysteine protease inhibitors were injected into muscle early post-mortem. The protease enzyme inhibitor E-64 was injected into the m. longissimus thoracis et lumborum (LTL) on the right side of 12 lamb carcasses within 15 min of death and in another 12 carcasses with the protease inhibitor Z-Phe-Ala-CHN 2. The left LTL (control) was injected with saline (0.25 M NaCl). To create variation in the rate of pH decline alternate carcasses were electrically stimulated (low voltage). The LTL was divided into cranial and caudal portions and aged for 1 or 2 days. Muscle samples at 1 day post-mortem were used for measurement of osmolality and sarcomere length ( n=48), and others at 1 and 2 days post-mortem for shear force determination ( n=96). The myofibrillar fragmentation index (MFI) was determined on samples taken at pH 6.2 and 1 and 2 days post-mortem ( n=144). Other muscle samples were obtained at death, pH 6.2 and 6.0 and then at 1 and 2 days post-mortem ( n=215). These samples were used for determination of protein solubility and the concentration of free amino acids. Stimulation caused a faster ( P<0.05) decline in pH. There was no effect of stimulation ( P>0.05) on shear force values, but injection of inhibitor and ageing both had effects ( P<0.001). The inhibitor E-64 prevented any improvement in tenderness with ageing, whereas the inhibitor Z-Phe-Ala-CHN 2 and the control samples showed a similar ageing response. In the latter two treatments there was an average reduction of 1 kg in shear between 1 and 2 days post-mortem, whilst the inhibitor E-64 maintained shear force on average 2 kg higher than control samples. Injection and ageing had an effect on MFI ( P<0.001) and there was an interaction ( P<0.05) between stimulation and ageing for MFI, such that as stimulated muscle aged the rate of change of MFI was greater. There was an interaction between injection and ageing ( P<0.05) for protein solubility such that samples treated with E-64 showed a minimal increase in protein solubility with ageing, whereas in samples treated with Z-Phe-Ala-CHN 2 and the control samples there was a significant increase. There was also an interaction between stimulation and ageing such that between sampling at pH 6.0 and 2 days post-mortem, stimulated muscle exhibited greater solubility ( P<0.05). There were no effects ( P>0.05) on the concentration of free amino acids. The evidence indicated that the cysteine proteases were responsible for post-mortem proteolysis and tenderisation, in particular the calpains, whereas the cathepsins (B and L) were unlikely to contribute to proteolysis and subsequent tenderisation in meat.

Measuring muscle and joint geometry parameters of a shoulder for modeling purposes

An extensive set of muscle and joint geometry parameters was measured of the right shoulder of an embalmed male. For all muscles the optimal muscle fiber length was determined by laser diffraction measurements of sarcomere length. In addition, tendon length and physiological cross-sectional area were determined. The parameter set was needed to enhance the reliability of a computer model of the shoulder ( Van der Helm, 1994a,b Journal of Biomechanics 27, 527–550, 551–569). With the model, an abduction of the arm was simulated in seven positions, at 30° intervals. In each of the simulated arm positions, actual sarcomere lengths were calculated from the lengths of 104 muscle elements, distributed over 16 shoulder muscles. For most muscle elements, the simulated abduction appeared to take place within the sarcomere length range in which the muscle elements can exert force. The muscle elements can then act on the ascending limb as well as on the plateau and on the descending limb of the relative force–length curves of sarcomeres. The produced data set is not only important for the refinement of shoulder modeling, but also for functional analyses of shoulder movements in general.

Measurement of sarcomere length during fast contraction of muscle fibers by digital image analysis

A low-cost, high-resolution (spatial and temporal) image analysis system was developed to measure sarcomere length (Sl) during fast twitch of isolated striated muscle fibers at different temperatures. Fiber images were examined during twitch with an imaging rate of 220 Hz. To increase temporal resolution beyond 220 Hz, consecutive temporally shifted image sequences (N sequences) were acquired. Individual or average Sl was directly measured from a horizontal profile without spatial-frequency assessment. Measurement precision ( E) was determined and expressed as: E(%)=100× P s/( I s×Sl), where P s is the pixel size and I s the involved sarcomere number. At 18°C during isometric twitch, Sls were measured with 220 Hz temporal and 0.2% spatial resolutions. Sl shortened in the central region (0.21±0.12 μm) as tension developed, reaching a maximal shortening of 8.09±2.05% (at rest, Sl=2.59±0.05 μm, n=4) in 32.5±1.96 ms. At 30°C, Sl variations were examined with 880 Hz temporal resolution, in which case maximal Sl shortening was reached in 15.74±1.99 ms, and then decreased to 5.19±1.97% (at rest, Sl=2.6±0.06 μm). The twitch tension developed by the whole fiber was recorded and compared with sarcomere length behavior. Sarcomere length variations in the central region were representative of overall developed tensions at 18 and 30°C.

SARCOMERE SHORTENING DURING FIXED-END CONTRACTION OF FROG SEMITENDINOSUS MUSCLES

1051 Activation of an intact muscle-tendon unit at a fixed length results in sarcomere shortening at the expense of tendon lengthening. Series elasticity permits muscle-tendon units to increase active range, absorb length changes, and to absorb energy thus increasing locomotion efficiency. However, sarcomere shortening at the expense of tendon elongation has not been quantified. The purpose of this study was to measure sarcomere shortening as a function of initial sarcomere length in the intact muscle-tendon-bone (MTB) complex of Rana pipiens semitendinosus (ST) muscles. MTB complexes (n=6) were dissected from R. pipiens after double pithing. Muscles were activated at nine lengths (−3 mm to +6 mm relative to optimal length in 1 mm intervals), while measuring sarcomere length by laser diffraction. Sarcomere shortening with muscles at long sarcomere lengths (∼3.3 μm) was significantly smaller (0.039±.014 μm) compared to sarcomere shortening at short (∼2.0 μm) sarcomere lengths (0.32±.024 μm). The functional consequence of this differential shortening as a function of sarcomere length was to skew the muscle length-tension relationship to longer lengths and to slightly underestimate maximum tetanic tension. The effect was relatively small for the ST muscle but simulations reveal the result would be much greater for muscles with long in-series tendons. This work was supported by NIH grant AR40050 the VA and the Mizuno International Sports Exchange Foundation.

Sarcomere length during contraction of isolated guinea-pig ventricular myocytes

An improved method was developed for measuring sarcomere length (SML) during twitch contractions of single cardiac ventricular myocytes, using a charge-coupled photodiode array self-scanning at a rate of 1.5 ms/element. The average resting SML of 111 cells was 1.88+/-0.04 microm (mean +/-SD). When contractions were triggered by action potentials under perforated-patch conditions, the time course of SML shortening closely followed changes in cell length. A large variation was observed in contraction time course between myocytes, some cells having a phasic component with a duration at 50% shortening (full-width at half-maximum; FWHM) of approximately 40 ms, while others shortened more slowly (FWHM of phasic component @100 ms). FWHM was highly correlated with relaxation half-time, but with neither action potential duration nor resting SML. The kinetics of slowly contracting cells could not be converted to the rapid type by using conditioning trains or applying isoprenaline. The steady-state SML/pCa relation in ventricular myocytes was measured by applying solutions of various pCa immediately after localized punctures of the surface membrane using a focal laser beam. The Hill coefficient, nH, was @4-5 and K1/2@400-500 nM, but there was no evidence of two populations of cells with different Ca2+ sensitivities.

The effect of phosphate on the relaxation of frog skeletal muscle.

The effect of phosphate on the relaxation of isometrically contracting single skinned fibres from the semitendinosus muscle of the frog Rana temporaria has been investigated using laser pulse photolysis of the photolabile caged calcium-chelator diazo-2 to rapidly reduce the Ca2+ (<2 ms) within the fibre and produce >90% relaxation of force. Relaxation occurred in two phases - an initial linear shoulder which lasted approximately 20 ms followed by a double-exponential phase which gave two rate constants, k1 (43.4+/-1. 8 s-1, mean +/-SEM, n=14) and k2 (15.6+/-0.3 s-1, mean +/-SEM, n=14) at 12 degreesC. Increased phosphate concentrations did not affect the linear phase, but slowed the double-exponential phase following photolysis of diazo-2 in a dose-dependent fashion (k50= 0.9 mM for k1, 1.15 mM for k2). Reducing the concentration of contaminating phosphate (from 640 microM to 100 microM) led to an increase in the rate of the double-exponential phase (k1=67.1+/-4.4 s-1, k2=19.7+/-0. 6 s-1, mean +/-SEM, n=12). Time-resolved measurements of sarcomere length during relaxation, both in control fibres and in the presence of a raised phosphate concentration, reveal a <2% change throughout the whole relaxation transient, and less than 0.1% at the end of the linear phase. This finding implies that gross changes in sarcomere length do not contribute to the decay of the relaxation transient seen upon diazo-2 photolysis. Our results suggest that cross-bridges in states prior to phosphate release are already committed to force generation and must relax by releasing phosphate, rather than by a reversal of the force-generating step to a weakly bound, low-force phosphate-bound state. These findings also indicate that an increase in the phosphate concentration within muscle fibres plays an important part in the slowing of relaxation observed in skeletal muscle fatigue and that the relaxation transients observed upon diazo-2 photolysis represent a disengagement of the cross-bridges.

Bleedout efficiency, carcass damage, and rigor mortis development following electrical stunning or carbon dioxide stunning on a shackle line

In Experiment 1, 400 male broilers were stunned using a gradient of 40 to 60% CO2 over a period of 25 s or a 1% brine solution that was electrically charged (35 mA) for 7 s. Blood loss during bleeding was measured in 30-s intervals for a total of 120 s. After conventional processing and chilling, carcass damage was subjectively evaluated. Results indicated that the birds stunned with electricity bled faster than the CO2-stunned birds until 60 s. However, the cumulative blood loss was not different after 90 s. Carcass damage evaluation indicated that birds stunned with CO2 had a significantly lower percentage of broken clavicles, and had fewer hemorrhages on the surface of the Pectoralis. However, there was no difference between the two stunning methods in the frequency of damage at the shoulder. In Experiment 2, 256 broilers were stunned using the same conditions as in Experiment 1. Measurements of pH, R-value, sarcomere length (SL), and fragmentation index (FI) were evaluated from the left breast fillets harvested at 0, 1, 2, and 6 h postmortem. Shear values (SV) were determined using the right fillets harvested at the same four postmortem times and aged on ice until 24 h. No significant difference in breast muscle pH value was observed at 0, 2, and 6 h postmortem. However, CO2-stunned fillets had significantly higher pH values than the ES fillets at 1 h postmortem. Carbon dioxide produced greater R values than electricity at 2 and 6 h. Sarcomere length, FI, and SV were not significantly different at any time tested. These data suggest that CO2 stunning reduced carcass damage but did not reduce the need for aging before deboning when compared to the electrical stunning method used.

ATPase and Shortening Rates in Frog Fast Skeletal Myofibrils by Time-Resolved Measurements of Protein-Bound and Free P i

Shortening and ATPase rates were measured in Ca 2+-activated myofibrils from frog fast muscles in unloaded conditions at 4°C. ATPase rates were determined using the phosphate-binding protein method (free phosphate) and quench flow (total phosphate). Shortening rates at near zero load ( V o) were estimated by quenching reaction mixtures 50 ms to 10 s old at pH 3.5 and measuring sarcomere lengths under the optical microscope. As with the rabbit psoas myofibrils (C. Lionne, F. Travers, and T. Barman, 1996, Biophys. J. 70:887–895), the ATPase progress curves had three phases: a transient P i burst, a fast linear phase ( k F), and a deceleration to a slow phase ( k S). Evidence is given that k F is the ATPase rate of shortening myofibrils. V o is in good agreement with mechanical measurements in myofibrils and fibers. Under the same conditions and at saturation in ATP, V o and k F are 2.4 μm half-sarcomere −1 s −1 and 4.6 s −1, and their K m values are 33 and 200 μM, respectively. These parameters are higher than found with rabbit psoas myofibrils. The myofibrillar k F is higher than the fiber ATPase rates obtained previously in frog fast muscles but considerably lower than obtained in skinned fibers by the phosphate-binding protein method (Z. H. He, R. K. Chillingworth, M. Brune, J. E. T. Corrie, D. R. Trentham, M. R. Webb, and M. R. Ferenczi, 1997, J. Physiol. 50:125–148). We show that, with frog as with rabbit myofibrillar ATPase, phosphate release is the rate-limiting step.

Musculoskeletal balance of the human wrist elucidated using intraoperative laser diffraction

This review describes a series of experiments in which sarcomere length was measured in human wrist muscles to understand their design. Sarcomere length measurements were combined with studies on cadaveric extremities to generate biomechanical models of human wrist function and to provide insights into the mechanism by which wrist strength balance is achieved. Intraoperative measurements of the human extensor carpi radialis brevis (ECRB) muscle during wrist joint rotation reveal that this muscle appears to be designed to operate on the descending limb of its length–tension curve and generates maximum tension with the wrist fully extended. Interestingly, the synergistic extensor carpi radialis longus (ECRL) also operates on its descending limb but over a much narrower sarcomere length range. This is due to the longer fibers and smaller wrist extension moment arm of the ECRL compared to the ECRB. Sarcomere lengths measured from wrist flexors are shorter compared to the extensors. Using a combination of intraoperative measurements on the flexor carpi ulnaris (FCU) and mechanical measurements of wrist muscles, joints and tendons, the general design of the prime wrist movers emerges: both muscle groups generate maximum force with the wrist fully extended. As the wrist flexes, force decreases due to extensor lengthening along the descending limb of their length–tension curve and flexor shortening along the ascending limb of their length–tension curve. The net result is a nearly constant ratio of flexor to extensor torque over the wrist range of motion and a wrist that is most stable in full extension. These experiments demonstrate the elegant match between muscle, tendon and joints acting at the wrist. Overall, the wrist torque motors appear to be designed for balance and control rather than maximum torque generating capacity.

In vivo microvascular structural and functional consequences of muscle length changes.

As muscles are stretched, blood flow and oxygen delivery are compromised, and consequently muscle function is impaired. We tested the hypothesis that the structural microvascular sequellae associated with muscle extension in vivo would impair capillary red blood cell hemodynamics. We developed an intravital spinotrapezius preparation that facilitated direct on-line measurement and alteration of sarcomere length simultaneously with determination of capillary geometry and red blood cell flow dynamics. The range of spinotrapezius sarcomere lengths achievable in vivo was 2.17 +/- 0.05 to 3.13 +/- 0.11 microns. Capillary tortuosity decreased systematically with increases of sarcomere length up to 2.6 microns, at which point most capillaries appeared to be highly oriented along the fiber longitudinal axis. Further increases in sarcomere length above this value reduced mean capillary diameter from 5.61 +/- 0.03 microns at 2.4-2.6 microns sarcomere length to 4.12 +/- 0.05 microns at 3.2-3.4 microns sarcomere length. Over the range of physiological sarcomere lengths, bulk blood flow (radioactive microspheres) decreased approximately 40% from 24.3 +/- 7.5 to 14.5 +/- 4.6 ml.100 g-1.min-1. The proportion of continuously perfused capillaries, i.e., those with continuous flow throughout the 60-s observation period, decreased from 95.9 +/- 0.6% at the shortest sarcomere lengths to 56.5 +/- 0.7% at the longest sarcomere lengths and was correlated significantly with the reduced capillary diameter (r = 0.711, P < 0.01; n = 18). We conclude that alterations in capillary geometry and luminal diameter consequent to increased muscle sarcomere length are associated with a reduction in mean capillary red blood cell velocity and a greater proportion of capillaries in which red blood cell flow is stopped or intermittent. Thus not only does muscle stretching reduce bulk blood (and oxygen) delivery, it also alters capillary red blood cell flow dynamics, which may further impair blood-tissue oxygen exchange.

Uncontrolled sarcomere shortening increases intracellular Ca2+ transient in rat cardiac trabeculae.

Isolated cardiac muscle preparations suffer from damaged-end compliance that allows for substantial shortening of central sarcomeres during contractions in which the overall length of muscle is kept constant. The impact of uncontrolled sarcomere shortening during a twitch on the intracellular calcium transient in myocardium is unknown. Accordingly, in the present study we developed an iterative laser-diffraction feedback system that allowed for the accurate control of central-segment sarcomere length and simultaneous measurement of iontophoretically injected fura 2 fluorescence in isolated cardiac trabeculae. We compared fura 2 fluorescence signals recorded during regular twitches with twitches in which central sarcomere length (SL) was held constant by feedback control ("SL clamp" twitches). We found that uncontrolled sarcomere shortening was associated with a significant (P = 0.005) increase in the peak of the calcium transient and that the amount of this increase was directly correlated to the extent of central-segment sarcomere shortening (r2 = 0.92; P < 0.01). The time course of the calcium transient, however, was unaffected by the mode of contraction (P = 0.64). These findings have important implications for the interpretation of studies of myocardial calcium handling in which uncontrolled sarcomere shortening takes place during the twitch.

Intraoperative sarcomere length measurements reveal differential design of human wrist extensor muscles.

The design of the wrist extensor muscles was studied using a combination of intraoperative laser diffraction and biomechanical modelling of data obtained from human patients and previously published data. Intraoperatively, the change in sarcomere length per degree joint angle rotation (i.e. dSL/d omega) was measured as the wrist was moved from neutral to full flexion in both the extensor carpi radialis brevis (ECRB) and extensor carpi radialis longus (ECRL) muscles. Sarcomere length change per degree rotation was approximately twice as great for the ECRB compared with the ECRL muscle (9.06 +/- 1.06 versus 4.69 +/- 1.20 nm degree-1, mean +/- S.E.M., N = 7). Muscle fibre length and wrist extensor moment arms were obtained from published data and dSL/d omega calculated. The experimental values for dSL/d omega were extremely close to the calculated values. These data demonstrate that architectural differences between the ECRB and ECRL are accentuated by differences between their wrist extensor moment arms. This differential design may permit the extensor muscles, as a group, to generate high force over a wider range of velocities than would be possible with a single muscle or it may permit conservation of mass such that the two muscles together can generate approximately the same force and excursion as a single muscle but with approximately 30% less mass.

Histological, Biophysical, Physical and Chemical Characteristics of Different Ostrich Muscles

Some characteristics generally related to meat quality were evaluated in several muscles from the leg of the ostrich. Ostrich meat is characterised by a relatively high pH f and low intramuscular fat content. Although standard slaughtering procedures of ostriches at present comprise the removal of legs within 30 min post mortem from the carcass, relatively long sarcomere lengths and the absence of a linear relationship (P > 0.05) between sarcomere length and Warner-Bratzler shear measurements (WBS) show that no cold shortening occurs in the different muscles. The means obtained for the Iliotibialis lateralis are representative of the mean of all the individual muscles combined regarding the characteristics muscle fibre diameter, WBS and pigment content. The overall mean for sarcomere length, pH f and collagen content of all the individual muscles combined can be deduced from determination of these characteristics in the Femorotibialis medius.

POSTMORTEM CHANGES IN THE MECHANICAL PROPERTIES AND ULTRASTRUCTURE OF THE LONGISSIMUS IN TWO PORCINE BREEDS

ABSTRACTKinetics of postmortem muscle changes of two pig breeds were studied. pH was measured at 30 min (pH1) and 26 h (pH2) after slaughter. The longissimus thoracis et lumborum was used at 1, 2, 3, 6 and 14 days postmortem for measurements of sarcomere length and mechanical properties in the raw state, and cooking loss and mechanical properties after cooking. Electron microscopy was performed at 1, 6 and 14 days postmortem. pH1 was higher and the maximum stress lower in Large Whites than in Pietrains. Maximum stress of cooked meat decreased with aging time in Large Whites but not in Pietrains. Ultrastructural changes were more obvious in Large Whites than in Pietrains through day 6, but breed differences were no longer observed at day 14. Meat from Pietrain pigs was tougher than that from Large White pigs and tenderized at a slower rate and to a lesser extent during aging.

Myosin and actin filament lengths in diaphragms from emphysematous hamsters

In vitro studies of the diaphragm from emphysematous animals have, in some instances, shown an alteration in its sarcomere length-tension relationship and a decreased maximal specific tension. To our knowledge, it has never been determined whether such functional changes may be indicative of ultrastructural adaptations, e.g., changes in filament lengths and thus cross-bridge number. To address this, we compared filament lengths in diaphragms from hamsters in which emphysema was induced by endotracheal instillation of elastase (E) 5 mo before the hamsters were killed with those from control hamsters (C; saline instillation). Diaphragms were then fixed by vascular perfusion with buffered glutaraldehyde in situ at airway pressures set to approximate the physiological range of lung volumes from residual volume (RV) to total lung capacity (TLC). Ultrathin sections (50-70 nm) were taken parallel to the muscle fiber axis and examined by electron microscopy (x33,000). Sarcomere and filament length measurements were calibrated using an actin periodicity of 39 nm and an M-band width of 86 nm to correct for dimensional changes during preparation. Emphysema increased the change in lung volume from -20 to +25 cmH2O airway pressure (from RV to TLC) by approximately 88%, and the displacement volume of excised lung at 0 cmH2O airway pressure was increased by approximately 138% on average. Neither myosin (C = 1.592 +/- 0.027; E = 1.572 +/- 0.035 micron; P = 0.72) nor actin (C = 1.210 +/- 0.035; E = 1.221 +/- 0.014 micron; P = 0.76) filament lengths were affected by emphysema. Thus, filament length changes do not underlie the diaphragm functional adaptations observed previously in emphysema.

Evaluation of Longissimus dorsi muscle pH at three hours Post mortem as a predictor of beef tenderness

The objective of this study was to determine the relationship of beef longissimus dorsi muscle (LM) pH at 3 h post mortem (pH 3) and aged LM tenderness. The cattle ( n = 444) sampled for this experiment represented various breed types, sex classes, feeding regimes, and post-mortem handling practices. The phenotypic diversity of the cattle used provided a great amount of variation in Warner-Bratzler shear (WBS) force (Coefficient of variation for pH, temperature, and sarcomere length measurements were much smaller than the coefficient of variation for WBS force). None of the parameters measured (LM pH at 3 and 48 h post mortem, temperature at 3 h post mortem and sarcomere length) was strongly related to tenderness. These results do not support the use of pH 3 as a criterion for sorting beef carcasses into expected tenderness groups.