Differences In Contractile Properties Research Articles

Sex-Related and Performance Differences in Contractile Properties, ROM, Strength, and Dynamometry Performance of World-Class Flatwater Canoeists

The inclusion of women canoe in the Olympic Games reflects the growth and development that women have ahead of them in this modality. Therefore, the aim of this study was to explore sex and performance level differences in muscle contractile properties through Tensiomyography (TMG), range of motion (ROM), strength, and canoe-specific functional electromechanical dynamometry (FEMD) performance and establish performance differences between international medalists and non-medalists. Twelve male and nine female canoeists from the Spanish and Portuguese national canoe teams were assessed through TMG, ROM, strength, and canoe-specific isometric and incremental FEMD tests. Few sex and performance level differences were found in TMG and ROM; however, significant sex differences were found in the strength and FEMD tests. Male canoeists had a greater Fmax in Leg Press, Pm and Pmax in canoe position cable row, 1RM bench press and bench pull, Fm and Fpeak canoe-specific isometric FEMD test and number of strokes, and Fpeak and Pmax on the incremental FEMD test than females. International medalists showed a lower time until reaching Vmax and Pmax in Leg Press on both sides and a greater number of strokes and Fpeak in the maximal incremental FEMD test than non-medalists. This study reinforces the utility of the use of TMG and FEMD for assessing and monitoring world-class athletes.

Mechanical Dyssynchrony of Isolated Left and Right Ventricular Human Myocardium in End-Stage Heart Failure.

The left and right ventricles of the human heart differ in embryology, shape, thickness, and function. Ventricular dyssynchrony often occurs in cases of heart failure. Our objectives were to assess whether differences in contractile properties exist between the left and right ventricles and to evaluate signs of left/right ventricular mechanical synchrony in isolated healthy and diseased human myocardium. Myocardial left and right ventricular trabeculae were dissected from nonfailing and end-stage failing human hearts. Baseline contractile force and contraction/relaxation kinetics of the left ventricle were compared to those of the right ventricle in the nonfailing group (n=41) and in the failing group (n=29). Correlation analysis was performed to assess the mechanical synchrony between left and right ventricular myocardium isolated from the same heart, in nonfailing (n=41) and failing hearts (n=29). The failing right ventricular myocardium showed significantly higher developed force (Fdev; P=0.001; d=0.98), prolonged time to peak (P<0.001; d=1.14), and higher rate of force development (P=0.002; d=0.89) and force decline (P=0.003; d=0.82) compared to corresponding left ventricular myocardium. In healthy myocardium, a strong positive relationship was present between the left and right ventricles in time to peak (r=0.58, P<0.001) and maximal kinetic rate of contraction (r=0.63, P<0.001). These coefficients were much weaker, often nearly absent, in failing myocardium. At the level of isolated cardiac trabeculae, contractile performance, specifically of contractile kinetics, is correlated in the nonfailing myocardium between the left and right ventricles' but this correlation is significantly weaker, or even absent, in end-stage heart failure, suggesting an interventricular mechanical dyssynchrony.

Contractile Properties of MHC I and II Fibers From Highly Trained Arm and Leg Muscles of Cross-Country Skiers.

IntroductionLittle is known about potential differences in contractile properties of muscle fibers of the same type in arms and legs. Accordingly, the present study was designed to compare the force-generating capacity and Ca2+ sensitivity of fibers from arm and leg muscles of highly trained cross-country skiers.MethodSingle muscle fibers of m. vastus lateralis and m. triceps brachii of eight highly trained cross-country skiers were analyzed with respect to maximal Ca2+-activated force, specific force and Ca2+ sensitivity.ResultThe maximal Ca2+-activated force was greater for myosin heavy chain (MHC) II than MHC I fibers in both the arm (+62%, P < 0.001) and leg muscle (+77%, P < 0.001), with no differences between limbs for each MHC isoform. In addition, the specific force of MHC II fibers was higher than that of MHC I fibers in both arms (+41%, P = 0.002) and legs (+95%, P < 0.001). The specific force of MHC II fibers was the same in both limbs, whereas MHC I fibers from the m. triceps brachii were, on average, 39% stronger than fibers of the same type from the m. vastus lateralis (P = 0.003). pCa50 was not different between MHC I and II fibers in neither arms nor legs, but the MHC I fibers of m. triceps brachii demonstrated higher Ca2+ sensitivity than fibers of the same type from m. vastus lateralis (P = 0.007).ConclusionComparison of muscles in limbs equally well trained revealed that MHC I fibers in the arm muscle exhibited a higher specific force-generating capacity and greater Ca2+ sensitivity than the same type of fiber in the leg, with no such difference in the case of MHC II fibers. These distinct differences in the properties of fibers of the same type in equally well-trained muscles open new perspectives in muscle physiology.

Gestational age at initial exposure to in utero inflammation influences the extent of diaphragm dysfunction in preterm lambs.

In utero infection may critically influence diaphragm development and predispose preterm infants to postnatal respiratory failure. We aimed to determine how frequency and gestational age (GA) at time of intra-amniotic (IA) lipopolysaccharide (LPS) exposure affects preterm diaphragm function. Pregnant ewes received IA injections of saline or 10-mg LPS at 7 days or 21 days or weekly injections 21, 14 and 7 days before delivery at 121-day GA. Foetal lambs were killed with pentobarbitone (150 mg/kg; intravenous). Diaphragm contractile function was measured in vitro. Muscle fibre type, activation of protein synthesis and degradation pathways, pro-inflammatory signalling and oxidative stress were evaluated using immunofluorescence staining, RT-qPCR, ELISA, Western blotting and biochemical assay. In utero LPS exposure significantly impaired diaphragm contractile function. LPS exposure 7 days before delivery caused maximum specific twitch and tetanic forces 30% lower than controls. When the initial LPS exposure occurred 21 days before delivery maximum specific forces were 40% lower than controls. Earlier LPS exposure also prolonged twitch contraction time, increased fatigue resistance and elevated protein carbonyl content. Despite increased white blood cell counts and interleukin-6 mRNA expression following weekly LPS exposure, there were no significant differences in contractile properties between exposure 21 days before delivery and repeated LPS groups suggesting that frequency of inflammatory exposure does not influence the severity of contractile dysfunction. GA at time of initial LPS exposure, rather than frequency of exposure, determines the extent of inflammation-induced diaphragm dysfunction.

Editorial: Therapeutic implications of circadian rhythms.

Editorial: Therapeutic implications of circadian rhythms.

Inappropriate interpretation of surface EMG signals and muscle fiber characteristics impedes understanding of the control of neuromuscular function

as the final common pathway from the nervous system to muscle, the motor unit transmits an activation signal generated by the nervous system to engage the contractile proteins and produce the muscle forces needed for reflex responses, automatic behaviors, and voluntary actions ([11][1]). The net

Slow motor units in female rat soleus are slower and weaker than their male counterparts.

The aim of the study was to investigate sex-related differences in contractile properties, parameters of action potentials, and mechanisms of force regulation of motor units in the rat soleus muscle, which is a frequent experimental model in animal research. It was revealed that the mean mass of the muscle in males was bigger than in females, by approximately 80%. However, the relation of the muscle mass to the body mass was not significantly different. These results correspond to approximately twice as much tetanic force per motor unit in male rats, and higher maximal contractile output, reflected by the force-time area per stimulus pulse. On the other hand, no differences were observed with respect to twitch forces of motor units. Thus the twitch-to-tetanus ratio was significantly higher in females. Additionally, the contraction and the half-relaxation times were significantly longer in female motor units, which might be due to differences in muscle architecture. The force-frequency curve in males was shifted rightwards with respect to females, indicating that the same relative level of tetanic force could be achieved at considerably lower stimulation frequency in females. The analysis of motor unit action potentials revealed about four times higher amplitudes in male rats, whereas the time parameters of action potentials were similar. The motor units in male and female rat soleus are considerably different and these observations should be taken in the consideration in various experiments on the muscle.

Evaluation of mouse urinary bladder smooth muscle for diurnal differences in contractile properties.

Most physiological systems show daily variations in functional output, entrained to the day–night cycle. Humans exhibit a daily rhythm in urinary voiding (micturition), and disruption of this rhythm (nocturia) has significant clinical impact. However, the underlying mechanisms are not well-understood. Recently, a circadian rhythm in micturition was demonstrated in rodents, correlated with functional changes in urodynamics, providing the opportunity to address this issue in an animal model. Smooth muscle cells from mouse bladder have been proposed to express a functional and autonomous circadian clock at the molecular level. In this study, we addressed whether a semi-intact preparation of mouse urinary bladder smooth muscle (UBSM) exhibited measurable differences in contractility between day and night. UBSM tissue strips were harvested at four time points over the diurnal cycle, and spontaneous (phasic) and nerve-evoked contractions were assessed using isometric tension recordings. During the active period (ZT12-24) when micturition frequency is higher in rodents, UBSM strips had no significant differences in maximal- (high K+) or nerve-evoked contractions compared to strips harvested from the resting period (ZT0-12). However, a diurnal rhythm in phasic contraction was observed, with higher amplitudes at ZT10. Consistent with the enhanced phasic amplitudes, expression of the BK K+ channel, a key suppressor of UBSM excitability, was lower at ZT8. Higher expression of BK at ZT20 was correlated with an enhanced effect of the BK antagonist paxilline (PAX) on phasic amplitude, but PAX had no significant time-of-day dependent effect on phasic frequency or nerve-evoked contractions. Overall, these results identify a diurnal difference for one contractile parameter of bladder muscle. Taken together, the results suggest that autonomous clocks in UBSM make only a limited contribution to the integrated control of diurnal micturition patterns.

An active learning mammalian skeletal muscle lab demonstrating contractile and kinetic properties of fast- and slow-twitch muscle

The fact that humans possess fast- and slow-twitch muscle in the ratio of ∼50% has profound implications for designing exercise training strategies for power and endurance activities. With the growth of exercise and sport science courses, we have seen the need to develop an undergraduate student laboratory that demonstrates the basic properties of fast- and slow-twitch mammalian skeletal muscle. This laboratory illustrates the major differences in contractile properties and fatigue profiles exhibited by the two muscle types. Students compare and contrast twitch kinetics, fused tetanus characteristics, force-frequency relationships, and fatigue properties of fast- and slow-twitch muscles. Examples of results collected by students during class are used to illustrate the type of data collected and analysis performed. During the laboratory, students are encouraged to connect factual information from their skeletal muscle lectures to their laboratory findings. This enables student learning in an active fashion; in particular, the isolated muscle preparation demonstrates that much of what makes muscle fast or slow is myogenic and not the product of the nervous or circulatory systems. This has far-reaching implications for motor control and exercise behavior and therefore is a crucial element in exercise science, with its focus on power and endurance sport activities. To measure student satisfaction with this active learning technique, a questionnaire was administered after the laboratory; 96% of the comments were positive in their support of active versus passive learning strategies.

Effects of testosterone on contractile properties of sexually dimorphic forelimb muscles in male bullfrogs (Rana catesbeiana, Shaw 1802)

SummaryThis study examined the effects of testosterone (T) on the contractile properties of two sexually dimorphic forelimb muscles and one non-dimorphic muscle in male bullfrogs (Rana catesbeiana, Shaw 1802). The dimorphic muscles in castrated males with testosterone replacement (T+) achieved higher forces and lower fatigability than did castrated males without replaced testosterone (T0 males), but the magnitude of the differences was low and many of the pair-wise comparisons of each muscle property were not statistically significant. However, when taken as a whole, the means of seven contractile properties varied in the directions expected of masculine values in T+ animals in the sexually dimorphic muscles. Moreover, these data, compared with previous data on male and female bullfrogs, show that values for T+ males are similar to normal males and are significantly different from females. The T0 males tended to be intermediate in character between T+ males and females, generally retaining masculine values. This suggests that the exposure of young males to T in their first breeding season produces a masculinizing effect on the sexually dimorphic muscles that is not reversed between breeding seasons when T levels are low. The relatively minor differences in contractile properties between T+ and T0 males may indicate that as circulating T levels rise during breeding season in normal males, contractile properties can be enhanced rapidly to maximal functional levels for breeding success.

Fatigue resistance of the knee extensor muscles is not reduced in post-polio syndrome

The present study investigated whether intrinsic fatigability of the muscle fibers is reduced in patients with post-polio syndrome (PPS). This may contribute to the muscle fatigue complaints reported by patients with PPS. For this purpose, we assessed contractile properties and fatigue resistance of the knee extensor muscles using repeated isometric electrically evoked contractions in 38 patients with PPS and 19 age-matched healthy subjects. To determine whether any difference in fatigue resistance between both groups could be attributed to differences in aerobic capacity of the muscle fibers, 9 patients with PPS and 11 healthy subjects performed the same protocol under arterial occlusion. Results showed that fatigue resistance of patients with PPS was comparable to that in controls, both in the situation with intact circulation and with occluded blood flow. Together, our findings suggest that there are no differences in contractile properties and aerobic muscle capacity that may account for the increased muscle fatigue perceived in PPS.

A Comparison of the Contractile Properties of Myometrium from Singleton and Twin Pregnancies

ObjectiveOver half of twin pregnancies in US and UK deliver prematurely but the reasons for this are unclear. The contractility of myometrium from twin pregnancies has not been directly investigated. The objective of this research was to determine if there are differences in the contractile activity and response to oxytocin, between myometrium from singleton and twin pregnancies, across a range of gestational ages. Furthermore, we wished to determine if contractile activity correlates with increasing level of stretch, using neonatal birth weights as a marker of uterine stretch.MethodsThis was an in vitro, laboratory based study of myometrial contractility in women pregnant with one or two babies, using biopsies obtained from non-labouring women undergoing Caesarean section. Spontaneous, oxytocin-stimulated and depolarization induced contractile activity was compared.ResultsDirect measurements of myometrial contractility under controlled conditions show that the frequency of contractions and responses to oxytocin are significantly increased in twins compared to singletons. The duration of contraction however was significantly reduced. We find that contractile activity correlates with increasing levels of stretch, using neonatal birth weights as a surrogate for uterine stretch, with response to oxytocin being significantly positively correlated with birth weight.ConclusionsWe have found significant differences in contractile properties between myometrium from singleton and twin pregnancies and that increasing uterine stretch can alter the contractile properties of myometrium. We discuss the implication of these findings to preterm delivery and future studies.

Characterization of human vocal fold fibroblasts derived from chronic scar

In vitro modeling of cell-matrix interactions that occur during human vocal fold scarring is uncommon, as primary human vocal fold scar fibroblast cell lines are difficult to acquire. The purpose of this study was to characterize morphologic features, growth kinetics, contractile properties, α-smooth muscle actin (α-SMA) protein expression and gene expression profile of human vocal fold fibroblasts derived from scar (sVFF) relative to normal vocal fold fibroblasts (nVFF). In vitro. We successfully cultured human vocal fold fibroblasts from tissue explants of scarred vocal folds from a 56-year-old female and compared these to normal fibroblasts from a 59-year-old female. Growth and proliferation were assessed by daily cell counts, and morphology was compared at 60% confluence for 5 days. Gel contraction assays were evaluated after seeding cells within a collagen matrix. α-SMA was measured using western blotting and immunocytochemistry (ICC). Quantitative reverse-transcriptase polymerase chain reaction (qRT-PCR) was used to assess differential extracellular matrix gene expression between the two cell types. sVFF were morphologically indistinguishable from nVFF. sVFF maintained significantly lower proliferation rates relative to nVFF on days 3 to 6 (day 3: P = .0138; days 4, 5, and 6: P < .0001). There were no significant differences in contractile properties between the two cell types at any time point (0 hours: P = .70, 24 hours: P = .79, 48 hours: P = .58). ICC and western blot analyses revealed increased expression of α-SMA in sVFF as compared with nVFF at passages 4 and 5, but not at passage 6 (passage 4: P = .006, passage 5: P = .0015, passage 6: P = .8860). Analysis of 84 extracellular matrix genes using qRT-PCR revealed differential expression of 15 genes (P < .01). nVFF and sVFF displayed differences in proliferation rates, α-SMA expression, and gene expression, whereas no differences were observed in contractile properties or morphology. Further investigation with a larger sample size is necessary to confirm these findings.

Effects of temperature on power output and contraction kinetics in the locomotor muscle of the regionally endothermic common thresher shark (Alopias vulpinus)

The common thresher shark (Alopias vulpinus) is a pelagic species with medially positioned red aerobic swimming musculature (RM) and regional RM endothermy. This study tested whether the contractile characteristics of the RM are functionally similar along the length of the body and assessed how the contractile properties of the common thresher shark compare with those of other sharks. Contractile properties of the RM were examined at 8, 16 and 24°C from anterior and posterior axial positions (0.4 and 0.6 fork length, respectively) using the work loop technique. Experiments were performed to determine whether the contractile properties of the RM are similar along the body of the common thresher shark and to document the effects of temperature on muscle power. Axial differences in contractile properties of RM were found to be small or absent. Isometric twitch kinetics of RM were ~fivefold slower than those of white muscle, with RM twitch durations of about 1s at 24°C and exceeding 5s at 8°C, a Q(10) of nearly 2.5. Power increased approximately tenfold with the 16°C increase in temperature, while the cycle frequency for maximal power only increased from about 0.5-1.0Hz over this temperature range. These data support the hypothesis that the RM is functionally similar along the body of the common thresher shark and corroborate previous findings from shark species both with and without medial RM. While twitch kinetics suggest the endothermic RM is not unusually temperature sensitive, measures of power suggest that the RM is not well suited to function at cool temperatures. The cycle frequency at which power is maximized appeared relatively insensitive to temperature in RM, which may reflect the relatively cooler temperature of the thresher RM compared to that observed in lamnid sharks as well as the relatively slow RM phenotype in these large fish.

Expression of SERCA and phospholamban in rainbow trout (Oncorhynchus mykiss) heart: comparison of atrial and ventricular tissue and effects of thermal acclimation

In the heart of rainbow trout (Oncorhynchus mykiss), the rate of contraction and Ca(2+) uptake into the sarcoplasmic reticulum (SR) are faster in atrial than ventricular muscle, and contraction force relies more on SR Ca(2+) stores after acclimation to cold. This study tested the hypothesis that differences in contractile properties and Ca(2+) regulation between atrial and ventricular muscle, and between warm-(WA) and cold-acclimated (CA) trout hearts, are associated with differences in expression of sarco(endo)plasmic reticulum Ca(2+) ATPase (SERCA) and/or phospholamban (PLN), an inhibitor of the cardiac SERCA. Quantitative PCR (SERCA only) and antibodies raised against SERCA and PLN were used to determine abundances of SERCA2 transcripts and SERCA and PLN proteins, respectively, in atrium and ventricle of trout acclimated to cold (+4°C, CA) and warm (+18°C, WA) temperatures. Expression of SERCA2 transcripts was 1.6 and 2.1 times higher in atrium than ventricle of WA and CA trout, respectively (P<0.05). At the protein level, differences in SERCA expression between atrium and ventricle were 6.1- and 23-fold for WA and CA trout, respectively (P<0.001). Acclimation to cold increased SERCA2 transcripts 2.6- and 2.0-fold in atrial and ventricular muscle, respectively (P<0.05). At the protein level, cold-induced elevation of SERCA (4.6-fold) was noted only in atrial (P<0.05) but not in ventricular tissue (P>0.05). The expression pattern of PLN was similar to that of the SERCA protein, but chamber-specific and temperature-induced differences were much smaller than in the case of SERCA. In the ventricle, PLN/SERCA ratio was 2.1 and 7.0 times higher than in the atrium for WA and CA fish, respectively. These findings are consistent with the hypothesis that low PLN/SERCA ratio in atrial tissue enables faster SR Ca(2+) reuptake and thus contributes to faster kinetics of contraction in comparison with ventricular muscle. Similarly, cold-induced decrease in PLN/SERCA ratio may be associated with faster contraction kinetics of the CA trout heart, in particular in the atrial muscle.

In Vivo Imaging of Tumor Cell Migration

The process of metastasis formation involves the migration and 3-D invasion of tumor cells from a primary tumor to distant sites. We propose that the dynamics of the migration and invasion process of magnetically labeled tumor cells can be monitored in animal models over prolonged time periods using magnetic resonance imaging (MRI). Human breast carcinoma cells (MB-MDA-231) were labeled with superparamagnetic Fe2O3 iron oxide nanoparticles coated with poly-L-lysine. The particles are readily taken up by cancer cells and stored in intracellular clusters. During cell division, the nanoparticle clusters are divided and split unevenly between daughter cells (mean partitioning fraction 0.85 to 0.15). Nanoparticles are non-toxic, are not degraded by the cell and remain stable for at least 3 weeks. In vitro collagen gel assays show no differences in contractile properties and invasion behavior of magnetically labeled vs. non-labeled tumor cells. MRI of cells suspended in agarose gave a detection limit of the spin-spin-relaxation-rate above the agar background of approximately 70 cells per 1 mm3. The minimal detection volume of tumor cells in agarose was 25 μl. Detection limit and minimal volume were verified by injecting labeled cancer cells in mice. Spin-spin-relaxation-weighted (T2-weighted) and susceptibility-weighted images show a rapid relaxation behavior and pronounced phase shifts in the vicinity of the injection area compared to control scans. These studies demonstrate the feasibility of the method for long-term observation of cancer cell migration in vivo with MRI.

Sex differences in contractile properties and fatigue resistance of human skeletal muscle

To explore the cause of higher skeletal muscle fatigue resistance in women than men, we used electrically evoked contractions (1 s on, 1 s off, 30 Hz, 2 min), which circumvent motivational bias and allow examination of contractile properties. We compared 29 men [26.5 (7.0) years old; mean (s.d.)] with 35 women [25.4 (7.6) years old]. Strength of the quadriceps muscle was higher in men than women (P < 0.001). The lower maximal rate of relaxation in women (P = 0.002) indicates that their muscles were slower than those of men. The torque declined less in women than in men [37.7 (10.7) versus 29.9 (10.0)%; P = 0.002], and was not related to muscle strength or size, as determined with magnetic resonance imaging. The sex difference in fatigability was also seen when the circulation to the leg was occluded [torque declined 76.9 (10.8) versus 59.5 (16.9)% in men versus women, respectively; P = 0.008]. The maximal rate of relaxation correlated with the fatigability of the muscle under all conditions (correlations ranging from 0.34 to 0.51, P < 0.02). We conclude that the sex-related difference in skeletal muscle fatigue resistance is not explicable by differences in motivation, muscle size, oxidative capacity and/or blood flow between sexes, but might be related to differences in fibre type composition.

Muscle specialization in the squid motor system

Although muscle specialization has been studied extensively in vertebrates, less is known about the mechanisms that have evolved in invertebrate muscle that modulate muscle performance. Recent research on the musculature of squid suggests that the mechanisms of muscle specialization in cephalopods may differ from those documented in vertebrates. Muscle diversity in the development and the evolution of cephalopods appears to be characterized by modulation of the dimensions of the myofilaments, in contrast to the relatively fixed myofilament dimensions of vertebrate muscle. In addition, the arrangement of the myofilaments may also be altered, as has been observed in the extensor muscle fibres of the prey capture tentacles of squid and cuttlefish, which show cross-striation and thus differ from the obliquely striated pattern of most cephalopod locomotor muscle fibres. Although biochemical specializations that reflect differences in aerobic capacity have been documented previously for specific layers of the mantle muscle of squid, comparison of protein profiles of myofilament preparations from the fast cross-striated tentacle fibres and slow obliquely striated fibres from the arms has revealed remarkably few differences in myofilament lattice proteins. In particular, previous studies using a variety of SDS-PAGE techniques and peptide mapping of the myosin heavy chain were unable to resolve differences in the myosin light and heavy chains. Since these techniques cannot exclude the presence of a highly conserved variant that differs in only a few amino acids, in this study semi-quantitative reverse transcription polymerase chain reaction (RT-PCR) analysis of myosin heavy chain messenger RNAs (mRNAs) from the cross-striated tentacle and obliquely striated arm muscle fibres was conducted. This analysis showed that a previously reported alternatively spliced isoform of the squid myosin motor domain is present only in low abundance in both muscle types and therefore differential expression of the two myosins cannot explain the difference in contractile properties. It thus appears that modulation of the contractile properties of the musculature of squid and other cephalopods occurs primarily through variation in the arrangement and dimensions of the myofilaments.

Contractile properties of inner and outer smooth muscle bundles from pig urinary detrusor

Like in the human detrusor, the pig urinary detrusor muscle consists of two layers: compactly arranged smooth muscle bundles on the mucosal side (inner layer) and loosely arranged smooth muscle bundles on the serosal side (outer layer). The contractile properties of muscle bundles of both layers were measured using the stop test followed by an isometric contraction. Total and passive forces were measured in ten muscle bundles from the inner and outer muscle layers. Active force was defined as the difference between total and passive force. The curvature and the unloaded shortening velocity of the force-velocity relation were calculated from the shortening forces measured during the stop test. The rate of force development was calculated from the isometric contraction. Differences in contractile properties between both layers were pairwise tested using the Wilcoxon Signed Ranks test. Percentage wise, the outer layer muscle bundles produced the highest active isometric force. The shortening forces were also higher in the outer layer bundles. As a result, both the curvature and the unloaded shortening velocity, derived from the average force-velocity relations fitted to the data sets, were higher in the muscle bundles from this layer. Finally, the outer layer muscle bundles contracted significantly faster than those of the inner layer. Muscle bundles from the outer layer of pig detrusor were found to be faster and stronger (more phasic) than the weaker and slower (more tonic) bundles from the inner layer, suggesting that during bladder contraction the outer layer of the detrusor does more work than the inner layer.

The scaling of acceleratory aquatic locomotion: body size and tail-flip performance of the california spiny lobster panulirus interruptus

Tail-flipping is a crucial escape locomotion of crustaceans which has been predicted to be limited by increased body mass (M(b)). Given isometric growth, one may predict that with growth event duration will decrease as M(b)(-)(1/3), translational distances will increase as M(b)(1/3), translational velocity will be independent of M(b), translational acceleration will decrease as M(b)(-)(1/3), angular displacement will be independent of M(b) and angular velocity and angular acceleration will decrease as M(b)(-)(1/3). We tested these hypotheses by examining the scaling of 12 morphological variables, five kinematic variables and six performance variables of tail-flipping by the California spiny lobster Panulirus interruptus. Growth approximated isometry, which validated the use of the proposed scaling hypotheses. For animals from 1 to 1000 g M(b), the predicted scaling relationships for tail-flip duration and translational distance and velocity variables were supported; however, translational acceleration performance was much better than predicted. Predictions for rotation and rotational velocity variables were not supported, while the rotational acceleration data closely matched the predicted relationship. The increase in tail-flip duration as predicted suggests that muscle shortening velocity decreases with growth; the sustained acceleration performance (similar to findings for shrimp and fish fast-starts) suggests that muscle force output may increase at a greater rate than predicted by isometry. The scaling of rotational acceleration indicates that the torque produced during the tail-flip scales with a mass exponent greater than 1. Comparison of the tail-flip performance of Panulirus interruptus with those of other crustacean species reveals a wide range in performance by animals of similar body size, which suggests that the abdominal muscle may show interesting differences in contractile properties among different species.