The Interaction of Compliance and Activation on the Force-Length Operating Range and Force Generating Capacity of Skeletal Muscle: A Computational Study using a Guinea Fowl Musculoskeletal Model.

A muscle's performance is influenced by where it operates on its force-length (F-L) curve. Here we explore how activation and tendon compliance interact to influence muscle operating lengths and force-generating capacity. To study this, we built a musculoskeletal model of the lower limb of the guinea fowl and simulated the F-L operating range during fixed-end fixed-posture contractions for 39 actuators under thousands of combinations of activation and posture using three different muscle models: Muscles with non-compliant tendons, muscles with compliant tendons but no activation-dependent shift in optimal fiber length (L0), and muscles with both compliant tendons and activation-dependent shifts in L0. We found that activation-dependent effects altered muscle fiber lengths up to 40% and increased or decreased force capacity by up to 50% during fixed-end contractions. Typically, activation-compliance effects reduce muscle force and are dominated by the effects of tendon compliance at high activations. At low activation, however, activation-dependent shifts in L0 are equally important and can result in relative force changes for low compliance muscles of up to 60%. There are regions of the F-L curve in which muscles are most sensitive to compliance and there are troughs of influence where these factors have little effect. These regions are hard to predict, though, because the magnitude and location of these areas of high and low sensitivity shift with compliance level. In this study we provide a map for when these effects will meaningfully influence force capacity and an example of their contributions to force production during a static task, namely standing.

A Interação de Conformidade e Ativação na Faixa de Operação Força-Comprimento e Capacidade de Geração de Força do Músculo Esquelético: Um Estudo Computacional Usando um Modelo Musculoesquelético de Galinhas-D'angola O desempenho muscular é influenciado por onde ele opera na sua curva de força-comprimento. Aqui, exploramos como a ativação e a conformidade do tendão interagem para influenciar os comprimentos musculares e a capacidade de geração de força. Para estudar isso, construímos um modelo musculoesquelético do membro inferior da galinha-d'angola e simulamos a faixa de operação força-comprimento durante contrações fixas de postura e extremidade para 39 atuadores sob milhares de combinações de ativação e postura usando três modelos musculares diferentes: músculos com tendões não-complacentes, músculos com tendões complacentes, mas sem desvio dependente de ativação no comprimento ideal de fibra (L0), e músculos com tendões complacentes e desvios dependentes de ativação em L0. Descobrimos que os efeitos dependentes da ativação alteraram os comprimentos da fibra muscular em até 40% e aumentaram ou diminuíram a capacidade de força em até 50% durante as contrações de extremidade fixas. Normalmente, os efeitos de ativação e conformidade reduzem a força muscular e são dominados pelos efeitos de complacência do tendão em altas ativações. Em baixa ativação, no entanto, desvios dependentes de ativação em L0 são igualmente importantes e podem resultar em mudanças de força relativas de até 60% para músculos de baixa complacência. Existem regiões da curva de força-comprimento em que os músculos são mais sensíveis à complacência e há baixas de influência onde esses fatores têm pouco efeito. Essas regiões são difíceis de prever porque a magnitude e a localização dessas áreas de alta e baixa sensibilidade mudam com o nível de conformidade. Neste estudo, fornecemos um mapa para quando esses efeitos influenciarão significativamente a capacidade de força e um exemplo de suas contribuições para a produção de forças durante uma tarefa estática, ou seja, em pé. Translated to Portuguese by G. Sobral (gabisobral@gmail.com).

The microbiota in bronchoalveolar lavage from young children with chronic lung disease includes taxa present in both the oropharynx and nasopharynx.

BACKGROUND: Invasive methods requiring general anaesthesia are needed to sample the lung microbiota in young children who do not expectorate. This poses substantial challenges to longitudinal study of paediatric airway microbiota. Non-invasive upper airway sampling is an alternative method for monitoring airway microbiota; however, there are limited data describing the relationship of such results with lung microbiota in young children. In this study, we compared the upper and lower airway microbiota in young children to determine whether non-invasive upper airway sampling procedures provide a reliable measure of either lung microbiota or clinically defined differences. RESULTS: The microbiota in oropharyngeal (OP) swabs, nasopharyngeal (NP) swabs and bronchoalveolar lavage (BAL) from 78 children (median age 2.2 years) with and without lung disease were characterised using 16S rRNA gene sequencing. Permutational multivariate analysis of variance (PERMANOVA) detected significant differences between the microbiota in BAL and those in both OP swabs (p = 0.0001, Pseudo-F = 12.2, df = 1) and NP swabs (p = 0.0001; Pseudo-F = 21.9, df = 1) with the NP and BAL microbiota more different than the OP and BAL, as indicated by a higher Pseudo-F value. The microbiota in combined OP and NP data (upper airways) provided a more comprehensive representation of BAL microbiota, but significant differences between the upper airway and BAL microbiota remained, albeit with a considerably smaller Pseudo-F (PERMANOVA p = 0.0001; Pseudo-F = 4.9, df = 1). Despite this overall difference, paired BAL and upper airway (OP and NP) microbiota were >50 % similar among 69 % of children. Furthermore, canonical analysis of principal coordinates (CAP analysis) detected significant differences between the microbiota from clinically defined groups when analysing either BAL (eigenvalues >0.8; misclassification rate 26.5 %) or the combined OP and NP data (eigenvalues >0.8; misclassification rate 12.2 %). CONCLUSIONS: Upper airway sampling provided an imperfect, but reliable, representation of the BAL microbiota for most children in this study. We recommend inclusion of both OP and NP specimens when non-invasive upper airway sampling is needed to assess airway microbiota in young children who do not expectorate. The results of the CAP analysis suggest lower and upper airway microbiota profiles may differentiate children with chronic suppurative lung disease from those with persistent bacterial bronchitis; however, further research is needed to confirm this observation.

Respiratory bacterial culture from two sequential bronchoalveolar lavages of the same lobe in children with chronic cough.

Identification of bacteria causing lower-airway infections is important to determine appropriate antimicrobial therapy. Flexible bronchoscopy with bronchoalveolar lavage (BAL) is used to obtain lower-airway specimens in young children. The first lavage (lavage-1) is typically used for bacterial culture. However, no studies in children have compared the detection of cultivable bacteria from sequential lavages of the same lobe. BAL fluid was collected from two sequential lavages of the same lobe in 79 children enrolled in our prospective studies of chronic cough. The respiratory bacteria Streptococcus pneumoniae, Haemophilus influenzae, Moraxella catarrhalis, Staphylococcus aureus and Haemophilus parainfluenzae were isolated and identified using standard published methods. H. influenzae was differentiated from Haemophilus haemolyticus using PCR assays. Lower-airway infection was defined as ≥ 104 c.f.u. ml- 1 BAL fluid. We compared cultivable bacteria from lavage-1 with those from the second lavage (lavage-2) using the κ statistic. Lower-airway infections by any pathogen were detected in 46% of first lavages and 39% of second lavages. Detection was similar in both lavages for all pathogens; the κ statistic was 0.7-0.8 for all bacteria except H. parainfluenzae. Of all infections detected in either lavage, 90% were detected in lavage-1 and 78 in lavage-2. However, culture of lavage-2 identified infections that would have been missed in 8% of children, including infections by additional Streptococcus pneumoniae serotypes. Our findings support the continued use of lavage-1 for bacterial culture; however, culture of lavage-2 may yield additional identifications of bacterial pathogens in lower-airway infections.

Lung disease in indigenous children.

Children in indigenous populations have substantially higher respiratory morbidity than non-indigenous children. Indigenous children have more frequent respiratory infections that are, more severe and, associated with long-term sequelae. Post-infectious sequelae such as chronic suppurative lung disease and bronchiectasis are especially prevalent among indigenous groups and have lifelong impact on lung function. Also, although estimates of asthma prevalence among indigenous children are similar to non-indigenous groups the morbidity of asthma is higher in indigenous children. To reduce the morbidity of respiratory illness, best-practice medicine is essential in addition to improving socio-economic factors, (eg household crowding), tobacco smoke exposure, and access to health care and illness prevention programs that likely contribute to these issues. Although each indigenous group may have unique health beliefs and interfaces with modern health care, a culturally sensitive and community-based comprehensive care system of preventive and long term care can improve outcomes for all these conditions. This article focuses on common respiratory conditions encountered by indigenous children living in affluent countries where data is available.

Chasing maximal performance: a cautionary tale from the celebrated jumping frogs of Calaveras County.

Maximal performance is an essential metric for understanding many aspects of an organism's biology, but it can be difficult to determine because a measured maximum may reflect only a peak level of effort, not a physiological limit. We used a unique opportunity provided by a frog jumping contest to evaluate the validity of existing laboratory estimates of maximum jumping performance in bullfrogs (Rana catesbeiana). We recorded video of 3124 bullfrog jumps over the course of the 4-day contest at the Calaveras County Jumping Frog Jubilee, and determined jump distance from these images and a calibration of the jump arena. Frogs were divided into two groups: 'rental' frogs collected by fair organizers and jumped by the general public, and frogs collected and jumped by experienced, 'professional' teams. A total of 58% of recorded jumps surpassed the maximum jump distance in the literature (1.295 m), and the longest jump was 2.2 m. Compared with rental frogs, professionally jumped frogs jumped farther, and the distribution of jump distances for this group was skewed towards long jumps. Calculated muscular work, historical records and the skewed distribution of jump distances all suggest that the longest jumps represent the true performance limit for this species. Using resampling, we estimated the probability of observing a given jump distance for various sample sizes, showing that large sample sizes are required to detect rare maximal jumps. These results show the importance of sample size, animal motivation and physiological conditions for accurate maximal performance estimates.

Molecular characterisation of pneumococcal serotype 16F: Established predominant carriage and otitis media serotype in the 7vPCV era.

Young Australian Aboriginal children in remote communities experience very high rates of pneumococcal carriage and otitis media. Prior to introduction of the 7-valent pneumococcal conjugate vaccine (7vPCV, Prevenar), serotype 16F was an important type found in nasal and ear discharge swabs. Since commencement of pneumococcal immunisation for Aboriginal infants in 2001, 16F has become the predominant established serotype in carriage and otitis media in young Aboriginal children. BOX typing and multi-locus sequence typing revealed a diverse population of serotype 16F strains, and evidence of potential capsule switching from a vaccine serotype 4 to a serotype 16F.

ISOFIT: a model-based method to measure muscle-tendon properties simultaneously.

Estimation of muscle parameters specifying force-length and force-velocity behavior requires in general a large number of sophisticated experiments often including a combination of isometric, isokinetic, isotonic, and quick-release experiments. This study validates a simpler method (ISOFIT) to determine muscle properties by fitting a Hill-type muscle model to a set of isovelocity data. Muscle properties resulting from the ISOFIT method agreed well with muscle properties determined separately in in vitro measurements using frog semitendinosus muscles. The force-length curve was described well by the results of the model. The force-velocity curve resulting from the model coincided with the experimentally determined curve above approximately 20% of maximum isometric force (correlation coefficient R>0.99). At lower forces and thus higher velocities the predicted curve underestimated velocity. The stiffness of the series elastic component determined with direct experiments was approximately 10% lower than that determined by the ISOFIT method. Use of the ISOFIT method can decrease experimental time up to 80% and reduce potential changes in muscle parameters due to fatigue.

Percutaneous embolization of varicoceles: outcomes and correlation of semen improvement with pregnancy.

OBJECTIVES: To assess the technical feasibility and compare the semen quality in men with or without pregnancy after percutaneous embolization of varicoceles in the management of infertility. METHODS: The records of 102 patients who underwent retrograde varicocele embolization between January 1997 and January 2002 were reviewed through the Hospital Information Support System. Infertility was the indication for embolization in 71 cases. The present study consisted of this group of patients. The size of the varicoceles, the size of the testis, the pre-embolization semen analysis parameters, the technical details of embolization procedure, any anomalous vessels seen on venography, and, if unsuccessful, the reason for failure of the procedure were noted. A record of postembolization semen parameters (at least two) was made. Patients were divided into four groups depending on the pre-embolization semen density, and a correlation of this was assessed with improvements in morphology and motility. Follow-up was performed using a questionnaire to evaluate the success rate of the procedure, complications, and any treatment for infertility by the patient or his partner after the procedure. Patients who had a successful pregnancy were compared with those who did not to determine the correlation between the changes in semen quality and pregnancy rate. RESULTS: Between January 1997 and January 2002, 71 patients underwent retrograde varicocele embolization, using an embolizing coil, for infertility. In 68 (95.7%), it was technically successful. Nineteen patients (26.7%) had various anomalous vessels on venography. A statistically significant improvement (P = 0.002) was noted in the motility parameters in patients with a pre-embolization semen density between 10 and 30 million/mL. All patients were followed up by questionnaire. Follow-up was possible in 51 patients (75%). One patient had varicocele recurrence and underwent open inguinal surgical ligation. Of 45 patients, the partners of 18 (40%) had a successful pregnancy. A comparison of the postembolization semen quality between those with and without a successful pregnancy found no correlation between the changes in the semen parameters and the pregnancy rate. CONCLUSIONS: Varicocele embolization is a technically feasible, minimally invasive, outpatient procedure that improves semen quality significantly in patients with a pre-embolization semen density of 10 to 30 million/mL. However, no correlation was found between the improvements in semen quality and the pregnancy rate.

Botulinum toxin injection for failed tracheo-oesophageal voice in laryngectomees: the Sunderland experience.

Spasm of the pharyngo-oesophageal segment is one of the important causes of tracheo-oesophageal voice failure. Traditionally it has been managed by either prolonged speech therapy, surgical pharyngeal myotomy or pharyngeal plexus neurectomy with varying degrees of success. Botulinum neurotoxin has been found to be effective in relieving pharyngo-oesophageal segment spasm. Since 1995, we have used botulinum toxin injection on 10 laryngectomees with either aphonia or hypertonicity due to pharyngo-oesophageal segment spasm. Early results were analysed by the Sunderland Surgical Voice Restoration Rating scale. Seven of the 10 patients, who were previously completely aphonic, developed voice following this therapy and are using their valve choice as their only method of communication. Out of the three patients who were treated for hypertonic voice, two did derive some benefit from the procedure. One patient developed a hypotonic voice, which lasted for a few months.

The mechanical power output of the pectoralis muscle of blue-breasted quail (Coturnix chinensis): the in vivo length cycle and its implications for muscle performance.

Sonomicrometry and electromyographic (EMG) recordings were made for the pectoralis muscle of blue-breasted quail (Coturnix chinensis) during take-off and horizontal flight. In both modes of flight, the pectoralis strain trajectory was asymmetrical, with 70 % of the total cycle time spent shortening. EMG activity was found to start just before mid-upstroke and continued into the downstroke. The wingbeat frequency was 23 Hz, and the total strain was 23 % of the mean resting length. Bundles of fibres were dissected from the pectoralis and subjected in vitro to the in vivo length and activity patterns, whilst measuring force. The net power output was only 80 W kg(-1) because of a large artefact in the force record during lengthening. For more realistic estimates of the pectoralis power output, we ignored the power absorbed by the muscle bundles during lengthening. The net power output during shortening averaged over the entire cycle was approximately 350 W kg(-1), and in several preparations over 400 W kg(-1). Sawtooth cycles were also examined for comparison with the simulation cycles, which were identical in all respects apart from the velocity profile. The power output during these cycles was found to be 14 % lower than during the in vivo strain trajectory. This difference was due to a higher velocity of stretch, which resulted in greater activation and higher power output throughout the later part of shortening, and the increase in shortening velocity towards the end of shortening, which facilitated deactivation. The muscle was found to operate at a mean length shorter than the plateau of the length/force relationship, which resulted in the isometric stress measured at the mean resting length being lower than is typically reported for striated muscle.

The mechanical power output of the flight muscles of blue-breasted quail (Coturnix chinensis) during take-off.

Blue-breasted quail (Coturnix chinensis) were filmed during take-off flights. By tracking the position of the centre of mass of the bird in three dimensions, we were able to calculate the power required to increase the potential and kinetic energy. In addition, high-speed video recordings of the position of the wings over the course of the wing stroke, and morphological measurements, allowed us to calculate the aerodynamic and inertial power requirements. The total power output required from the pectoralis muscle was, on average, 390 W kg(-1), which was similar to the highest measurements made on bundles of muscle fibres in vitro (433 W kg(-1)), although for one individual a power output of 530 W kg(-1) was calculated. The majority of the power was required to increase the potential energy of the body. The power output of these muscles is the highest yet found for any muscle in repetitive contractions. We also calculated the power requirements during take-off flights in four other species in the family Phasianidae. Power output was found to be independent of body mass in this family. However, the precise scaling of burst power output within this group must await a better assessment of whether similar levels of performance were measured across the group. We extended our analysis to one species of hawk, several species of hummingbird and two species of bee. Remarkably, we concluded that, over a broad range of body size (0.0002-5 kg) and contractile frequency (5-186 Hz), the myofibrillar power output of flight muscles during short maximal bursts is very high (360-460 W kg(-1)) and shows very little scaling with body mass. The approximate constancy of power output means that the work output varies inversely with wingbeat frequency and reaches values of approximately 30-60 J kg(-1) in the largest species.

Output monitoring tests reveal false memories of memories that never existed.

Three experiments were conducted to assess participants' beliefs about potential false memories that might have occurred during free recall tests. An input-output monitoring test was administered that required participants to discriminate between items that were studied and recalled, studied and not recalled, or were entirely new. Critical lures from Roediger and McDermott's (1995) paradigm were inserted into this test. The results demonstrated that participants believed erroneously recalled items were both studied and recalled. The intriguing finding was that unrecalled items were believed to have been studied approximately 80% of the time, and half of those were also believed to have been recalled. This result represents a dual false memory effect in which items were believed to have been studied and also to have been recalled. The ramifications of this new procedure are discussed in terms of proposed experiments that might clarify the genesis of these false memories.

False recognition occurs more frequently during source identification than during old-new recognition.

A number of recent reports have investigated false memories using variants of the Deese-Roediger-McDermott (DRM) paradigm. Because these false memories have been difficult to eliminate, this study investigated whether false recognition could be reduced by incorporating source-monitoring criteria into decision processes. Making claims about the manner in which items were learned should require more careful scrutiny of memories, and therefore false recognition should be minimized with source instructions as compared with old-new recognition instructions. In 3 experiments that varied the combination of sources, false recognition was increased rather than reduced by applying source-monitoring processes. The theoretical implications of these counterintuitive results are discussed in terms of the old-new detection component of source judgments.

Effects of long-term exercise on the biomechanical properties of the Achilles tendon of guinea fowl.

The purpose of this study was to determine the effect of long-term exercise on tendon compliance and to ascertain whether tendons adapt differently to downhill running vs. running on a level surface. We carried out this investigation on the gastrocnemius tendon of helmeted guinea fowl (Numida meleagris) that were trained for 8-12 wk before commencing experimental procedures. We used an in situ technique to measure tendon stiffness. The animals were deeply anesthetized with isofluorane during all in situ procedures. Our results indicate that long-term exercise increased tendon stiffness. This finding held true after normalization for the cross-sectional area of the free tendon, likely reflecting a change in the material properties of the exercised tendons. Whether training consisted of level or downhill running did not appear to influence response of the tendon to exercise. We hypothesize that the increased stiffness observed in tendons after a long-term running program may be a response to repeated stress and may function as a mechanism to resist tendon damage due to mechanical fatigue.

The properties of retention intervals and their affect on retaining prospective memories.

Five experiments were conducted to explore how the character of the retention interval affected event-based prospective memory. According to the canons of retrospective memory, prospective performance should have been worse with increasing delays between intention formation and the time it was appropriate to complete an action. That result did not occur. Rather, prospective memory was better with increasing retention intervals in Experiments 1A, 1B, and 3. In manipulating the nature of the retention interval, the authors found that there were independent contributions of retention interval length and the number of intervening activities, with more activities leading to better prospective memory (Experiments 2 and 3). The identical retention intervals did not improve retrospective memory in Experiment 4. Theoretical explanations for these dissociations between prospective and retrospective memory are considered.

Dissociation of two kinds of source attributions.

We examined the relationship between two different source attribution errors. One error found primarily in the cognitive psychology literature is the belief that one is an author of an idea when one is not. The other error, reported in the social psychology literature, occurs when people overestimate how long they have known an idea. Although somewhat different, both errors are a form of misappropriation of ideas to oneself. We investigated both attributions and found that when participants performed a more elaborate encoding task, erroneous claims of authorship were reduced but length-of-knowing judgments increased. The results are discussed in terms of the cognitive processing that is likely to give rise to each source attribution.

Toward specifying the attentional demands of recognition memory.

Previous studies have concluded that recognition memory is immune to disruption from divided attention and therefore is a relatively automatic process (A. Baddeley, V. Lewis, M. Eldridge, & N. Thomson, 1984; F. I. M. Craik, R. Govoni, M. Naveh-Benjamin, & N. D. Anderson, 1996). Because costs have been found on the concurrent task used to divide attention, recognition may nevertheless require some attentional resources (M. Naveh-Benjamin, F. I. M. Craik, J. Guez, & H. Dori, 1998). The present authors used attention-demanding concurrent tasks to demonstrate significant costs on both the concurrent task and recognition memory performance. Decrements in recognition accuracy were found for classes of items that were studied deeply but not for more shallowly learned materials. The present findings suggest that recognition processes can require significant attentional resources when tested under the appropriate conditions. The results are discussed in terms of the requirements both at encoding and at test that are needed to observe dual-task decrements to recognition accuracy.

Contractile properties of muscles used in sound production and locomotion in two species of gray tree frog.

The sound-producing muscles of frogs and toads are interesting because they have been selected to produce high-power outputs at high frequencies. The two North American species of gray tree frog, Hyla chrysoscelis and Hyla versicolor, are a diploid-tetraploid species pair. They are morphologically identical, but differ in the structure of their advertisement calls. H. chrysoscelis produces very loud pulsed calls by contracting its calling muscles at approximately 40 Hz at 20 degrees C, whereas, H. versicolor operates the homologous muscles at approximately 20 Hz at this temperature. This study examined the matching of the intrinsic contractile properties of the calling muscles to their frequency of use. I measured the isotonic and isometric contractile properties of two calling muscles, the laryngeal dilator, which presumably has a role in modulating call structure, and the external oblique, which is one of the muscles that provides the mechanical power for calling. I also examined the properties of the sartorius as a representative locomotor muscle. The calling muscles differ greatly in twitch kinetics between the two species. The calling muscles of H. chrysoscelis reach peak tension in a twitch after approximately 15 ms, compared with 25 ms for the same muscles in H. versicolor. The muscles also differ significantly in isotonic properties in the direction predicted from their calling frequencies. However, the maximum shortening velocities of the calling muscles of H. versicolor are only slightly lower than those of the comparable muscles of H. chrysoscelis. The calling muscles have similar maximum shortening velocities to the sartorius, but have much flatter force-velocity curves, which may be an adaptation to their role in cyclical power output. I conclude that twitch properties have been modified more by selection than have intrinsic shortening velocities. This difference corresponds to the differing roles of shortening velocity and twitch kinetics in determining power output at differing frequencies.

Power output of sound-producing muscles in the tree frogs Hyla versicolor and Hyla chrysoscelis.

Sound-producing muscles provide the opportunity of studying the limits of power production at high contractile frequencies. We used the work loop technique to determine the power available from the external oblique muscles in two related species of North American gray tree frog, Hyla chrysoscelis and Hyla versicolor. These trunk muscles contract cyclically, powering high-intensity sound production in anuran amphibians. The external oblique muscles in H. chrysoscelis have an in vivo operating frequency of 40-55 Hz at 20-25 degrees C, whereas in H. versicolor these muscles contract with a frequency of 20-25 Hz at these temperatures. In vivo investigations have shown that these muscles use an asymmetrical sawtooth length trajectory (with a longer shortening phase compared with the lengthening phase) during natural cycles. To study the influence of this particular length trajectory on power output, we subjected the muscles to both sinusoidal and sawtooth length trajectories. In both species, the sawtooth trajectory yielded a significantly higher power output than the sinusoidal length pattern. The maximum power output during sawtooth cycles was similar in both species (54 W kg(-)(1) in H. chrysoscelis and 58 W kg(-)(1) in H. versicolor). These values are impressive, particularly at the operating frequencies and temperatures of the muscle. The sinusoidal length trajectory yielded only 60 % of the total power output compared with the sawtooth trajectory (34 W kg(-)(1) for H. chrysoscelis and 36 W kg(-)(1) for H. versicolor). The optimum cycle frequencies maximizing the power output using a sawtooth length pattern were approximately 44 Hz for H. chrysoscelis and 21 Hz for H. versicolor. These frequencies are close to those used by the two species during calling. Operating at higher frequencies, H. chrysoscelis maximized power at a strain amplitude of only 8 % compared with a value of 12 % in H. versicolor. These strains match those used in vivo during calling. The stimulus timing observed in vivo during calling was also similar to that yielding maximum power at optimal frequency in both species (6 ms and 8 ms before the start of shortening in H. chrysoscelis and H. versicolor, respectively). As expected, twitch duration in H. chrysoscelis is much shorter than that in H. versicolor (23 ms and 37 ms, respectively). There was a less remarkable difference between their maximum shortening velocities (V(max)) of 13.6 L(0 )s(-)(1) in H. chrysoscelis and 11.1 L(0 )s(-)(1) in H. versicolor, where L(0) is muscle length. The force-velocity curves are very flat, which increases power output. At the myofibrillar level, the flat force-velocity curves more than compensate for the lower peak isometric force found in these muscles. The data presented here emphasize the importance of incorporating in vivo variables in designing in vitro studies.