Author Correction: Rubber bands reduce the cost of carrying loads.

In this Brief Communication, the scale bar in Fig. 2a should be '0.1 m' rather than '0.01 m'. This has not been corrected online.

Small Ca2+ releases enable hour-long high-frequency contractions in midshipman swimbladder muscle.

Type I males of the Pacific midshipman fish (Porichthys notatus) vibrate their swimbladder to generate mating calls, or "hums," that attract females to their nests. In contrast to the intermittent calls produced by male Atlantic toadfish (Opsanus tau), which occur with a duty cycle (calling time divided by total time) of only 3-8%, midshipman can call continuously for up to an hour. With 100% duty cycles and frequencies of 50-100 Hz (15°C), the superfast muscle fibers that surround the midshipman swimbladder may contract and relax as many as 360,000 times in 1 h. The energy for this activity is supported by a large volume of densely packed mitochondria that are found in the peripheral and central regions of the fiber. The remaining fiber cross section contains contractile filaments and a well-developed network of sarcoplasmic reticulum (SR) and triadic junctions. Here, to understand quantitatively how Ca2+ is managed by midshipman fibers during calling, we measure (a) the Ca2+ pumping-versus-pCa and force-versus-pCa relations in skinned fiber bundles and (b) changes in myoplasmic free [Ca2+] (Δ[Ca2+]) during stimulated activity of individual fibers microinjected with the Ca2+ indicators Mag-fluo-4 and Fluo-4. As in toadfish, the force-pCa relation in midshipman is strongly right-shifted relative to the Ca2+ pumping-pCa relation, and contractile activity is controlled in a synchronous, not asynchronous, fashion during electrical stimulation. SR Ca2+ release per action potential is, however, approximately eightfold smaller in midshipman than in toadfish. Midshipman fibers have a larger time-averaged free [Ca2+] during activity than toadfish fibers, which permits faster Ca2+ pumping because the Ca2+ pumps work closer to their maximum rate. Even with midshipman's sustained release and pumping of Ca2+, however, the Ca2+ energy cost of calling (per kilogram wet weight) is less than twofold more in midshipman than in toadfish.

Intracellular calcium movements during relaxation and recovery of superfast muscle fibers of the toadfish swimbladder.

The mating call of the Atlantic toadfish is generated by bursts of high-frequency twitches of the superfast twitch fibers that surround the swimbladder. At 16°C, a calling period can last several hours, with individual 80-100-Hz calls lasting ∼ 500 ms interleaved with silent periods (intercall intervals) lasting ∼ 10 s. To understand the intracellular movements of Ca(2+) during the intercall intervals, superfast fibers were microinjected with fluo-4, a high-affinity fluorescent Ca(2+) indicator, and stimulated by trains of 40 action potentials at 83 Hz, which mimics fiber activity during calling. The fluo-4 fluorescence signal was measured during and after the stimulus trains; the signal was also simulated with a kinetic model of the underlying myoplasmic Ca(2+) movements, including the binding and transport of Ca(2+) by the sarcoplasmic reticulum (SR) Ca(2+) pumps. The estimated total amount of Ca(2+) released from the SR during a first stimulus train is ∼ 6.5 mM (concentration referred to the myoplasmic water volume). At 40 ms after cessation of stimulation, the myoplasmic free Ca(2+) concentration ([Ca(2+)]) is below the threshold for force generation (∼ 3 µM), yet the estimated concentration of released Ca(2+) remaining in the myoplasm (Δ[CaM]) is large, ∼ 5 mM, with ∼ 80% bound to parvalbumin. At 10 s after stimulation, [Ca(2+)] is ∼ 90 nM (three times the assumed resting level) and Δ[CaM] is ∼ 1.3 mM, with 97% bound to parvalbumin. Ca(2+) movements during the intercall interval thus appear to be strongly influenced by (a) the accumulation of Ca(2+) on parvalbumin and (b) the slow rate of Ca(2+) pumping that ensues when parvalbumin lowers [Ca(2+)] near the resting level. With repetitive stimulus trains initiated at 10-s intervals, Ca(2+) release and pumping come quickly into balance as a result of the stability (negative feedback) supplied by the increased rate of Ca(2+) pumping at higher [Ca(2+)].

Vocal production complexity correlates with neural instructions in the oyster toadfish (Opsanus tau).

Sound communication is fundamental to many social interactions and essential to courtship and agonistic behaviours in many vertebrates. The swimbladder and associated muscles in batrachoidid fishes (midshipman and toadfish) is a unique vertebrate sound production system, wherein fundamental frequencies are determined directly by the firing rate of a vocal-acoustic neural network that drives the contraction frequency of superfast swimbladder muscles. The oyster toadfish boatwhistle call starts with an irregular sound waveform that could be an emergent property of the peripheral nonlinear sound-producing system or reflect complex encoding in the central nervous system. Here, we demonstrate that the start of the boatwhistle is indicative of a chaotic strange attractor, and tested whether its origin lies in the peripheral sound-producing system or in the vocal motor network. We recorded sound and swimbladder muscle activity in awake, freely behaving toadfish during motor nerve stimulation, and recorded sound, motor nerve and muscle activity during spontaneous grunts. The results show that rhythmic motor volleys do not cause complex sound signals. However, arrhythmic recruitment of swimbladder muscle during spontaneous grunts correlates with complex sounds. This supports the hypothesis that the irregular start of the boatwhistle is encoded in the vocal pre-motor neural network, and not caused by peripheral interactions with the sound-producing system. We suggest that sound production system demands across vocal tetrapods have selected for muscles and motorneurons adapted for speed, which can execute complex neural instructions into equivalently complex vocalisations.

Optimized ratiometric calcium sensors for functional in vivo imaging of neurons and T lymphocytes.

The quality of genetically encoded calcium indicators (GECIs) has improved dramatically in recent years, but high-performing ratiometric indicators are still rare. Here we describe a series of fluorescence resonance energy transfer (FRET)-based calcium biosensors with a reduced number of calcium binding sites per sensor. These 'Twitch' sensors are based on the C-terminal domain of Opsanus troponin C. Their FRET responses were optimized by a large-scale functional screen in bacterial colonies, refined by a secondary screen in rat hippocampal neuron cultures. We tested the in vivo performance of the most sensitive variants in the brain and lymph nodes of mice. The sensitivity of the Twitch sensors matched that of synthetic calcium dyes and allowed visualization of tonic action potential firing in neurons and high resolution functional tracking of T lymphocytes. Given their ratiometric readout, their brightness, large dynamic range and linear response properties, Twitch sensors represent versatile tools for neuroscience and immunology.

Paying the piper: the cost of Ca2+ pumping during the mating call of toadfish.

Superfast fibres of toadfish swimbladder muscle generate a series of superfast Ca(2+) transients, a necessity for high-frequency calling. How is this accomplished with a relatively low rate of Ca(2+) pumping by the sarcoplasmic reticulum (SR)? We hypothesized that there may not be complete Ca(2+) saturation and desaturation of the troponin Ca(2+) regulatory sites with each twitch during calling. To test this, we determined the number of regulatory sites by measuring the concentration of troponin C (TNC) molecules, 33.8 µmol per kg wet weight. We then estimated how much SR Ca(2+) is released per twitch by measuring the recovery oxygen consumption in the presence of a crossbridge blocker, N-benzyl-p-toluene sulphonamide (BTS). The results agreed closely with SR release estimates obtained with a kinetic model used to analyse Ca(2+) transient measurements. We found that 235 µmol of Ca(2+) per kg muscle is released with the first twitch of an 80 Hz stimulus (15(o)C). Release per twitch declines dramatically thereafter such that by the 10th twitch release is only 48 µmol kg(-1) (well below the concentration of TNC Ca(2+) regulatory sites, 67.6 µmol kg(-1)). The ATP usage per twitch by the myosin crossbridges remains essentially constant at ∼25 µmol kg(-1) throughout the stimulus period. Hence, for the first twitch, ∼80% of the energy goes into pumping Ca(2+) (which uses 1 ATP per 2 Ca(2+) ions pumped), but by the 10th and subsequent twitches the proportion is ∼50%. Even though by the 10th stimulus the Ca(2+) release per twitch has dropped 5-fold, the Ca(2+) remaining in the SR has declined by only ∼18%; hence dwindling SR Ca(2+) content is not responsible for the drop. Rather, inactivation of the Ca(2+) release channel by myoplasmic Ca(2+) likely explains this reduction. If inactivation did not occur, the SR would run out of Ca(2+) well before the end of even a 40-twitch call. Hence, inactivation of the Ca(2+) release channel plays a critical role in swimbladder muscle during normal in vivo function.

A simple experimentally based model using proprioceptive regulation of motor primitives captures adjusted trajectory formation in spinal frogs.

Spinal circuits may organize trajectories using pattern generators and synergies. In frogs, prior work supports fixed-duration pulses of fixed composition synergies, forming primitives. In wiping behaviors, spinal frogs adjust their motor activity according to the starting limb position and generate fairly straight and accurate isochronous trajectories across the workspace. To test whether a compact description using primitives modulated by proprioceptive feedback could reproduce such trajectory formation, we built a biomechanical model based on physiological data. We recorded from hindlimb muscle spindles to evaluate possible proprioceptive input. As movement was initiated, early skeletofusimotor activity enhanced many muscle spindles firing rates. Before movement began, a rapid estimate of the limb position from simple combinations of spindle rates was possible. Three primitives were used in the model with muscle compositions based on those observed in frogs. Our simulations showed that simple gain and phase shifts of primitives based on published feedback mechanisms could generate accurate isochronous trajectories and motor patterns that matched those observed. Although on-line feedback effects were omitted from the model after movement onset, our primitive-based model reproduced the wiping behavior across a range of starting positions. Without modifications from proprioceptive feedback, the model behaviors missed the target in a manner similar to that in deafferented frogs. These data show how early proprioception might be used to make a simple estimate initial limb state and to implicitly plan a movement using observed spinal motor primitives. Simulations showed that choice of synergy composition played a role in this simplicity. To generate froglike trajectories, a hip flexor synergy without sartorius required motor patterns with more proprioceptive knee flexor control than did patterns built with a more natural synergy including sartorius. Such synergy choices and control strategies may simplify the circuitry required for reflex trajectory construction and adaptation.

Is high concentration of parvalbumin a requirement for superfast relaxation?

It is generally thought that the rapid relaxation of fast muscles is facilitated by the Ca(2+) binding protein parvalbumin (Parv). Indeed superfast swimbladder (SWB) muscle of toadfish contains the largest concentration of this protein ever observed (up to 1.5 mM). At 15 degrees C toadfish perform a 100 Hz call, 400 ms in duration, followed by a long (5-15 s) intercall interval. It has been proposed that Parv helps sequester the Ca(2+) during the call, and then Ca(2+) unbinds and is pumped back into the sarcoplasmic reticulum during the long intercall interval. Midshipman (Porichthys notatus) is another fish which calls at a high frequency; 80-100 Hz at a temperature of 12-15 degrees C. However, unlike toadfish, midshipman call with a 100% duty cycle. Without an intercall interval, Parv would seem of little use as it would become saturated early in calling. Here we show that the midshipman SWB has only about 1/8th of the Parv in toadfish. Moreover, total Parv content in calling male midshipman SWB was not different from that in the non-calling female and the much slower locomotory muscles. These data suggest that Parv does not play a large role in the calling of midshipman, which is accomplished without a high concentration of this protein. Native gel-electrophoresis also revealed presence of three major (PA-I, PA-II and PA-III) and two minor (PA-Ia and PA-IIIa, <5% of total content) Parv isoforms in adult toadfish SWB. Midshipman SWB contained about equal amounts of PA-I and PA-II and also a small (approximately 10%) amount of PA-III. By amino acid composition, toadfish PA-Ia and PA-I isoforms were different from PA-II and PA-III isoforms (by 24 and 14 residues, respectively).

Superfast vocal muscles control song production in songbirds.

Birdsong is a widely used model for vocal learning and human speech, which exhibits high temporal and acoustic diversity. Rapid acoustic modulations are thought to arise from the vocal organ, the syrinx, by passive interactions between the two independent sound generators or intrinsic nonlinear dynamics of sound generating structures. Additionally, direct neuromuscular control could produce such rapid and precisely timed acoustic features if syringeal muscles exhibit rare superfast muscle contractile kinetics. However, no direct evidence exists that avian vocal muscles can produce modulations at such high rates. Here, we show that 1) syringeal muscles are active in phase with sound modulations during song over 200 Hz, 2) direct stimulation of the muscles in situ produces sound modulations at the frequency observed during singing, and that 3) syringeal muscles produce mechanical work at the required frequencies and up to 250 Hz in vitro. The twitch kinematics of these so-called superfast muscles are the fastest measured in any vertebrate muscle. Superfast vocal muscles enable birds to directly control the generation of many observed rapid acoustic changes and to actuate the millisecond precision of neural activity into precise temporal vocal control. Furthermore, birds now join the list of vertebrate classes in which superfast muscle kinetics evolved independently for acoustic communication.

Controlling the freezing process: a robotic device for rapidly freezing biological tissues with millisecond time resolution.

A robotic cryogenic device was developed which allows freezing of thick biological tissues with millisecond time resolution. The device consists of two horizontally oriented hammers (pre-cooled with liquid N(2)) driven by two linear servo-motors. The tissue sample is bathed in Ringers contained in a chamber which drops rapidly out of the way just as the hammers approach. A third linear motor is vertically oriented, and permits the rapidly dropping chamber to smoothly decelerate. All movements were performed by the three motors and four solenoids controlled by a PC. Mechanical adjustments, that change the size of the gap between the hammers at the end position, permit the final thickness of the frozen tissue to be varied. Here we show that the freezing time increased with the square of the final thickness of the frozen bundle. However, when bundles of different original thicknesses (up to at least 1mm) were compressed to the same final thickness (e.g., 0.2mm), they exhibited nearly equal freezing times. Hence, by being able to adjust the final thickness of the frozen bundles, the device not only speeds the rate of freezing, but standardizes the freezing time for different diameter samples. This permits the use of freezing for accurate determination of the kinetics of cellular processes in biological tissue.

The effect of temperature and thermal acclimation on the sustainable performance of swimming scup.

There is a significant reduction in overall maximum power output of muscle at low temperatures due to reduced steady-state (i.e. maximum activation) power-generating capabilities of muscle. However, during cyclical locomotion, a further reduction in power is due to the interplay between non-steady-state contractile properties of muscle (i.e. rates of activation and relaxation) and the stimulation and the length-change pattern muscle undergoes in vivo. In particular, even though the relaxation rate of scup red muscle is slowed greatly at cold temperatures (10 degrees C), warm-acclimated scup swim with the same stimulus duty cycles at cold as they do at warm temperature, not affording slow-relaxing muscle any additional time to relax. Hence, at 10 degrees C, red muscle generates extremely low or negative work in most parts of the body, at all but the slowest swimming speeds. Do scup shorten their stimulation duration and increase muscle relaxation rate during cold acclimation? At 10 degrees C, electromyography (EMG) duty cycles were 18% shorter in cold-acclimated scup than in warm-acclimated scup. But contrary to the expectations, the red muscle did not have a faster relaxation rate, rather, cold-acclimated muscle had an approximately 50% faster activation rate. By driving cold- and warm-acclimated muscle through cold- and warm-acclimated conditions, we found a very large increase in red muscle power during swimming at 10 degrees C. As expected, reducing stimulation duration markedly increased power output. However, the increased rate of activation alone produced an even greater effect. Hence, to fully understand thermal acclimation, it is necessary to examine the whole system under realistic physiological conditions.

Biomechanics: rubber bands reduce the cost of carrying loads.

Vertical movement of the hip during locomotion causes a loaded backpack to be accelerated with each step, which imposes large peak forces on the wearer. Here we show that using bungee cords to suspend the load from a backpack frame reduces not only its vertical movement, and hence its vertical force on the carrier, but also the energetic cost of walking with the pack. This permits larger loads to be carried while moving rapidly, and at the same time reduces the risk of orthopaedic and muscular injury.

Design and function of superfast muscles: new insights into the physiology of skeletal muscle.

Superfast muscles of vertebrates power sound production. The fastest, the swimbladder muscle of toadfish, generates mechanical power at frequencies in excess of 200 Hz. To operate at these frequencies, the speed of relaxation has had to increase approximately 50-fold. This increase is accomplished by modifications of three kinetic traits: (a) a fast calcium transient due to extremely high concentration of sarcoplasmic reticulum (SR)-Ca2+ pumps and parvalbumin, (b) fast off-rate of Ca2+ from troponin C due to an alteration in troponin, and (c) fast cross-bridge detachment rate constant (g, 50 times faster than that in rabbit fast-twitch muscle) due to an alteration in myosin. Although these three modifications permit swimbladder muscle to generate mechanical work at high frequencies (where locomotor muscles cannot), it comes with a cost: The high g causes a large reduction in attached force-generating cross-bridges, making the swimbladder incapable of powering low-frequency locomotory movements. Hence the locomotory and sound-producing muscles have mutually exclusive designs.

Generating electricity while walking with loads.

We have developed the suspended-load backpack, which converts mechanical energy from the vertical movement of carried loads (weighing 20 to 38 kilograms) to electricity during normal walking [generating up to 7.4 watts, or a 300-fold increase over previous shoe devices (20 milliwatts)]. Unexpectedly, little extra metabolic energy (as compared to that expended carrying a rigid backpack) is required during electricity generation. This is probably due to a compensatory change in gait or loading regime, which reduces the metabolic power required for walking. This electricity generation can help give field scientists, explorers, and disaster-relief workers freedom from the heavy weight of replacement batteries and thereby extend their ability to operate in remote areas.

Effect of chronic obstructive pulmonary disease on calcium pump ATPase expression in human diaphragm.

We have previously demonstrated that human diaphragm remodeling elicited by severe chronic obstructive pulmonary disease (COPD) is characterized by a fast-to-slow myosin heavy chain isoform transformation. To test the hypothesis that COPD-induced diaphragm remodeling also elicits a fast-to-slow isoform shift in the sarcoendoplasmic reticulum Ca(2+) ATPase (SERCA), the other major ATPase in skeletal muscle, we obtained intraoperative biopsies of the costal diaphragm from 10 severe COPD patients and 10 control subjects. We then used isoform-specific monoclonal antibodies to characterize diaphragm fibers with respect to the expression of SERCA isoforms. Compared with control diaphragms, COPD diaphragms exhibited a 63% decrease in fibers expressing only fast SERCA (i.e., SERCA1; P < 0.001), a 190% increase in fibers containing both fast and slow SERCA isoforms (P < 0.01), and a 19% increase (P < 0.05) in fibers expressing only the slow SERCA isoform (i.e., SERCA2). Additionally, immunoblot experiments carried out on diaphragm homogenates indicated that COPD diaphragms expressed only one-third the SERCA1 content noted in control diaphragms; in contrast, COPD and control diaphragms did not differ with respect to SERCA2 content. The combination of these histological and immunoblot results is consistent with the hypothesis that diaphragm remodeling elicited by severe COPD is characterized by a fast-to-slow SERCA isoform transformation. Moreover, the combination of these SERCA data and our previously reported myosin heavy chain isoform data (Levine S, Nguyen T, Kaiser LR, Rubinstein NA, Maislin G, Gregory C, Rome LC, Dudley GA, Sieck GC, and Shrager JB. Am J Respir Crit Care Med 168: 706-713, 2003) suggests that diaphragm remodeling elicited by severe COPD should decrease ATP utilization by the diaphragm.

Human diaphragm remodeling associated with chronic obstructive pulmonary disease: clinical implications.

Diaphragm remodeling associated with chronic obstructive pulmonary disease (COPD) consists of a fast-to-slow fiber type transformation as well as adaptations within each fiber type. To try to explain disparate findings in the literature regarding the relationship between fiber type proportions and FEV1, we obtained costal diaphragm biopsies on 40 subjects whose FEV1 ranged from 118 to 16% of the predicted normal value. First, we noted that our exponential regression model indicated that changes in FEV1 can account for 72% of the variation in the proportion of Type I fibers. Second, to assess the impact of COPD on diaphragm force generation, we measured maximal specific force generated by single permeabilized fibers prepared from the diaphragms of two patients with normal pulmonary function tests and two patients with severe COPD. We noted that fibers prepared from the diaphragms of severe COPD patients generated a lower specific force than control fibers (p < 0.001) and Type I fibers generated a lower specific force than Type II fibers (p < 0.001). Our finding of an exponential relationship between the proportion of Type I fibers and FEV1 accounts for discrepancies in the literature. Moreover, our single-fiber results suggest that COPD-associated diaphragm remodeling decreases diaphragmatic force generation by adaptations within each fiber type as well as by fiber type transformations.

Cross-bridge blocker BTS permits direct measurement of SR Ca2+ pump ATP utilization in toadfish swimbladder muscle fibers.

Because the major processes involved in muscle contraction require rapid utilization of ATP, measurement of ATP utilization can provide important insights into the mechanisms of contraction. It is necessary, however, to differentiate between the contribution made by cross-bridges and that of the sarcoplasmic reticulum (SR) Ca2+ pumps. Specific and potent SR Ca2+ pump blockers have been used in skinned fibers to permit direct measurement of cross-bridge ATP utilization. Up to now, there was no analogous cross-bridge blocker. Recently, N-benzyl-p-toluene sulfonamide (BTS) was found to suppress force generation at micromolar concentrations. We tested whether BTS could be used to block cross-bridge ATP utilization, thereby permitting direct measurement of SR Ca2+ pump ATP utilization in saponin-skinned fibers. At 25 microM, BTS virtually eliminates force and cross-bridge ATP utilization (both <4% of control value). By taking advantage of the toadfish swimbladder muscle's unique right shift in its force-Ca2+ concentration ([Ca2+]) relationship, we measured SR Ca2+ pump ATP utilization in the presence and absence of BTS. At 25 microM, BTS had no effect on SR pump ATP utilization. Hence, we used BTS to make some of the first direct measurements of ATP utilization of intact SR over a physiological range of [Ca2+]at 15 degrees C. Curve fits to SR Ca2+ pump ATP utilization vs. pCa indicate that they have much lower Hill coefficients (1.49) than that describing cross-bridge force generation vs. pCa (approximately 5). Furthermore, we found that BTS also effectively eliminates force generation in bundles of intact swimbladder muscle, suggesting that it will be an important tool for studying integrated SR function during normal motor behavior.

The design of vertebrate muscular systems: comparative and integrative approaches.

For more than 50 years, it has been known that vertebrates engage in a wide range of motor activities and that they possess muscle types with a similarly large range of contractile properties. However, only during the past 15 years has it been shown experimentally that the contractile properties of muscle fibers are well adjusted to their in vivo function. Arriving at this conclusion has required an integrative approach, that is, comparing measurements of muscle fiber properties with measurements of fiber use during normal motor activity. Because the muscles of mammals (and humans) generally are heterogenous in fiber type, this makes it technically very difficult to measure either the contractile properties of different fiber types or their use during normal motor activity. Therefore, many of the advances in the understanding of the design and function of vertebrate muscular systems have come from work on lower vertebrates. Fish, because of the anatomic separation of different muscle fiber types, have provided a key experimental model on which much of what is known about muscle design has been determined. Frogs, because of the near homogeneity of their large extensor muscles used during jumping, also provide an important model which will, in the near future, serve as the first platform where molecular properties of muscle (calcium and cross-bridge kinetics) can be related to whole body movement in a meaningful and predictive manner.

Jumping in frogs: assessing the design of the skeletal system by anatomically realistic modeling and forward dynamic simulation.

Comparative musculoskeletal modeling represents a tool to understand better how motor system parameters are fine-tuned for specific behaviors. Frog jumping is a behavior in which the physical properties of the body and musculotendon actuators may have evolved specifically to extend the limits of performance. Little is known about how the joints of the frog contribute to and limit jumping performance. To address these issues, we developed a skeletal model of the frog Rana pipiens that contained realistic bones, joints and body-segment properties. We performed forward dynamic simulations of jumping to determine the minimal number of joint degrees of freedom required to produce maximal-distance jumps and to produce jumps of varied take-off angles. The forward dynamics of the models was driven with joint torque patterns determined from inverse dynamic analysis of jumping in experimental frogs. When the joints were constrained to rotate in the extension-flexion plane, the simulations produced short jumps with a fixed angle of take-off. We found that, to produce maximal-distance jumping, the skeletal system of the frog must minimally include a gimbal joint at the hip (three rotational degrees of freedom), a universal Hooke's joint at the knee (two rotational degrees of freedom) and pin joints at the ankle, tarsometatarsal, metatarsophalangeal and iliosacral joints (one rotational degree of freedom). One of the knee degrees of freedom represented a unique kinematic mechanism (internal rotation about the long axis of the tibiofibula) and played a crucial role in bringing the feet under the body so that maximal jump distances could be attained. Finally, the out-of-plane degrees of freedom were found to be essential to enable the frog to alter the angle of take-off and thereby permit flexible neuromotor control. The results of this study form a foundation upon which additional model subsystems (e.g. musculotendon and neural) can be added to test the integrative action of the neuromusculoskeletal system during frog jumping.

Functional morphology of proximal hindlimb muscles in the frog Rana pipiens.

Musculoskeletal models have become important tools in understanding motor control issues ranging from how muscles power movement to how sensory feedback supports movements. In the present study, we developed the initial musculotendon subsystem of a realistic model of the frog Rana pipiens. We measured the anatomical properties of 13 proximal muscles in the frog hindlimb and incorporated these measurements into a set of musculotendon actuators. We examined whether the interaction between this musculotendon subsystem and a previously developed skeleton/joint subsystem captured the passive behavior of the real frog's musculoskeletal system. To do this, we compared the moment arms of musculotendon complexes measured experimentally with moment arms predicted by the model. We also compared sarcomere lengths measured experimentally at the starting and take-off positions of a jump with sarcomere lengths predicted by the model at these same limb positions. On the basis of the good fit of the experimental data, we used the model to describe the multi-joint mechanical effects produced by contraction of each hindlimb muscle and to predict muscle trajectories during a range of limb behaviors (wiping, defensive kicking, swimming and jumping). Through these analyses, we show that all hindlimb muscles have multiple functions with respect to accelerating the limb in its three-dimensional workspace and that the balance of functions depends greatly on limb configuration. In addition, we show that muscles have multiple, task-specific functions with respect to the type of contraction performed. The results of this study provide important data regarding the multifunctional role of hindlimb muscles in the frog and form a foundation upon which additional model subsystems (e.g. neural) and more sophisticated muscle models can be appended.