Lawrence C. Rome

Maximum velocity of shortening of three fibre types from horse soleus muscle: implications for scaling with body size.

1. To explore how maximum velocity of shortening (Vmax) of fibres varies within one muscle and how Vmax varies with body size, we measured Vmax of muscle fibres from soleus muscle of a large animal, the horse. 2. Vmax was determined by the slack test on skinned single muscle fibres at 15 degrees C during maximal activation (pCa = 5.2). The fibre type was subsequently determined by a combination of single‐cell histochemistry and gel electrophoresis of the myosin light chains. 3. Vmax values for the type I, IIA and IIB muscle fibres were 0.33 +/‐ 0.04 muscle lengths/s (ML/s) (+/‐ S.E.M., n = 6), 1.33 +/‐ 0.08 ML/s (n = 7) and 3.20 +/‐ 0.26 ML/s (n = 6), respectively. It is likely that the large range in Vmax is due to differences observed in the myosin heavy chains and light chains associated with the three fibre types. 4. Comparison of Vmax over a 1200‐fold range (450 kg horse vs. 0.38 kg rat) of body mass (Mb) suggests that slow fibres scale more dramatically (Mb‐0.18) than do fast glycolytic fibres (Mb‐0.07). This difference may enable the slow fibres to work at high efficiencies in the large animal while the fast fibres can still generate a large mechanical power when necessary.

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.

The influence of temperature on mechanics of red muscle in carp.

1. We measured the influence of temperature on maximum velocity of shortening (Vmax) of red muscle in carp in order to better understand the influence of temperature on locomotory performance. 2. A stable red muscle bundle preparation containing about 100 muscle fibres was developed. The bundles could not be activated directly by electrical stimulation, but rather contained sufficient nervous tissue so that acetylcholine released from the nerve terminals caused activation of the muscle. A high level of activation was achieved (116 kN/m2) by adding a combination of a 1 mM‐caffeine and 10(‐5) g/ml eserine to physiological Ringer solution and electrically stimulating the preparation. 3. Force‐velocity characteristics were determined at 10 and 20 degrees C by the force clamp method. The data were well fitted by a hyperbola not constrained to pass through P0 = 1 (where P0 is the isometric force). The mean Vmax at 10 degrees C was 3.55 +/‐ 0.26 muscle lengths/s (ML/s) (n = 6) and at 20 degrees C, 5.71 +/‐ 0.29 ML/s (n = 6). The mean Q10 for Vmax was 1.63 +/‐ 0.07 (n = 6). The a/P0* (Hill constant) and Po* (where P0* is the extrapolated load at zero velocity) were 0.49 +/‐ 0.06 (n = 6) and 1.19 +/‐ 0.04 (n = 6) respectively at 10 degrees C and 0.29 +/‐ 0.06 (n = 6) and 1.51 +/‐ 0.20 (n = 6) respectively at 20 degrees C. 4. The mean Q10 for maximum isometric tension was 1.13 +/‐ 0.02 (n = 6). The maximal power generation was 59.7 +/‐ 2.3 W/kg (n = 6) at 10 degrees C and 94.3 +/‐ 3.2 W/kg (n = 6) at 20 degrees C representing a Q10 of 1.58. The Q10 is less than the product of Q10s for P0 and Vmax because of the greater curvature of the force‐velocity curve at 20 degrees C. 5. The 1.63‐fold higher Vmax at 20 degrees C than at 10 degrees C enables fish to swim with a 1.6‐fold faster muscle shortening velocity, V, at the higher temperature. Thus at both 10 and 20 degrees C, red muscle is used only over the same narrow range of V/Vmax (0.18‐0.36), where isolated muscle experiments suggest that power and efficiency are maximal. Thus V/Vmax appears to be an effective design constraint which limits the range of velocities over which muscle is used in vivo at different temperatures.

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.

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.

Quantitative analysis of muscle fibre type and myosin heavy chain distribution in the frog hindlimb: implications for locomotory design.

To investigate the design of the frog muscular system for jumping, fibre type distribution and myosin heavy chain (MHC) isoform composition were quantified in the hindlimb muscles of Rana pipiens. Muscles were divided into two groups: five large extensor muscles which were predicted to shorten and produce mechanical power during jumping (JP), and four much smaller muscles commonly used in muscle physiology studies, but that do not shorten or produce power during jumping (NJP). Fibres were classified as one of four different types (type 1, 2, 3 or tonic) or an intermediate type (type 1–2) based on␣their relative myosin-ATPase reactivity and MHC immunoreactivity in muscle cross-sections according to previous nomenclature established for amphibian skeletal muscle. Type 1 fibres correspond to the fastest and most powerful of the twitch fibres, and type 3 fibres are the slowest and least powerful. Myosin-ATPase histochemistry revealed that the JP muscles were co mposed primarily of type 1 fibres (89%) with a small percentage of type 2 (7%) and intermediate type 1–2 fibres (4%). The fibre type composition of NJP muscles was more evenly distributed between type 1 (29%), type 2 (46%) and type 1–2 (24%) fibres. Tonic fibres comprised less than 2% of the muscle cross-section in both JP and NJP groups. Similarly, MHC composition determined by quantitative SDS–PAGE revealed that JP muscles were composed predominantly of type 1 MHC (86%), with a balance of type 2 MHC (14%). The opposite pattern was found for MHC composition in the NJP muscles: type 1 (28%), type 2 (66%) and type 3 (6%). These results demonstrate that the large extensor muscles that produce the power required for jumping have a fibre type distribution that enables them to generate high levels of mechanical power, with the type 1 isoform accounting for 85–90% of the total M HC content.

Some advances in integrative muscle physiology.

Integrative muscle physiology has evolved from black box correlations to an understanding of how muscular systems are designed at the molecular level. This paper traces some of the obstacles facing integrative muscle physiology and some of the intellectual and technological breakthroughs which led to the fields development. The ability to determine (1) which fiber types are active, (2) over what sarcomere lengths and velocities they shorten during locomotion and (3) their respective force-velocity relationships, enabled us to show that many muscular systems are designed so that muscles operate at optimal myofilament overlap and at optimal V/Vmax (where maximum power is generated). The ability to impose the in vivo length change and stimulation pattern on isolated muscle has further showed that fish muscle has a relatively slow relaxation rate, and thus rather than generating maximum power during swimming, the muscle appears designed to generate power efficiently. By contrast, during the single shot jump, frog muscle remains maximally activated during shortening and generates maximum power. Recently biophysical techniques have shown that relaxation rate can be altered during evolution by changing (1) Ca2+ transient duration; (2) Ca(2+)-troponin kinetics, and (3) crossbridge kinetics. New technologies will soon enable us to better appreciate how different animal designs evolved.

Superfast contractions without superfast energetics: ATP usage by SR-Ca2+ pumps and crossbridges in toadfish swimbladder muscle

1 The rate at which an isometrically contracting muscle uses energy is thought to be proportional to its twitch speed. In both slow and fast muscles, however, a constant proportion (25‐40 %) of the total energy has been found to be used by SR‐Ca2+ pumps and the remainder by crossbridges. We examined whether SR‐Ca2+ pumps account for a larger proportion of the energy in the fastest vertebrate muscle known (the toadfish swimbladder), and whether the swimbladder muscle utilizes energy at the superfast rate one would predict from its mechanics. 2 The ATP utilization rates of the SR‐Ca2+ pumps and crossbridges were measured using a coupled assay system on fibres skinned with saponin. Surprisingly, despite its superfast twitch speed, the ATP utilization rate of swimbladder was no higher than that of much slower fast‐twitch amphibian muscles. 3 The swimbladder achieves tremendous twitch speeds with a modest steady‐state ATP utilization rate by employing two mechanisms: having a small number of attached crossbridges and probably utilizing intracellular Ca2+ buffers (parvalbumin) to spread out the time over which Ca2+ pumping can occur. 4 Finally, although the total ATP utilization rate was not as rapid as expected, the relative proportions used by SR‐Ca2+ pumps and the crossbridges were similar to other muscles.

VARIATIONS OF PULSE REPETITION RATE IN BOATWHISTLE SOUNDS FROM OYSTER TOADFISH OPSANUS TAU AROUND WAQUOIT BAY, MASSACHUSETTS

ABSTRACT The boatwhistle sound produced by male toadfish during the reproductive season attracts females to the nest site. Boatwhistles consist of a series of rapidly produced, amplitude modulated ‘pulses’ of sound that are generated by specific muscles of the gas bladder. Previous studies have shown that boatwhistle characteristics vary with temperature and geographic location. This study investigated the normal range of variation in boatwhistles produced by males in a northern population of O. tau. Multiple boatwhistles were recorded from individuals at different sites around Waquoit Bay, Massachusetts. Multiple boatwhistles were recorded at different sites to assess the range of variation in boatwhistle production from this population. Pulse repetition rates varied from a low of 125 pulses/sec at 16 °C to a high of 219 pulses/sec at 21 °C. Careful examination of recordings from different sites indicated that individuals vary in the durations of their boatwhistles as well as in the pulse repetition rates during the very consistently produced second segment of the call. In particular, pulse repetition rate (PRR) varied significantly (p < 0.001) among most individuals recorded at the same temperatures. Psychophysical testing as well as behavioral choice experiments are needed to assess the relative importance of PRR and spectral cues in species recognition and/or mate choice.

Muscle Activity in Steady Swimming Scup, Stenotomus chrysops, Varies With Fiber Type and Body Position

The red and pink aerobic muscle fibers are used to power steady swimming in fishes. We examined red and pink muscle recruitment and function during swimming in scup, Stenotomus chrysops, through electromyography and high-speed ciné. Computer analysis of electromyograms (EMGs) allowed determination of initial speed of muscle recruitment and duty cycle and phase of muscle electromyographic activity for both fiber types. This analysis was carried out for three longitudinal positions over a range of swimming speeds. Fiber type and longitudinal position both affected swimming speed of initial recruitment. Posterior muscle is recruited at the lowest swimming speed, whereas more anterior muscle is not initially recruited until higher speeds. At more anterior positions, the initial recruitment of pink muscle occurs at a higher swimming speed than the recruitment of red muscle. The duty cycle of pink muscle EMG activity is significantly shorter than that of red muscle, reflecting a difference in the onset time of activation during each cycle of length change: pink muscle onset time follows that of red. The different patterns of usage of red and pink muscle reflect differences in their contraction kinetics. Because pink muscle generates force more rapidly than red muscle, it can be activated later in each tailbeat cycle. Pink muscle is used to augment red muscle power production at higher swimming speeds, allowing a higher aerobically based steady swimming speed than that possible by red muscle alone.