Assessment of Neuromuscular Function Using Percutaneous Electrical Nerve Stimulation

Percutaneous electrical nerve stimulation is a non-invasive method commonly used to evaluate neuromuscular function from brain to muscle (supra-spinal, spinal and peripheral levels). The present protocol describes how this method can be used to stimulate the posterior tibial nerve that activates plantar flexor muscles. Percutaneous electrical nerve stimulation consists of inducing an electrical stimulus to a motor nerve to evoke a muscular response. Direct (M-wave) and/or indirect (H-reflex) electrophysiological responses can be recorded at rest using surface electromyography. Mechanical (twitch torque) responses can be quantified with a force/torque ergometer. M-wave and twitch torque reflect neuromuscular transmission and excitation-contraction coupling, whereas H-reflex provides an index of spinal excitability. EMG activity and mechanical (superimposed twitch) responses can also be recorded during maximal voluntary contractions to evaluate voluntary activation level. Percutaneous nerve stimulation provides an assessment of neuromuscular function in humans, and is highly beneficial especially for studies evaluating neuromuscular plasticity following acute (fatigue) or chronic (training/detraining) exercise.     

Coexistence of twitch potentiation and tetanic force decline in rat hindlimb muscle

An experimental protocol designed to assess fatigability in motor units has been applied to two hindlimb muscles of anesthetized adult rats to study the effects of whole-muscle fatigue on the isometric twitch. Both soleus and extensor digitorum longus exhibited a linear relationship between fatigability (i.e., force decline after a 360-s fatigue test) and the magnitude of the twitch force following the fatigue test. Twitch force after the fatigue test was potentiated (i.e., greater than the value before the fatigue test) in many muscles, despite the development of considerable fatigue. This coexistence of fatigue and twitch potentiation was observed in 7% (5/70) of soleus and 48% (31/64) of extensor digitorum longus muscles. The coexistence was exhibited only by the least fatigable muscles of the fast-contracting extensor digitorum longus. The extensor digitorum longus muscles that did not exhibit twitch potentiation probably experienced a higher proportion of muscle-fiber inactivation, such as due to failure of neuromuscular propagation, that was induced by the fatigue regimen.     

Effect of aging on fatigue characteristics of elbow flexor muscles during sustained submaximal contraction.

The purpose of this study was to compare fatigue-related measures of central and peripheral mechanisms between young and elderly subjects for a task performed with elbow flexor muscles. Ten young and nine elderly subjects performed a sustained submaximal fatigue task at 35% of their maximum voluntary contraction torque. Measures of neuromuscular function, reflecting changes in neuromuscular propagation, voluntary activation, excitation-contraction-relaxation processes, and metabolite buildup, were taken before, during, and after the fatigue task. The main results were the absence of neuromuscular propagation failure in either young or elderly subjects, the presence of central fatigue at the end of the fatigue task in 7 of 9 elderly but only 3 of 10 young subjects, and lesser changes in twitch torque contraction-relaxation variables and electromyographic median frequency in elderly compared with young subjects. The lesser fatigue-related changes in twitch contraction speed and median frequency in elderly compared with young subjects could reflect the increase in type I-to-type II fiber area reported with old age. The presence of significant central fatigue can apparently minimize some of the potential differences present in peripheral fatigue sites.     

Neuromuscular fatigue of elbow flexor muscles of dominant and non-dominant arms in healthy humans.

The purpose of this study was to assess differences in fatigue-related changes in variables related to structures within the neuromuscular system, between the dominant and non-dominant elbow flexor muscles of right-handed individuals. Two experimental sessions were performed on the right arm and one on the left arm. For each session, maximum voluntary torque, level of voluntary activation, M-wave amplitude, twitch/train or twitch/doublet torque ratio and EMG median frequency were obtained before and up to 20 min after a sustained maximum isometric fatigue task. Our main results were: 1) reproducible fatigue-induced changes in all variables of interest between the two sessions performed with the right arm, 2) significantly greater failure in voluntary activation and neuromuscular propagation with sustained activity for the non-dominant compared with dominant side, and 3) no effect of dominance on MVC torque, endurance time, and fatigue-induced changes in EMG median frequency and elicited torques. These results suggest that the preferential use of elbow flexor muscles with the dominant arm leads to more fatigue resistance in certain structures/mechanisms of the neuromuscular system, but not in others.     

Anatomy and histochemistry of hindlimb flight posture in birds. I. The extended hindlimb posture of shorebirds

Birds utilize one of two hindlimb postures during flight: an extended posture (with the hip and knee joints flexed, while the ankle joint is extended caudally) or a flexed posture (with the hip, knee, and ankle joints flexed beneath the body). American Avocets (Recurvirostra americana) and Black‐necked Stilts (Himantopus mexicanus) extend their legs caudally during flight and support them for extended periods. Slow tonic and slow twitch muscle fibers are typically found in muscles functioning in postural support due to the fatigue resistance of these fibers. We hypothesized that a set of small muscles composed of high percentages of slow fibers and thus dedicated to postural support would function in securing the legs in the extended posture during flight. This study examined the anatomy and histochemical profile of eleven hindlimb muscles to gain insight into their functional roles during flight. Contrary to our hypothesis, all muscles possessed both fast twitch and slow twitch or slow tonic fibers. We believe this finding is due to the versatility of dynamic and postural functions the leg muscles must facilitate, including standing, walking, running, swimming, and hindlimb support during flight. Whether birds use an extended or flexed hindlimb flight posture may be related to the aerodynamic effect of leg position or may reflect evolutionary history. J. Morphol., 2008. © 2008 Wiley‐Liss, Inc.     

Fatiguing Exercise Intensity Influences the Relationship between Parameters Reflecting Neuromuscular Function and Postural Control Variables

The purpose of this study was to investigate the influence of fatiguing exercise intensity on the nature and extent of fatigue-induced changes in neuromuscular function and postural stability in quiet standing. We also explored the contribution of selected neuromuscular mechanisms involved in force production to postural stability impairment observed following fatigue using an approach based on multivariate regressions. Eighteen young subjects performed 30-s postural trials on one leg with their eyes closed. Postural trials were performed before and after fatiguing exercises of different intensities: 25, 50 and 75% of maximal isometric plantarflexor torque. Fatiguing exercises consisted of sustaining a plantarflexor isometric contraction at the target intensity until task failure. Maximal isometric plantarflexor torque, electromyographic activity of plantarflexor and dorsiflexor muscles, activation level (twitch interpolation technique) and twitch contractile properties of plantarflexors were used to characterize neuromuscular function. The 25% exercise was associated with greater central fatigue whereas the 50 and 75% exercises involved mostly peripheral fatigue. However, all fatiguing exercises induced similar alterations in postural stability, which was unexpected considering previous literature. Stepwise multiple regression analyses showed that fatigue-related changes in selected parameters related to neuromuscular function could explain more than half (0.51≤R²≤0.82) of the changes in postural variables for the 25% exercise. On the other hand, regression models were less predictive (0.17≤R²≤0.73) for the 50 and 75% exercises. This study suggests that fatiguing exercise intensity does not influence the extent of postural stability impairment, but does influence the type of fatigue induced and the neuromuscular function predictors explaining changes in postural variables.     

Mass and functional capacity of regenerating muscle is enhanced by myoblast transfer

Morphology and functional capacity of homotopically transplanted extensor digitorum longus muscles (EDL) of adult SCID mice that received 1 × 10⁶ myoblasts [stably transfected to express nuclear localizing β-galactosidase under the control of the myosin light-chain 3F promoter/enhancer] 2 days posttransplantation were evaluated 9 weeks after transplantation, to determine whether the injection of exogenous myoblasts had an effect on muscle regeneration. Regenerated muscles that received exogenous myoblasts were compared to similarly transplanted muscles that received (a) no further treatment, or (b) sham injection of the vehicle (without myoblasts) and to unoperated EDL. Nine weeks after myoblast transfer, myofibers containing donor-derived nuclei could be identified after staining with X-gal solution. Judging from its size and poor functional performance compared to muscles subjected to transplantation only, sham injection provided a secondary trauma to the regenerating muscle from which it failed to fully recover. In comparison to the sham-injected muscle, the myoblast-injected muscles weighed 61% more and had 50% more myofibers and 82% more cross-sectional area occupied by myofibers at the muscles' widest girths. Their absolute twitch and tetanic tensions were threefold and twofold greater, respectively, and their specific twitch and tetanic tensions were 71% and 50% greater, respectively, than those of sham-injected muscles. In many parameters, the regenerating muscle subjected to myoblast transfer equaled or exceeded those of muscles that were transplanted only received only one trauma). Absolute twitch and tetanic tensions were 73% and 65% greater, respectively, and specific twitch tensions of the muscles receiving myoblasts were 50% greater than forces generated by muscles subjected to whole-muscle transplantation only. © 1997 John Wiley & Sons, Inc. J Neurobiol 33: 185–198, 1997     

α-Tocopherol Modifies Lead Induced Functional Changes at Murine Neuromuscular Junction

Lead impacts neuromuscular junction and might induce skeletal muscle weakness. Antioxidants may prevent toxic actions of lead on muscle. In this study, resting membrane potentials, endplate potentials, miniature endplate potentials (MEPPs) and isometric twitch tensions were recorded to investigate effects of α-tocopherol (Vitamin E) on lead induced changes at murine dorsiflexor muscle. Moreover, levels of endplate nicotinic receptors were measured by receptor autoradiography. Forty rats were divided into four groups (lead alone, α-tocopherol, lead plus α-tocopherol and saline). Lead (1?mg/kg, i.p.), was administered daily for 2 weeks and α-tocopherol (100?mg/kg, i.p.) was given daily for 3 weeks. Lead treatment significantly reduced twitch tension (from 4.4±0.4 to 2.2±0.3?g) and delayed half time of decay. MEPP frequencies and quantal content were also significantly reduced after lead treatment. Pretreatment with α-tocopherol reversed twitch tension reduction (4.1±0.3?g) and modified lead induced delay in half time of decay. Similarly, α-tocopherol modified the negative actions of lead exposure on MEPP frequencies and quantal content. Receptor autoradiographic studies revealed significant increase of nicotinic receptor levels at the endplate region of flexor muscle in lead treated mice. However, animals treated with lead plus α-tocopherol showed significantly decreased levels of nicotinic receptors. α-Tocopherol appears to protect against lead induced neuromuscular dysfunction. These effects of α-tocopherol are possibly mediated via a free radical mechanism or modification of calcium homeostasis.     

Fatigability of rat hindlimb muscles after acute irreversible acetylcholinesterase inhibition.

The purpose of this study was to investigate the functional impact of acute irreversible inhibition of acetylcholinesterase (AChE) on the fatigability of medial gastrocnemius and plantaris muscles of Sprague-Dawley rats. After treatment with methanesulfonyl fluoride (a lipid-soluble anticholinesterase), which reduced their AChE activity by >90%, these muscles were subjected to an in situ indirect stimulation protocol, including a series of isolated twitch and tetanic contractions preceding a 3-min fatigue regimen (100-ms trains at 75 Hz applied every 1.5 s). During the first minute of the fatigue regimen, the effects of AChE inhibition were already near maximal, including marked reductions in peak tension and the force-time integral (area), as well as a decrement of compound muscle action potential amplitudes within a stimulus train. Neuromuscular transmission failure was the major contributor of the force decreases in the AChE-inhibited muscles. However, despite this neuromuscular transmission failure, muscles of which all AChE molecular forms were nearly completely inhibited were still able to function, although abnormally, during 3 min of intermittent high-frequency nerve stimulation.     

Genetic inactivation of acetylcholinesterase causes functional and structural impairment of mouse soleus muscles

Acetylcholinesterase (AChE) plays an essential role in neuromuscular transmission. Not surprisingly, neuromuscular transmission during repetitive nerve stimulation is severely depressed in the AChE knockout mouse (KO). However, whether this deficit in AChE leads to skeletal muscle changes is not known. We have studied the in vitro contractile properties of the postural and locomotor soleus muscles of adult KO and normal (wildtype, WT) mice, and this was completed by histological and biochemical analyses. Our results show that muscle weight, cross-sectional area of muscle fibres and absolute maximal isometric force are all reduced in KO mice compared with WT mice. Of interest, the relative amount of slow myosin heavy chain (MHC-1) in muscle homogenates and the percentage of muscle fibres expressing MHC-1 are decreased in the KO mice. Surprisingly, AChE ablation does not modify twitch kinetics, absolute maximal power, fatigue resistance or citrate synthase activity, despite the reduced number of slow muscle fibres. Thus, a deficit in AChE leads to alterations in the structure and function of muscles but these changes are not simply related to the reduced body weight of KO mice. Our results also suggest that this murine model of congenital myasthenic syndrome with endplate AChE deficiency combines alterations in both neurotransmission and intrinsic muscle properties.     

Relationship between numbers and frequencies of stimuli in human muscle fatigue

The effect of stimulus frequency on the rate of muscle fatigue has been studied on dorsiflexor muscles of the human ankle. It was found that significantly fewer stimuli were required to abolish twitch and tetanic torque when the stimuli were delivered at 15 Hz rather than 30 Hz. At both stimulus frequencies twitch torque disappeared before tetanic torque. The difference in numbers of stimuli required for fatigue was not due to impaired excitation of muscle fibers at either of the two frequencies. At both stimulating frequencies, twitch fatigue appeared to be due to a defect in excitation-contraction coupling and/or the contractile machinery.     

Recovery after intense chronic stimulation: a physiological study of cat's fast muscle

Adult cats were used to study the recovery of muscles that had become altered by long-term electrical stimulation. Chronic activation was delivered to the deafferented common peroneal nerve (no pain, no reflexes), and contractile properties were measured for peroneus longus muscle. After 4 wk of great daily amounts of treatment at moderately high pulse rates (30-40 Hz delivered during 50% of daily time), the peroneus longus became considerably weaker, demonstrated a longer time course of twitches and a slower rate of rise of tetanic force, and became less fatigable. Furthermore, its twitch-to-tetanus ratio decreased, and there was no longer any depression of electromyogram (EMG) amplitude during fatigue tests. After 4 wk of subsequent rest it was found that 1) twitch speed and maximum tetanic force had returned to nearly normal values, 2) fatigue resistance showed some return toward normal but was still significantly enhanced, and 3) no significant recovery had yet occurred of the altered twitch-to-tetanus ratio, the abolished EMG depression, or the slowed rate of rise of tetanic tension. During the poststimulation recovery period, the progressive increase of isometric twitch speed was not promoted by the administration of small daily amounts of high-rate stimulation (100-Hz bursts). The results support the conclusions that 1) the time course of recovery differs among physiological properties, 2) the EMG and force reactions that occur during a fatigue test are not strongly coupled, as demonstrated by the alterations of their relationship during poststimulation recovery, and 3) in cat's fast muscles, there is still no evidence for rate-specific effects of chronic stimulation on isometric twitch speed.     

The peripheral nerve allograft in the primate immunosuppressed with Cyclosporin A: II. Functional evaluation of reinnervated muscle.

Isometric contractile function was evaluated in primates receiving peripheral nerve allografts and autografts. Twelve adult male cynomolgus monkeys received both sural nerve allografts and autografts to the ulnar nerve in opposite forearms. Half the animals received Cyclosporin A (CsA) immunosuppression (25 mg/kg per day); the remaining animals received placebo. One year following nerve engraftment, isometric contractile muscle function was evaluated in reinnervated abductor digiti quinti and intact abductor pollicis brevis muscles. Maximal twitch tension (Pt), tetanic tension (P(o)), time to peak tension (tpt), rate of rise of twitch tension (DP/dt), and muscle fatigue were evaluated at optimal muscle length (L(o)). All reinnervated muscles distal to nerve autografts and allografts in both Cyclosporin A-immunosuppressed and placebo-treated animals generated equivalent maximal twitch tension, tetanic tension, and time to peak tension, with no significant difference between groups (p > 0.05 by ANOVA). There was a tendency toward increased muscle fatiguability in Cyclosporin A-treated animals (p > 0.05). However, the rate of rise of twitch tension was significantly faster in the reinnervated and intact muscles of Cyclosporin A-treated primates (p < 0.05). Evidence of excellent functional reinnervation across nerve allografts and autografts similar to that seen in histologic and electrophysiologic studies was noted. Cyclosporin A immunosuppression did not significantly enhance recovery of muscle function distal to nerve allografts in this model.     

Twitch potentiation during fatiguing exercise in the elderly: the effects of training.

Twitch potentiation was studied during a fatigue paradigm involving intermittent maximum voluntary contractions (MVCs) of the tibialis anterior muscle in the elderly and in young adults. Resting twitch torques were similar between groups, but twitch potentiation was significantly greater (241% vs 166%) in the young; the recovery of the twitch after fatigue was similar between groups. Contraction time, time to peak torque and half-relaxation time were all significantly slower in the elderly. Following 12 weeks of resistance training in the elderly, there was no significant change in the twitch contractile properties at rest, but there was a significant main effect of training on the degree of twitch potentiation during the same fatigue protocol (peak potentiation 192% post-training vs 165% pretraining). These data suggest that the mechanism(s) responsible for twitch potentiation following MVCs may be influenced by both aging and training.     

Differential effects of post-natal development, animal strain and long term recovery on the restoration of neuromuscular function after neuromyotoxic injury in rat

We have analysed the effect of long term recovery, post-natal development and animal strain on the extent of restoration of neuromuscular function after neuromyotoxic injury in the rat (Rattus norvegicus). Muscle isometric contractile properties of soleus muscle in response to nerve stimulation were measured in situ in snake venom injured muscles and compared to contralateral uninjured muscles. We show here that neuromuscular function was not fully recovered until 24 weeks after injury in young adult (2-3 month old) Wistar rats. Moreover, the level of functional recovery 3 weeks after injury induced in juvenile rats (1 month old) was not globally different from that in younger adult, adult (10 month old) and older adult (24 month old) Wistar rats. Furthermore, the level of recovery of some contractile parameters differed between Wistar and Sprague-Dawley strains 3 weeks after injury. In conclusion, a very long time (>12 weeks) is required for full neuromuscular recovery following neuromyotoxic injury of young adult rats. Moreover, neuromuscular recovery during post-natal development is not markedly different from that during adult stage in the Wistar rat strain. Finally, some rat strain differences are observed in the recovery after injury of young adult rats.     

Effect of clonidine on neuromuscular transmission and the nicotinic acetylcholine receptor

The effects of clonidine on neuromuscular transmission were investigated in the mouse phrenic nerve-diaphragms and chicken biventer cervicis. Clonidine inhibited the indirect twitch response dose-dependently and reversibly without an effect on the direct response of the muscles to electrical stimulation and KCl. This effect was antagonized effectively by diaminopyridine but not by yohimbine, phentolamine or physostigmine. The quantal content was not affected although the amplitudes of end-plate potential (epp) and spontaneous miniature epp (mepp) were markedly depressed. Clonidine also decreased the slope of the ACh dose-response curve and maximal response in denervated mouse diaphragms as well as the carbachol response in the chinck muscle. In the latter, ACh response was not depressed by clonidine probably because of its inherent anticholinesterase activity. Clonidine facilitated the fading of ACh-contracture either in mouse or chick muscle. It is concluded that clonidine impairs the neuromuscular transmission by a noncompetitive blockade of ACh receptors, most likely affecting the ACh channel but not the recognition site of the ACh receptor. Its inhibitory effect is not mediated by alpha 2-adrenoceptor, suggesting that there is no alpha 2-adrenoceptor on the motor nerve terminal to modulate the transmitter release.     

Reappraisal of VAChT‐Cre: Preference in slow motor neurons innervating type I or IIa muscle fibers

VAChT‐Cre.Fast and VAChT‐Cre.Slow mice selectively express Cre recombinase in approximately one half of postnatal somatic motor neurons. The mouse lines have been used in various studies with selective genetic modifications in adult motor neurons. In the present study, we crossed VAChT‐Cre lines with a reporter line, CAG‐Syp/tdTomato, in which synaptophysin‐tdTomato fusion proteins are efficiently sorted to axon terminals, making it possible to label both cell bodies and axon terminals of motor neurons. In the mice, Syp/tdTomato fluorescence preferentially co‐localized with osteopontin, a recently discovered motor neuron marker for slow‐twitch fatigue‐resistant (S) and fast‐twitch fatigue‐resistant (FR) types. The fluorescence did not preferentially co‐localize with matrix metalloproteinase‐9, a marker for fast‐twitch fatigable (FF) motor neurons. In the neuromuscular junctions, Syp/tdTomato fluorescence was detected mainly in motor nerve terminals that innervate type I or IIa muscle fibers. These results suggest that the VAChT‐Cre lines are Cre‐drivers that have selectivity in S and FR motor neurons. In order to avoid confusion, we have changed the mouse line names from VAChT‐Cre.Fast and VAChT‐Cre.Slow to VAChT‐Cre.Early and VAChT‐Cre.Late, respectively. The mouse lines will be useful tools to study slow‐type motor neurons, in relation to physiology and pathology.     

Limiting mechanisms of force production after repetitive dynamic contractions in human triceps surae.

The influence of repetitive dynamic fatiguing contractions on the neuromuscular characteristics of the human triceps surae was investigated in 10 subjects. The load was 50% of the torque produced during a maximal voluntary contraction, and the exercise ended when the ankle range of motion declined to 50% of control. The maximal torque of the triceps surae and the electromyographic (EMG) activities of the soleus and medial gastrocnemius were studied in response to voluntary and electrically induced contractions before and after the fatiguing task and after 5 min of recovery. Reflex activities were also tested by recording the Hoffmann reflex (H reflex) and tendon reflex (T reflex) in the soleus muscle. The results indicated that whereas the maximal voluntary contraction torque, tested in isometric conditions, was reduced to a greater extent (P < 0.05) at 20 degrees of plantar flexion (-33%) compared with the neutral position (-23%) of the ankle joint, the EMG activity of both muscles was not significantly reduced after fatigue. Muscle activation, tested by the interpolated-twitch method or the ratio of the voluntary EMG to the amplitude of the muscle action potential (M-wave), as well as the neuromuscular transmission and sarcolemmal excitation, tested by the M-wave amplitude, did not change significantly after the fatiguing exercise. Although the H and T reflexes declined slightly (10-13%; P < 0.05) after fatigue, these adjustments did not appear to have a direct deleterious effect on muscle activation. In contrast, alterations in the mechanical twitch time course and postactivation potentiation indicated that intracellular Ca(2+)-controlled excitation-contraction coupling processes most likely played a major role in the force decrease after dynamic fatiguing contractions performed for short duration.     

Fatigue and contraction of slow and fast muscles in hypokinetic/hypodynamic rats

This investigation examined the effects of hypokinesia/hypodynamia (H/H) on fatigability and contractile properties of rat soleus (S) and gastrocnemius (G) muscles. Whole-body suspension for 1 wk was used to eliminate hindlimb load-bearing functions and simultaneously permit voluntary isotonic contractions. Train stimulations (45/min, 16 min) resulted in significantly (P less than 0.05) faster rates of fatigue to lower asymptotes in G from H/H rats. Fatigue in the S was minimal at this stimulation frequency and differences between H/H and control animals were not significant. Contractile properties (twitch and tetanic) were measured before and after train stimulations. H/H suspension resulted in an increased twitch tension in G. However, H/H did not change train or tetanic tensions per gram or other G contractile properties. Peak twitch, train, and tetanic tensions, time to peak tension, one-half relaxation time, and twitch and tetanic peak rates of tension development and decline were unchanged by H/H in S muscles. These results indicate that 1 wk of H/H-induced muscle atrophy significantly increases fatigability in G but does not effect contractile properties of fast-twitch (G) or slow-twitch (S) muscles.     

AXIAL MUSCULATURE IN THE DOLPHIN (TURSIOPS TRUNCATUS): SOME ARCHITECTURAL AND HISTOCHEMICAL CHARACTERISTICS

In view of the supposition that a dolphin can swim faster than would be predicted based on its physical features and presumed muscle power potential, studies were initiated to reevaluate the assumptions made in reaching these conclusions. Several previous studies have shown that the architectural and histochemical properties of a skeletal muscle dictate its force, velocity and displacement properties. This study examined the muscle fiber lengths and tendon arrangements of the dorsal and ventral axial muscles in dolphins ( Tursiops truncatus ). Fiber type and fiber size distributions were determined to reflect the general biochemical characteristics of the musculature. The dorsal muscles had a higher mean fiber length (167 Vs. 90 mm) and the range within and across different dorsal muscles was less (141–199 vs. 37–185 mm) than in the ventral muscles. Both the dorsal and ventral muscles consisted of an overall mean of 50 percent slow twitch and 50 percent fast twitch fiber types. The fast twitch fibers were 67 percent larger (2,200 vs. 1,317 μ m ²) than the slow twitch fibers in the ventral and 38 percent larger (1,213 Vs. 879 μm²) in the dorsal muscles. In addition, the mean cross sectional area of the fibers in the ventral muscles was approximately 65 percent greater (1,750 vs. 1,072 μm²) than those in the dorsal muscles. The shorter, larger-diameter fibers of the ventral musculature give it a greater potential for force production for a given amount of muscle mass. In contrast, the dorsal muscles appear to be designed to optimize velocity and displacement, ( i.e. , longer fibers). These findings contribute to the information necessary for the determination of the power potential of the musculature of the dolphin.