An Intermediate in the evolution of superfast sonic muscles

Background

Intermediate forms in the evolution of new adaptations such as transitions from water to land and the evolution of flight are often poorly understood. Similarly, the evolution of superfast sonic muscles in fishes, often considered the fastest muscles in vertebrates, has been a mystery because slow bladder movement does not generate sound. Slow muscles that stretch the swimbladder and then produce sound during recoil have recently been discovered in ophidiiform fishes. Here we describe the disturbance call (produced when fish are held) and sonic mechanism in an unrelated perciform pearl perch (Glaucosomatidae) that represents an intermediate condition in the evolution of super-fast sonic muscles.

Results

The pearl perch disturbance call is a two-part sound produced by a fast sonic muscle that rapidly stretches the bladder and an antagonistic tendon-smooth muscle combination (part 1) causing the tendon and bladder to snap back (part 2) generating a higher-frequency and greater-amplitude pulse. The smooth muscle is confirmed by electron microscopy and protein analysis. To our knowledge smooth muscle attachment to a tendon is unknown in animals.

Conclusion

The pearl perch, an advanced perciform teleost unrelated to ophidiiform fishes, uses a slow type mechanism to produce the major portion of the sound pulse during recoil, but the swimbladder is stretched by a fast muscle. Similarities between the two unrelated lineages, suggest independent and convergent evolution of sonic muscles and indicate intermediate forms in the evolution of superfast muscles.     

The activity pattern of shoulder muscles in subjects with and without subacromial impingement

Altered shoulder muscle activity is frequently believed to be a pathogenetic factor of subacromial impingement (SI) and therapeutic interventions have been directed towards restoring normal motor patterns. Still, there is a lack of scientific evidence regarding the changes in muscle activity in patients with SI. The aim of the study was to determine and compare the activity pattern of the shoulder muscles in subjects with and without SI. Twenty-one subjects with SI and 20 healthy controls were included. Electromyography (EMG) was assessed from eight shoulder muscles from both shoulders during motion. In the symptomatic shoulder, there was a significantly greater EMG activity during abduction in the supraspinatus and latissimus muscles and less activity in serratus anterior compared to the healthy subjects. During external rotation, there was significantly less activity of the infraspinatus and serratus anterior muscles on the symptomatic side compared to the healthy subjects. On the asymptomatic side, the groups showed different muscle activity during external rotation. Our findings of an altered shoulder muscle activity pattern on both the symptomatic and asymptomatic side in patients indicate that the different motor patterns might be a pathogenetic factor of SI, perhaps due to inappropriate neuromuscular strategies affecting both shoulders.     

New insights into pain-related changes in muscle activation revealed by high-density surface electromyography

High-density surface electromyography (HDEMG) is an electrophysiological technique that can be used to quantify the spatial distribution of activity within muscles. When pain-free individuals perform sustained or repetitive tasks, different regions within a muscle become progressively more active; this is thought to reflect a strategy to redistribute the load to different regions, thus limiting localised muscle fatigue. The use of HDEMG has revealed that when people with musculoskeletal pain perform the same tasks, the distribution of activity within the same muscle is usually different, and the same muscle region tends to be active throughout the whole task without progressive activation of different muscle regions. This potentially results in a focal overload of a muscle region, and may contribute to fatigue, localised muscle pain and potentially pain persistence and/or recurrence over time. Interestingly, not all patients with musculoskeletal pain present with this regional alteration in muscle activation, reflecting the heterogeneity of patient presentations. This article will briefly review the technique of HDEMG followed by a review of studies demonstrating spatial redistribution of muscle activity in asymptomatic people during both isometric and dynamic conditions, including functional tasks. Lastly, the article will provide a review of HDEMG studies with a focus on changes in the behaviour of the lumbar erector spine and upper trapezius in people with spinal pain. These studies have revealed subtle changes in the distribution of muscle activity in people with spinal pain, which may have relevance for onset, persistence or recurrence of symptoms and could become a target of novel therapeutic approaches.     

Muscular Activity and Fatigue in Lower-Limb and Trunk Muscles during Different Sit-To-Stand Tests

Sit-to-stand (STS) tests measure the ability to get up from a chair, reproducing an important component of daily living activity. As this functional task is essential for human independence, STS performance has been studied in the past decades using several methods, including electromyography. The aim of this study was to measure muscular activity and fatigue during different repetitions and speeds of STS tasks using surface electromyography in lower-limb and trunk muscles. This cross-sectional study recruited 30 healthy young adults. Average muscle activation, percentage of maximum voluntary contraction, muscle involvement in motion and fatigue were measured using surface electrodes placed on the medial gastrocnemius (MG), biceps femoris (BF), vastus medialis of the quadriceps (QM), the abdominal rectus (AR), erector spinae (ES), rectus femoris (RF), soleus (SO) and the tibialis anterior (TA). Five-repetition STS, 10-repetition STS and 30-second STS variants were performed. MG, BF, QM, ES and RF muscles showed differences in muscle activation, while QM, AR and ES muscles showed significant differences in MVC percentage. Also, significant differences in fatigue were found in QM muscle between different STS tests. There was no statistically significant fatigue in the BF, MG and SO muscles of the leg although there appeared to be a trend of increasing fatigue. These results could be useful in describing the functional movements of the STS test used in rehabilitation programs, notwithstanding that they were measured in healthy young subjects.     

Mutually exclusive muscle designs: the power output of the locomotory and sonic muscles of the oyster toadfish (Opsanus tau)

Animals perform a vast array of motor activities. Although it has generally been accepted that muscles are well suited to the function that they must perform, specialization for performing one function may compromise their ability for carrying out another. We examined this principle in the toadfish muscular system: slow-twitch red and fast-twitch white myotomal muscles are used for powering swimming at relatively low frequencies, while the superfast swimbladder muscle powers mating calls by contracting at 100 Hz. We measured muscle power output over a wide range of frequencies. The red and white locomotory muscles could not generate power over ca. 2.2 and 12 Hz, respectively and, hence, could not power sound production. In contrast, the swimbladder muscle has many specializations that permit it to generate power at frequencies in excess of 100 Hz. However, these specializations drastically reduce its power output at low frequencies: the swimbladder muscle generated only one-twentieth of the power of the red muscle and one-seventh of the power of the white muscle at the frequencies used during swimming. To generate the same total power needed for swimming would require unfeasibly large amounts of swimbladder muscle that could not fit into the fish. Hence, the designs of the swimbladder and locomotory muscles are mutually exclusive.     

FUNCTIONAL ANALYSIS OF SWIM-BLADDER MUSCLES ENGAGED IN SOUND PRODUCTION OF THE TOADFISH

A functional analysis of the striated swim-bladder muscles engaged in the sound production of the toadfish has been performed by simultaneous recording of muscle action potentials, mechanical effects, and sound. Experiments with electrical nerve stimulation were made on excised bladder, while decerebrate preparations were used for studies of reflex activation of bladders in situ. The muscle twitch in response to a single maximal nerve volley was found to be very fast. The average contraction time was 5 msec. with a range from 3 to 8 msec., the relaxation being somewhat slower. The analysis of muscle action potentials with surface electrodes showed that the activity of the muscle fibers running transversely to the long axis of the muscle was well synchronized both during artificial and reflex activation. With inserted metal microelectrodes monophasic potentials of 0.4 msec. rise time and 1.2 to 1.5 msec. total duration were recorded. The interval between peak of action potential and onset of contraction was only 0.5 msec. Microphonic recordings of the characteristic sound effect accompanying each contraction showed a high amplitude diphasic deflection during the early part of the contraction. During relaxation a similar but smaller deflection of opposite phase could sometimes be distinguished above the noise level. The output from the microphone was interpreted as a higher order derivative function of the muscle displacement. This interpretation was supported by complementary experiments on muscle sound in mammalian muscle. The dependence of the sound effects on the rate of muscle contraction was demonstrated by changing the temperature of the preparation and, in addition, by a special series of experiments with repeated stimulation at short intervals. Results obtained by varying the pressure within the bladder provided further evidence for the view that the sound initiated in the muscle is reinforced by bladder resonance. Analysis of spontaneous grunts confirmed the finding of a predominant sound frequency of about 100 per second, which was also found in reflexly evoked grunts. During these, muscle action potentials of the same rate as the dominant sound frequency were recorded, the activity being synchronous in the muscles on both sides. Some factors possibly contributing to rapid contraction are discussed.     

Insect Muscles Innervated by Single Motoneurons: Structural and Biochemical Features

SYNOPSIS. Three locomotory muscles of adult male cockroacheshave been biochemically and structurally compared. All fibersof two muscles are innervated by the same motoneuron; both musclesare monofunctional—used only in running. Fibers of thethird muscle are also innervated by a single, but differentmotoneuron. This muscle is bifunctional— used in bothwalking and flying. Histochemical observations of enzymes associatedwith energy production indicate that the three muscles are eachcomprised of a homogeneous population of fibers. However, qualitativedifferences do correlate with muscle use. The bifunctional muscleshows high oxidative and glycolytic enzyme localization; themonofunctional muscles show a low profile for these enzymes.Quantitative determinations of specific activity for similarenzymes corroborate the histochemical observations. Light andelectron microscope observations also indicate that fibers fromeach of the two muscle groups are structurally homogeneous anddistinct. The two monofunctional muscles parallel one anotherfor all structures examined and are different from the bifunctionalmuscle. The bifunctional muscle fibers have cross-sectionalareas twice that of the monofunctional muscles, have a greatermyofibrillar diffusion distance, and have about half as manyactin per myosin filaments. Stereometric analyses show volumedensities for mitochondria and tracheoles to be five times greater;membrane systems associated with excitation- contraction coupling,however, are half as extensive; and myofibrillar volume about1.3 times less. These data form the basis for studies of denervationand cross-reinnervation, and the role of individual motoneuronsin specifying muscle fiber properties.     

Alanine and aspartate aminotransferase activities in muscles of diabetic rats

Activities of alanine and aspartate aminotransferase in different muscle types and in the liver of streptozotocin diabetic rats were studied 1,2 and 3 days after administering of streptozotocin. It was shown that the activity of both enzymes was elevated in the "white" layer of the vastus lateralis, in the liver and in the heart, whereas it remained unchanged in the "red" layer of the same muscle, in the soleus and the diaphragm. It is concluded that the effect of acute insulin deficiency on the aminotransferase activity in skeletal muscles depends on the muscle fiber composition and does not appear in muscles with a high oxidative potential. These results indicate that muscle fiber composition should be taken into account when evaluating the role of insulin in amino acid metabolism in the muscle.     

No difference in 1RM strength and muscle activation during the barbell chest press on a stable and unstable surface

Exercise or Swiss balls are increasingly being used with conventional resistance exercises. There is little evidence supporting the efficacy of this approach compared to traditional resistance training on a stable surface. Previous studies have shown that force output may be reduced with no change in muscle electromyography (EMG) activity while others have shown increased muscle EMG activity when performing resistance exercises on an unstable surface. This study compared 1RM strength, and upper body and trunk muscle EMG activity during the barbell chest press exercise on a stable (flat bench) and unstable surface (exercise ball). After familiarization, 13 subjects underwent testing for 1RM strength for the barbell chest press on both a stable bench and an exercise ball, each separated by at least 7 days. Surface EMG was recorded for 5 upper body muscles and one trunk muscle from which average root mean square of the muscle activity was calculated for the whole 1RM lift and the concentric and eccentric phases. Elbow angle during each lift was recorded to examine any range-of-motion differences between the two surfaces. The results show that there was no difference in 1RM strength or muscle EMG activity for the stable and unstable surfaces. In addition, there was no difference in elbow range-of-motion between the two surfaces. Taken together, these results indicate that there is no reduction in 1RM strength or any differences in muscle EMG activity for the barbell chest press exercise on an unstable exercise ball when compared to a stable flat surface. Moreover, these results do not support the notion that resistance exercises performed on an exercise ball are more efficacious than traditional stable exercises.     

Electromyography of the gluteal muscles in Hylobates,Pongo, and pan: Implications for the evolution of hominid bipedality

The gluteal musculature of primates has been a focus of great research interest among those who study human evolution. Most current theorists agree that gluteus superficialis (= maximus) need not have changed its action in the step from pongid to hominid, but dispute has arisen over a purported change in action and role of the gluteus medius. To clarify the functions of gluteus medius, gluteus superficialis, and tensor fasciae femoris during ape locomotion, we conducted a telemetered electromyographic study of these muscles in two gibbons, one orangutan, and four chimpanzees as they walked bipedally on the ground and on a horizontal tree trunk, walked quadrupedally on the same substrates, and climbed a vertical tree trunk. The results indicate that the gluteus medius of apes is not, as has been previously suggested, primarily an extensor of the thigh; its action is chiefly that of medial rotation. The role of the gluteus medius during bipedality is the same in apes and humans–to provide side-to-side balance of the trunk at the hip. The change in the hominid lateral balance mechanism can be viewed as primarily osteological, allowing preservation of the same muscle function with an extended thigh. As a result, the stride length is increased and there occurs a diminution of the demands placed on other muscles to maintain anteroposterior balance at the hip and knee. Our data also support the view that vertical climbing may be specifically preadaptive to bipedalism. One may picture the earliest hominid as part biped, when on the ground traveling between scattered food trees, and part climber, when moving from the ground to food.     

Uncertainty of knee joint muscle activity during knee joint torque exertion: the significance of controlling adjacent joint torque.

In the single-joint torque exertion task, which has been widely used to control muscle activity, only the relevant joint torque is specified. However, the neglect of the neighboring joint could make the procedure unreliable, considering our previous result that even monoarticular muscle activity level is indefinite without specifying the adjacent joint torque. Here we examined the amount of hip joint torque generated with knee joint torque and its influence on the activity of the knee joint muscles. Twelve healthy subjects were requested to exert various levels of isometric knee joint torque. The knee and hip joint torques were obtained by using a custom-made device. Because no information about hip joint torque was provided to the subjects, the hip joint torque measured here was a secondary one associated with the task. The amount of hip joint torque varied among subjects, indicating that they adopted various strategies to achieve the task. In some subjects, there was a considerable internal variability in the hip joint torque. Such variability was not negligible, because the knee joint muscle activity level with respect to the knee joint torque, as quantified by surface electromyography (EMG), changed significantly when the subjects were requested to change the strategy. This change occurred in a very systematic manner: in the case of the knee extension, as the hip flexion torque was larger, the activity of mono- and biarticular knee extensors decreased and increased, respectively. These results indicate that the conventional single knee joint torque exertion has the drawback that the intersubject and/or intertrial variability is inevitable in the relative contribution among mono- and biarticular muscles because of the uncertainty of the hip joint torque. We discuss that the viewpoint that both joint torques need to be considered will bring insights into various controversial problems such as the shape of the EMG-force relationship, neural factors that help determine the effect of muscle strength training, and so on.     

The somitic level of origin of embryonic chick hindlimb muscles

Studies of avian chimeras made by transplanting groups of quail somites into chick embryos have consistently shown that the muscle cells of the hindlimb are derived from the adjacent somites, however, the pattern of cell distribution from individual somites to individual hindlimb muscles has not been characterized. I have mapped quail cell distribution in the chick hindlimb after single somite transplantation to determine if cells from an individual somite populate discrete limb muscle regions and if there is a spatial correspondence between a muscle's somitic level of origin and the known spinal cord position of its motoneuron pool. At stages 15-18 single chick somites or equivalent lengths of unsegmented somitic mesoderm adjacent to the prospective hindlimb region were replaced with the corresponding tissue from quail embryos. At stages 28-34, quail cell distribution was mapped within individual thigh muscles and shank muscle regions. A quail-specific antiserum and Feulgen staining were used to identify quail cells. Transplants from somite levels 26-33 each gave rise to consistent quail cell labeling in a unique subset of limb muscles. The anteroposterior positions of these subsets corresponded to that of the transplanted somitic tissue. For example, more anterior or anteromedial thigh muscles contained quail cells when more anterior somitic tissue had been transplanted. For the majority of thigh muscles studied and for shank muscle groups, there was also a clear correlation between somitic level of origin and motoneuron pool position. These data are compatible with the hypothesis that motoneurons and the muscle cells of their targets share axial position labels. The question of whether motoneurons from a specific spinal cord segment recognize and consequently innervate muscle cells derived from the same axial level during early axon outgrowth is addressed in the accompanying paper (C. Lance-Jones, 1988, Dev. Biol. 126, 408-419). Quail cell distribution was also mapped in chick embryos in which quail somites or unsegmented mesoderm had been placed 2-3 somites away from their position of origin. In all cases donor somitic tissues contributed to muscles in accord with their host position. These results indicate that muscle cell precursors within the somites are not specified to migrate to a predetermined target region.     

Myosin ATPase activity in somatic muscle fibers in different degree of pH of the pre-incubation medium

The problem on presence of muscle fibers (MF) types is an actual one, since it is a prerequisite to forecasting certain changes in MF under different conditions. The aim of the investigation has been to find out whether independent groups of MF exist or whether they react as a whole population to gradual changes in pH of the preincubational medium, when dynamic changes in m-ATPhase activity are studied. In serial transversal cryostate slices 10 mcm thick m-ATPhase activity has been defined in MF at preincubation of the slices in media having various pH. The experiment has had three series. In the I--the ventral denticulate muscle with universal biomechanical properties has been investigated; preincubation has been carried out in media having pH from 3.0 up to 11.0 with the intervals 0.5 pH. In the II--besides the muscle mentioned the fast brachial muscle and the force triceps muscle have been studied; preincubation has been performed only at pH 9.5, since in the I series two dissociated groups of MF have been distinguished. In the III series, at studying the same three muscles m-ATPhase activity has been detailed with their preincubation at pH from 3.8 up to 4.5 with the interval 0.1. The m-ATPase activity begins to manifest in various MF at pH of the preincubational medium within the limits of 3.8-4.2. A gradual increase in amount of MF that are revealing, as pH is gradually increasing, evidences most likely against independence of separate groups.(ABSTRACT TRUNCATED AT 250 WORDS)     

An introductory biology lab that uses enzyme histochemistry to teach students about skeletal muscle fiber types

One important goal of introductory biology laboratory experiences is to engage students directly in all steps in the process of scientific discovery. Even when laboratory experiences are built on principles discussed in the classroom, students often do not adequately apply this background to interpretation of results they obtain in lab. This disconnect has been described at the level of medical education (4), so it should not be surprising that educators have struggled with this same phenomenon at the undergraduate level. We describe a new introductory biology lab that challenges students to make these connections. The lab utilizes enzyme histochemistry and morphological observations to draw conclusions about the composition of functionally different types of muscle fibers present in skeletal muscle. We report that students were not only successful at making these observations on a specific skeletal muscle, the gastrocnemius of the frog Rana pipiens, but that they were able to connect their results to the principles of fiber type differences that exist in skeletal muscles in all vertebrates.     

Differences in the Developmental Fate of Cultured and Noncultured Myoblasts When Transplanted into Embryonic Limbs

Myoblasts from embryonic, fetal, and adult quail and chick muscles were transplanted into limb buds of chick embryos to determine if myoblasts can form muscle fibers in heterochronic limbs and to define the conditions that affect the ability of transplanted cells to populate newly developing limb musculature. Myoblasts from each developmental stage were either freshly isolated and transplanted or were cultured prior to transplantation into limb buds of 4- to 5-day (ED4-5) chick embryos. Transplanted myoblasts, regardless of the age of the donor from which they were derived, formed muscle fibers within embryonic limb muscles. Transplanted cloned myoblasts formed muscle fibers, although there was little evidence that the number of transplanted myoblasts significantly increased following transplantation or that they migrated any distance from the site of injection. The fibers that formed from transplanted clonal myoblasts often did not persist in the host limb muscles until ED10. Diminished fiber formation from myoblasts transplanted into host limbs was observed whether myoblasts were cloned or cultured at high density. However, when freshly isolated myoblasts were transplanted, the fibers they formed were numerous, widely dispersed within the limb musculature, and persisted in the muscles until at least ED10. These results indicate that transplanted myoblasts of embryonic, fetal, and adult origin are capable of forming fibers during early limb muscle formation. They also indicate that even in an embryonic chick limb where proliferation of endogenous myoblasts and muscle fiber formation is rapidly progressing, myoblasts that are cultured in vitro do not substantially contribute to long-term muscle fiber formation after they are transplanted into developing limbs. However, when the same myoblasts are freshly isolated and transplanted without prior cell culture, substantial numbers of fibers form and persist after transplantation into developing limbs. Thus, these studies demonstrate that the extent to which transplanted myoblasts fuse to form fibers which persist in host musculature depends upon whether donor myoblasts are freshly isolated or maintained in vitro prior to injection.     

Refined structure of bony fish muscle myosin filaments from low-angle X-ray diffraction data

Application of X-ray diffraction methods to the elucidation of the arrangement of the myosin heads on myosin filaments in resting muscles is made simpler when the muscles themselves are well ordered in 3D. Bony fish muscle for the vertebrates and insect flight muscle for the invertebrates are the muscles of choice for this analysis. The rich, well-sampled, low-angle X-ray diffraction pattern from bony fish muscle has previously been modelled with an R-factor of 3.4% between observed and calculated transforms on the assumption that the two heads in one myosin molecule have the same shape. However, recent evidence from other kinds of analysis of other muscles has shown that this assumption may not be valid. There is evidence that the motor domain of one head in each pair may interact with the neck region of the second head. This possibility has been tested directly in the present analysis which extends the X-ray modelling of fish muscle myosin filaments by permitting independent shape changes of the two heads in one molecule. The new model, with a computed R-factor of 1.19% against 56 independent reflections, shows that in fish muscle also there is a marked asymmetry in the organisation of each head pair.     

The relation between ATPASE activity and light chains of myosin in developing, adult and denervated muscles of several animals species.

Ca2+ATPase activity and light chains of myosin, fractionated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, in developing, adult and denervated fast, slow and cardiac muscles of the rat, guinea-pig, cat, rabbit and chick were studied. It has been shown that in normal adult muscles the electrophoretic pattern of light chains of myosin reflects the myosin ATPase activity only when muscles from the same animal species are compared. In homologous muscles from adult animals differing in size, the size-dependent difference in myosin ATPase activity is not revealed in the electrophoretic pattern. Both in developing and in denervated muscle, changes in myosin ATPase activity are either connected with changes in the pattern of light chains of myosin or this pattern does not change. This relation is different in fast and slow muscles and also differs in chick and rabbit muscles. There are several possibilities of explaining the relation between ATPase activity of myosin and the pattern of light chains of myosin. The observation that myosin from the soleus muscle of 1-month-old rabbit contains light chains corresponding to both fast and slow type of myosin, indicates that the change in myosin ATPase activity during development is due to changes in the ratio between the fast and slow type of myosin.     

Shortening velocity and myosin heavy chains of developing rabbit muscle fibers

The regulation of vertebrate muscle contraction with respect to the role of the different subunits of myosin remains somewhat uncertain. One approach to gaining a better understanding of the molecular basis of contraction is to study developing muscle which undergoes changes in myosin isozyme composition and contractile properties during the normal course of maturation. The present study utilizes single fibers from psoas muscles of rabbits at several ages as a model system for fast-twitch muscle development. This approach eliminates the inherent problems of interpreting results from studies on whole muscles which usually contain heterogeneous fiber types with respect to contractile properties and isoenzyme composition. Maximum velocity of shortening and tension-generating ability of individual fibers were measured and the myosin heavy chain composition of the same fibers was examined using an ultrasensitive sodium dodecyl sulfate-polyacrylamide gel system. The results indicate that 1) with regard to contractile properties, there is a transitional period from slow to fast shortening velocities within the first postnatal month; 2) a strong, positive correlation exists between the speed of shortening and tension-generating ability of individual postnatal day 7 fibers, suggesting that as more myosin is incorporated in these developing fibers it is of the fast type; and 3) there is a wide variation in maximum velocity of shortening among postnatal day 7 psoas fibers which is also a time when a mixture of heavy chain isoforms characterizes the myosin composition of single muscle fibers.     

Relationship between muscle length and moment arm on EMG activity of human triceps surae muscle.

PURPOSE: The purpose of this experiment was to evaluate the effects of both muscle length and moment arm (MA) on the electromyographic (EMG) and force output of the triceps surae (TS) muscle. RELEVANCE: It is well recognized that changes in muscle length affect both the muscle's force generating capacity as well as its twitch speed. This relationship is well established in animal preparations. Contrary to animal experiments where length can be directly manipulated in isolated muscles, human experiments require that all muscle length changes be secondary to changes in a joint angle. Such experimental manipulations therefore produce changes in not only muscle length, but also in the muscle's MA. The relative effect of muscle length and MA changes on muscle EMG has not been determined in previous experiments. METHODS: This study was executed in two phases. First, using fresh human cadaver lower limbs, data were gathered describing the relationship between knee and ankle angle changes for maintenance of a constant TS muscle length, while its MA at the ankle joint has been changed. In the second phase of the study, results obtained from phase one were applied to 10 healthy adult human subjects to measure the EMG (surface and fine wire) activity of TS at three different conditions: when both length and MA were shortened, when muscle length was decreased given a constant MA and when MA was shortened given a constant muscle length. RESULTS: A significant increase in muscle activity was found as both the length and MA of TS muscle were shortened. A similar pattern of increased muscle activity was observed when the MA was shortened given a constant muscle length. No significant change in TS activity was found when muscle length was shortened, given a constant MA at the ankle joint. CONCLUSIONS: The findings of this study indicate that changes in the Achilles tendon MA predominate over the muscle length variations in determining the level of TS activity when generating plantar flexion torque.     

Seasonal variation in sonic muscles in the fawn cusk-eel Lepophidium profundorum

The fawn cusk-eel Lepophidium profundorum (Ophidiidae) has an unusual sound-producing system with sexually dimorphic sets of antagonistic muscles. Outside the mating season, the dorsal and ventral muscles are well developed and larger in males than in females, but the tiny intermediate muscles are smaller, suggesting a minor role, if any, in male advertisement call production. We examined summer individuals with more developed gonads and find a fourfold hypertrophy of the intermediate but not the other muscles. This result suggests androgen dependence and an important role in sound production for the intermediate muscle. Even though both sexes gain weight in the summer, the ventral and dorsal muscles in females lose weight, suggesting that sound production is less important in females and that muscle mass may be used to support egg growth.