Alterations in skeletal muscle morphology and mechanics in juvenile male Sprague Dawley rats exposed to a high-fat high-sucrose diet

Although once a health concern largely considered in adults, the obesity epidemic is now prevalent in pediatric populations. While detrimental effects on skeletal muscle function have been seen in adulthood, the effects of obesity on skeletal muscle function in childhood is not clearly understood. The purpose of this study was to determine if the consumption of a high-fat high-sucrose (HFS) diet, starting in the post-weaning period, leads to changes in skeletal muscle morphology and mechanics after 14 weeks on the HFS diet. Eighteen 3-week-old male CD-Sprague Dawley rats were randomly assigned to a HFS (C-HFS, n = 10) or standard chow diet (C-CHOW, n = 8). Outcome measures included: weekly energy intake, activity levels, oxygen consumption, body mass, body composition, metabolic profile, serum protein levels, and medial gastrocnemius gene expression, morphology, and mechanics. The main findings from this study were that C-HFS rats: (1) had a greater body mass and percent body fat than control rats; (2) showed early signs of metabolic syndrome; (3) demonstrated potential impairment in muscle remodeling; (4) produced lower relative muscle force; and (5) had a shift in the force-length relationship, indicating that the medial gastrocnemius had shorter muscle fiber lengths compared to those of C-CHOW rats. Based on the results of this study, we conclude that exposure to a HFS diet led to increased body mass, body fat percentage, and early signs of metabolic syndrome, resulting in functional deficits in MG of childhood rats.

The influence of longitudinal muscle fascicle growth on mechanical function

Skeletal muscle has the remarkable ability to remodel and adapt, such as the increase in serial sarcomere number (SSN) or fascicle length (FL) observed after overstretching a muscle. This type of remodelling is termed longitudinal muscle fascicle growth, and its impact on biomechanical function has been of interest since the 1960s due to its clinical applications in muscle strain injury, muscle spasticity, and sarcopenia. Despite simplified hypotheses on how longitudinal muscle fascicle growth might influence mechanical function, existing literature presents conflicting results partly due to a breadth of methodologies. The purpose of this review is to outline what is currently known about the influence of longitudinal muscle fascicle growth on mechanical function and suggest future directions to address current knowledge gaps and methodological limitations. Various interventions indicate longitudinal muscle fascicle growth can increase the optimal muscle length for active force, but whether the whole force-length relationship widens has been less investigated. Future research should also explore the ability for longitudinal fascicle growth to broaden the torque-angle relationship's plateau region, and the relation to increased force during shortening. Without a concurrent increase in intramuscular collagen, longitudinal muscle fascicle growth also reduces passive tension at long muscle lengths; further research is required to understand whether this translates to increased joint range of motion. Lastly, some evidence suggests longitudinal fascicle growth can increase maximum shortening velocity and peak isotonic power, however, there has yet to be direct assessment of these measures in a neurologically intact model of longitudinal muscle fascicle growth.

Modeling muscle function using experimentally determined subject-specific muscle properties

Muscle models are commonly based on intrinsic properties pooled across a number of individuals, often from a different species, and rarely validated against directly measured muscle forces. Here we use a rich data set of rat medial gastrocnemius muscle forces recorded during in-situ and in-vivo isometric, isotonic, and cyclic contractions to test the accuracy of forces predicted using Hill-type muscle models. We identified force-length and force-velocity parameters for each individual, and used either these subject-specific intrinsic properties, or population-averaged properties within the models. The modeled forces for cyclic in-vivo and in-situ contractions matched with measured muscle-tendon forces with r² between 0.70 and 0.86, and root-mean square errors (RMSE) of 0.10 to 0.13 (values normalized to the maximum isometric force). The modeled forces were least accurate at the highest movement and cycle frequencies and did not show an improvement in r² when subject-specific intrinsic properties were used; however, there was a reduction in the RMSE with fewer predictions having higher errors. We additionally recorded and tested muscle models specific to proximal and distal regions of the muscle and compared them to measures and models from the whole muscle belly: there was no improvement in model performance when using data from specific anatomical regions. These results show that Hill-type muscle models can yield very good performance for cyclic contractions typical of locomotion, with small reductions in errors when subject-specific intrinsic properties are used.

In vivo force-length and activation dynamics of two distal rat hindlimb muscles in relation to gait and grade

Muscle function changes to meet the varying mechanical demands of locomotion across different gait and grade conditions. A muscle's work output is determined by time-varying patterns of neuromuscular activation, muscle force and muscle length change, but how these patterns change under different conditions in small animals is not well defined. Here, we report the first integrated in vivo force-length and activation patterns in rats, a commonly used small animal model, to evaluate the dynamics of two distal hindlimb muscles (medial gastrocnemius and plantaris) across a range of gait (walk, trot and gallop) and grade (level and incline) conditions. We use these data to explore how the pattern of force production, muscle activation and muscle length changes across conditions in a small quadrupedal mammal. As hypothesized, we found that the rat muscles show limited fascicle strains during active force generation in stance across gaits and grades, indicating that these distal rat muscles generate force economically but perform little work, similar to patterns observed in larger animals during level locomotion. Additionally, given differences in fiber type composition and variation in motor unit recruitment across the gait and grade conditions examined here for these muscles, the in vivo force-length behavior and neuromuscular activation data reported here can be used to validate improved two-element Hill-type muscle models.

Muscle proprioceptors in adult rat: mechanosensory signaling and synapse distribution in spinal cord

The characteristic signaling and intraspinal projections of muscle proprioceptors best described in the cat are often generalized across mammalian species. However, species-dependent adaptations within this system seem necessary to accommodate asymmetric scaling of length, velocity, and force information required by the physics of movement. In the present study we report mechanosensory responses and intraspinal destinations of three classes of muscle proprioceptors. Proprioceptors from triceps surae muscles in adult female Wistar rats anesthetized with isoflurane were physiologically classified as muscle spindle group Ia or II or as tendon organ group Ib afferents, studied for their firing responses to passive-muscle stretch, and in some cases labeled and imaged for axon projections and varicosities in spinal segments. Afferent projections and the laminar distributions of provisional synapses in rats closely resembled those found in the cat. Afferent signaling of muscle kinematics was also similar to reports in the cat, but rat Ib afferents fired robustly during passive-muscle stretch and Ia afferents displayed an exaggerated dynamic response, even after locomotor scaling was accounted for. These differences in mechanosensory signaling by muscle proprioceptors may represent adaptations for movement control in different animal species.NEW & NOTEWORTHY Muscle sensory neurons signal information necessary for controlling limb movements. The information encoded and transmitted by muscle proprioceptors to networks in the spinal cord is known in detail only for the cat, but differences in size and behavior of other species challenge the presumed generalizability. This report presents the first findings detailing specializations in mechanosensory signaling and intraspinal targets for functionally identified subtypes of muscle proprioceptors in the rat.

Movement Complexity and Neuromechanical Factors Affect the Entropic Half-Life of Myoelectric Signals

Appropriate neuromuscular functioning is essential for survival and features underpinning motor control are present in myoelectric signals recorded from skeletal muscles. One approach to quantify control processes related to function is to assess signal variability using measures such as Sample Entropy. Here we developed a theoretical framework to simulate the effect of variability in burst duration, activation duty cycle, and intensity on the Entropic Half-Life (EnHL) in myoelectric signals. EnHLs were predicted to be <40 ms, and to vary with fluctuations in myoelectric signal amplitude and activation duty cycle. Comparison with myoelectic data from rats walking and running at a range of speeds and inclines confirmed the range of EnHLs, however, the direction of EnHL change in response to altered locomotor demand was not correctly predicted. The discrepancy reflected different associations between the ratio of the standard deviation and mean signal intensity (Ist:It¯) and duty factor in simulated and physiological data, likely reflecting additional information in the signals from the physiological data (e.g., quiescent phase content; variation in action potential shapes). EnHL could have significant value as a novel marker of neuromuscular responses to alterations in perceived locomotor task complexity and intensity.

Crouching to fit in: the energetic cost of locomotion in tunnels

Animals that are specialized for a particular habitat or mode of locomotion often demonstrate locomotor efficiency in a focal environment when compared to a generalist species. However, measurements of these focal habitats or behaviors are often difficult or impossible to do in the field. In this study, the energetics and kinematics of simulated tunnel locomotion by two unrelated semi-fossorial mammals, the ferret and degu, were analyzed using open-flow respirometry and digital video. Animals were trained to move inside of normal (unconstrained, overground locomotion) and height-decreased (simulated tunnel, adjusted to tolerance limits for each species) Plexiglas chambers that were mounted flush onto a treadmill. Both absolute and relative tunnel performance differed between the species; ferrets tolerated a tunnel height that forced them to crouch at nearly 25% lower hip height than in an unconstrained condition, while degus would not perform on the treadmill past a ∼9% reduction in hip height. Both ferrets and degus exhibited significantly higher metabolic rates and cost of transport (CoT) values when moving in the tunnel condition relative to overgound locomotion. When comparing CoT values across small (&lt;10kg) mammals, ferrets demonstrated a lower than predicted metabolic cost during both tunnel and terrestrial locomotion, whereas degus were very close to line of best fit. Although tunnel locomotion requires a more striking change in posture for ferrets, ferrets are more efficient locomotors in both conditions than mammals of similar mass.

Molecular and metabolomic effects of voluntary running wheel activity on skeletal muscle in late middle-aged rats

We examined the molecular and metabolomic effects of voluntary running wheel activity in late middle-aged male Sprague Dawley rats (16–17 months). Rats were assigned either continuous voluntary running wheel access for 8 weeks (RW+) or cage-matched without running wheel access (RW−). The 9 RW+ rats averaged 83 m/day (range: 8–163 m), yet exhibited both 84% reduced individual body weight gain (4.3 g vs. 26.3 g, P = 0.02) and 6.5% reduced individual average daily food intake (20.6 g vs. 22.0 g, P = 0.09) over the 8 weeks. Hindlimb muscles were harvested following an overnight fast. Muscle weights and myofiber cross-sectional area showed no difference between groups. Western blots of gastrocnemius muscle lysates with a panel of antibodies suggest that running wheel activity improved oxidative metabolism (53% increase in PGC1α, P = 0.03), increased autophagy (36% increase in LC3B-II/-I ratio, P = 0.03), and modulated growth signaling (26% increase in myostatin, P = 0.04). RW+ muscle also showed 43% increased glycogen phosphorylase expression (P = 0.04) and 45% increased glycogen content (P = 0.04). Metabolomic profiling of plantaris and soleus muscles indicated that even low-volume voluntary running wheel activity is associated with decreases in many long-chain fatty acids (e.g., palmitoleate, myristoleate, and eicosatrienoate) relative to RW− rats. Relative increases in acylcarnitines and acyl glycerophospholipids were also observed in RW+ plantaris. These data establish that even modest amounts of physical activity during late middle-age promote extensive metabolic remodeling of skeletal muscle.

Post-immobilization eccentric training promotes greater hypertrophic and angiogenic responses than passive stretching in muscles of weanling rats

This study investigated how different types of remobilization after hind limb immobilization, eccentric exercise and passive static stretching, influenced the adaptive responses of muscles with similar function and fascicle size, but differing in their contractile characteristics. Female Wistar weanling rats (21 days old) were divided into 8 groups: immobilized for 10 days, maintaining the ankle in maximum plantar flexion; immobilized and submitted to eccentric training for 10 or 21 days on a declining treadmill for 40min; immobilized and submitted to passive stretching for 10 or 21 days for 40min by maintaining the ankle in maximum dorsiflexion; control of immobilized; and control of 10 or 21 days. The soleus and plantaris muscles were analyzed using fiber distribution, lesser diameter, capillary/fiber ratio, and morphology. Results showed that the immobilization reduced the diameter of all fiber types, caused changes in fiber distribution and decreased the number of transverse capillaries in both muscles. The recovery period of the soleus muscle is longer than that of the plantaris after detraining. Moreover, eccentric training induced greater hypertrophic and angiogenic responses than passive stretching, especially after 21 days of rehabilitation. Both techniques demonstrated positive effects for muscle rehabilitation with the eccentric exercise being more effective.

Acute effects of sex-specific sex hormones on heat shock proteins in fast muscle of male and female rats

Heat shock protein (HSP) expression and sex hormone levels have been shown to influence several aspects of skeletal muscle physiology (e.g., hypertrophy, resistance to oxidative stress), suggesting that sex hormone levels can effect HSP expression. This study evaluated the effects of differing levels of sex-specific sex hormones (i.e., testosterone in males and estrogen in females) on the expression of 4: HSP70, HSC70, HSP25, and αB-crystallin in the quadriceps muscles of male and female rats. Animals were assigned to 1 of 3 groups (n = 5 M and F/group). The first group (Ctl) consisted of typically cage-housed animals that served as controls. The second group (H) was gonadectomized and received either testosterone (males) or estradiol (females) via injection for 12 consecutive days. The third group (Gx) was gonadectomized and injected as above, but with vehicle only, rather than hormones. Significant sex by condition interactions (P < 0.05 by two-way MANOVA) were found for all 4 proteins studied, except for HSP70, which exhibited a significant effect of condition only. The expression of all HSPs was greater (1.9-2.5-fold) in males vs. females in the Ctl group, except for HSP70, which was no different. Generally, gonadectomy appeared to have greater effects in males than females, but administration of the exogenous sex hormones tended to produce more robust relative changes in females than males. There were no differences in myosin composition in any of the groups, suggesting that changes in fiber type were not a factor in the differential protein expression. These data may have implications for sex-related differences in muscular responses to exercise, disuse, and injury.

Recruitment of faster motor units is associated with greater rates of fascicle strain and rapid changes in muscle force during locomotion

Animals modulate the power output needed for different locomotor tasks by changing muscle forces and fascicle strain rates. To generate the necessary forces, appropriate motor units must be recruited. Faster motor units have faster activation-deactivation rates than slower motor units, and they contract at higher strain rates; therefore, recruitment of faster motor units may be advantageous for tasks that involve rapid movements or high rates of work. This study identified motor unit recruitment patterns in the gastrocnemii muscles of goats and examined whether faster motor units are recruited when locomotor speed is increased. The study also examined whether locomotor tasks that elicit faster (or slower) motor units are associated with increased (or decreased) in vivo tendon forces, force rise and relaxation rates, fascicle strains and/or strain rates. Electromyography (EMG), sonomicrometry and muscle-tendon force data were collected from the lateral and medial gastrocnemius muscles of goats during level walking, trotting and galloping and during inclined walking and trotting. EMG signals were analyzed using wavelet and principal component analyses to quantify changes in the EMG frequency spectra across the different locomotor conditions. Fascicle strain and strain rate were calculated from the sonomicrometric data, and force rise and relaxation rates were determined from the tendon force data. The results of this study showed that faster motor units were recruited as goats increased their locomotor speeds from level walking to galloping. Slow inclined walking elicited EMG intensities similar to those of fast level galloping but different EMG frequency spectra, indicating that recruitment of the different motor unit types depended, in part, on characteristics of the task. For the locomotor tasks and muscles analyzed here, recruitment patterns were generally associated with in vivo fascicle strain rates, EMG intensity and tendon force. Together, these data provide new evidence that changes in motor unit recruitment have an underlying mechanical basis, at least for certain locomotor tasks.

Task-dependent activity of motor unit populations in feline ankle extensor muscles

Understanding the functional significance of the morphological diversity of mammalian skeletal muscles is limited by technical difficulties of estimating the contribution of motor units with different properties to unconstrained motor behaviours. Recently developed wavelet and principal components analysis of intramuscular myoelectric signals has linked signals with lower and higher frequency contents to the use of slower and faster motor unit populations. In this study we estimated the relative contributions of lower and higher frequency signals of cat ankle extensors (soleus, medial and lateral gastrocnemii, plantaris) during level, downslope and upslope walking and the paw-shake response. This was done using the first two myoelectric signal principal components (PCI, PCII), explaining over 90% of the signal, and an angle θ, a function of PCI/PCII, indicating the relative contribution of slower and faster motor unit populations. Mean myoelectric frequencies in all walking conditions were lowest for slow soleus (234 Hz) and highest for fast gastrocnemii (307 and 330 Hz) muscles. Motor unit populations within and across the studied muscles that demonstrated lower myoelectric frequency (suggesting slower populations) were recruited during tasks and movement phases with lower mechanical demands on the ankle extensors--during downslope and level walking and in early walking stance and paw-shake phases. With increasing mechanical demands (upslope walking, mid-phase of paw-shake cycles), motor unit populations generating higher frequency signals (suggesting faster populations) contributed progressively more. We conclude that the myoelectric frequency contents within and between feline ankle extensors vary across studied motor behaviours, with patterns that are generally consistent with muscle fibre-type composition.

A muscle's force depends on the recruitment patterns of its fibers

Biomechanical models of whole muscles commonly used in simulations of musculoskeletal function and movement typically assume that the muscle generates force as a scaled-up muscle fiber. However, muscles are comprised of motor units that have different intrinsic properties and that can be activated at different times. This study tested whether a muscle model comprised of motor units that could be independently activated resulted in more accurate predictions of force than traditional Hill-type models. Forces predicted by the models were evaluated by direct comparison with the muscle forces measured in situ from the gastrocnemii in goats. The muscle was stimulated tetanically at a range of frequencies, muscle fiber strains were measured using sonomicrometry, and the activation patterns of the different types of motor unit were calculated from electromyographic recordings. Activation patterns were input into five different muscle models. Four models were traditional Hill-type models with different intrinsic speeds and fiber-type properties. The fifth model incorporated differential groups of fast and slow motor units. For all goats, muscles and stimulation frequencies the differential model resulted in the best predictions of muscle force. The in situ muscle output was shown to depend on the recruitment of different motor units within the muscle.

EMG analysis tuned for determining the timing and level of activation in different motor units

Recruitment patterns and activation dynamics of different motor units greatly influence the temporal pattern and magnitude of muscle force development, yet these features are not often considered in muscle models. The purpose of this study was to characterize the recruitment and activation dynamics of slow and fast motor units from electromyographic (EMG) recordings and twitch force profiles recorded directly from animal muscles. EMG and force data from the gastrocnemius muscles of seven goats were recorded during in vivo tendon-tap reflex and in situ nerve stimulation experiments. These experiments elicited EMG signals with significant differences in frequency content (p<0.001). The frequency content was characterized using wavelet and principal components analysis, and optimized wavelets with centre frequencies, 149.94 Hz and 323.13 Hz, were obtained. The optimized wavelets were used to calculate the EMG intensities and, with the reconstructed twitch force profiles, to derive transfer functions for slow and fast motor units that estimate the activation state of the muscle from the EMG signal. The resulting activation-deactivation time constants gave r values of 0.98-0.99 between the activation state and the force profiles. This work establishes a framework for developing improved muscle models that consider the intrinsic properties of slow and fast fibres within a mixed muscle, and that can more accurately predict muscle force output from EMG.

Ageing, but not yet senescent, rats exhibit reduced muscle quality and sarcoplasmic reticulum function

AIM

Reduced muscle force greater than expected from loss of muscle mass has been reported in ageing muscles. Impaired sarcoplasmic reticulum (SR) Ca(2+) release has been implicated as a possible mechanism, and attributed to several factors, including loss of ryanodine receptor (RYR) expression and protein binding. The aim of this study was to evaluate muscle quality and SR Ca(2+) release in ageing rats that were not so old that major atrophy had occurred.

METHODS

We collected in situ force data from the plantarflexor muscle group and muscle mass from the constituent muscles to determine muscle quality (force/mass) in adult (6-8 months) and ageing (24 months) rats (n=8/group). We evaluated SR Ca(2+) uptake and release, and determined expression of key proteins associated with Ca(2+) release [RYR and FK506 binding protein (FKBP)] and uptake (SERCA, parvalbumin, calsequestrin).

RESULTS

Plantarflexor force and muscle quality were reduced with ageing (approx. 28 and 34%, respectively), but atrophy was limited, and significant only in the medial gastrocnemius (approx. 15%). The fast phase of SR Ca(2+) release was reduced with ageing in both gastrocnemii, as was FKBP expression and FKBP-RYR binding, but RYR expression was not affected. Similar, but non-significant changes were present in the plantaris, but the soleus muscle generally showed no ageing-related changes.

CONCLUSION

These data suggest a possible role for impaired SR Ca(2+) release in ageing-related loss of muscle quality, although not through loss of RYR expression.