Muscle-specific Changes Research Articles

Neural Compensation Within the Human Triceps Surae During Prolonged Walking

During human walking, muscle activation strategies are approximately constant across consecutive steps over a short time, but it is unknown whether they are maintained over a longer duration. Prolonged walking may increase tendinous tissue (TT) compliance, which can influence neural activation, but the neural responses of individual muscles have not been investigated. This study investigated the hypothesis that muscle activity is up- or down-regulated in individual triceps surae muscles during prolonged walking. Thirteen healthy subjects walked on a treadmill for 60 min at 4.5 km/h, while triceps surae muscle activity, maximal muscle compound action potentials, and kinematics were recorded every 5 min, and fascicle lengths were estimated at the beginning and end of the protocol using ultrasound. After 1 h of walking, soleus activity increased by 9.3 ± 0.2% (P < 0.05) and medial gastrocnemius activity decreased by 9.3 ± 0.3% (P < 0.01). Gastrocnemius fascicle length at ground contact shortened by 4.45 ± 0.99% (P < 0.001), whereas soleus fascicle length was unchanged (P = 0.988). Throughout the stance phase, medial gastrocnemius fascicle lengthening decreased by 44 ± 13% (P < 0.001), whereas soleus fascicle lengthening amplitude was unchanged (P = 0.650). The data suggest that a compensatory neural strategy exists between triceps surae muscles and that changes in muscle activation are generally mirrored by changes in muscle fascicle length. These findings also support the notion of muscle-specific changes in TT compliance after prolonged walking and highlight the ability of the CNS to maintain relatively constant movement patterns in spite of neuromechanical changes in individual muscles.

In vivo Oxidative Capacity of Two Leg Muscles in Young and Older Women

Several studies have reported lower muscle oxidative capacity (Vmax) in older adults, suggesting impaired mitochondrial function with old age. However, others have shown that muscle V is maintained in older subjects. This discrepancy may be due to differences in subjects' health or physical activity, or result from muscle-specific changes in aging. PURPOSE: To quantify Vmax in two morphologically and functionally distinct muscles (tibialis anterior, TA; vastus lateralis, VL) of young and older healthy women. METHODS: 7 young (27±1 yrs, mean±SEM) and 5 old (70±1) activity-matched (by accelerometer) women were positioned in a 4.0 Tesla superconducting magnet where they performed a 16- and a 24-s maximal voluntary contraction of TA and VL, respectively. 31P magnetic resonance spectra were obtained before, during and after each contraction, and the rate constant of PCr recovery (kPCr) was determined from an exponential fit of the data. Vmax was calculated as (kPCr · resting [PCr]). Two-way (age, muscle) ANOVAs were applied. RESULTS: Physical activity was similar in both groups (p=0.21), and [PCr] did not differ by age or muscle at rest (p=0.77, p=0.29) or at the end of contractions (p=0.09, p=0.65). While kPCr and Vmax were higher in TAthan VL (p=0.010, p =0.007), there were no significant age-group differences (p =0.13, p =0.12) for these variables.TableCONCLUSION: The higher Vmax in TA compared to VL was expected, based on the larger proportion of oxidative type I fibers in the TA. Given the current results, there appears to be no effect of age on Vmax in either muscle, suggesting that mitochondrial function is maintained in vivo in healthy older women of comparable physical activity level as the young. Support: NIH/NIA R01 AG21094

Noninvasive monitoring of muscle damage during reloading following limb disuse

Cast immobilization causes skeletal muscle disuse atrophy and an increased susceptibility to muscle damage. The objective of this study was to explore the utility of noninvasive magnetic resonance (MR) imaging to monitor muscle damage in the lower hindlimb muscles of the mouse during reloading following cast immobilization and to compare the findings in different muscles. The hindlimbs of C57BL6 mice were immobilized for 2 weeks in plantarflexion using a bilateral casting model. Following immobilization the mice were allowed to reambulate and muscle damage was monitored at different times. Cage-restricted reloading following cast immobilization induced a significant shift (P < 0.0001) in the transverse (T2) relaxation characteristics of the postural slow-twitch soleus muscle, but not in the neighboring gastrocnemius. Soleus T2 values peaked at 2 days of reloading. Muscle-specific changes in MR T2 relaxation properties correlated with uptake of Evans blue dye, a histological marker of muscle damage. This study demonstrates that T2 MR imaging can be implemented to monitor noninvasively and sequentially muscle-specific damage during reloading following limb disuse.

Muscle fatigue and fatigue-related biomechanical changes during a cyclic lifting task.

Electromyographic and biomechanical methods were utilized to investigate correlations between indexes of localized muscle fatigue and changes in the kinematics and kinetics of motion during a cyclic lifting task. Recent advances in time-frequency analysis procedures for electromyographicic signal processing provide a new way of studying localized muscle fatigue during dynamic contractions. These methods provide a means to investigate fatigue-related functional impairments in patients with low back pain. To study the relationship between localized muscle fatigue and the biomechanics of lifting and lowering a weighted box. Fatigue-related changes in the electromyographicic signal of trunk and limb muscles were evaluated and compared to kinematic and kinetic measures in order to determine whether lifting strategy is modified with fatigue. A total of 14 healthy male subjects (26 +/- 5 years) cyclically lifted and lowered a 13 kg box (12 lifts/min) for 4.5 minutes. A 5-second static maximum lifting task was included immediately before and after the cyclic lifting task to measure changes in lifting strength and static electromyographicic fatigue indexes. Electromyographic signals from 14 muscle sites (including paravertebral and limb muscles) were measured. Changes in the electromyographicic Instantaneous Median Frequency, a fatigue index, were computed using time-frequency analysis methods. This index was compared with more standardized measures of fatigue, such as those based on electromyographicic median frequency acquired during a static trunk extension test, subjective fatigue measures, and maximal static lifting strength. Biomechanical measures were gathered using a motion analysis system to study kinematic and kinetic changes during the lifting task. During the cyclic lifting task, the electromyographic Instantaneous Median Frequency significantly decreased over time in the paravertebral muscles, but not in the limb muscles. Paravertebral electromyographicic Instantaneous Median Frequency changes were consistent with self-reports of fatigue as well as decreases in trunk extension strength. The magnitude of muscle-specific changes in electromyographicic Instantaneous Median Frequency was not significantly correlated with electromyographicic median frequency changes from the static trunk extension task. The load of the box relative to the maximal static lifting strength significantly affected the electromyographicic Instantaneous Median Frequency changes of paravertebral back muscles. Significant changes with fatigue during the task were found in the angular displacements at the knee, hip, trunk, and elbow. These biomechanical changes were associated with increased peak torque and forces at the L4-L5 vertebral segment. Our results demonstrate correlation between localized muscle fatigue and biomechanical adaptations that occur during a cyclic lifting task. This new technique may provide researchers and clinicians with a means to investigate fatigue-related effects of repetitive work tasks or assessment procedures that might be useful in improving education, lifting ergonomy, and back school programs. Although both the dynamic and static tasks resulted in spectral shifts in the electromyographicic data, the fact that these methods led to different muscle-specific findings indicates that they should not be considered as equivalent assessment procedures.

The effect of material characteristics of shoe soles on muscle activation and energy aspects during running

The purposes of this study were (a) to determine group and individual differences in oxygen consumption during heel–toe running and (b) to quantify the differences in EMG activity for selected muscle groups of the lower extremities when running in shoes with different mechanical heel characteristics. Twenty male runners performed heel–toe running using two shoe conditions , one with a mainly elastic and a visco-elastic heel. Oxygen consumption was quantified during steady state runs of 6 min duration, running slightly above the aerobic threshold providing four pairs of oxygen consumption results for comparison. Muscle activity was quantified using bipolar surface EMG measurements from the tibialis anterior, medial gastrocnemius, vastus medialis and the hamstrings muscle groups. EMG data were sampled for 5 s every minute for the 6 min providing 30 trials. EMG data were compared for the different conditions using an ANOVA ( α=0.05). The findings of this study showed that changes in the heel material characteristics of running shoes were associated with (a) subject specific changes in oxygen consumption and (b) subject and muscle specific changes in the intensities of muscle activation before heel strike in the lower extremities. It is suggested that further study of these phenomena will help understand many aspects of human locomotion, including work, performance, fatigue and possible injuries.

Effect of age and cold exposure on morphofunctional characteristics of skeletal muscle in neonatal pigs.

Muscular changes accompanying and/or promoting the rapid postnatal improvement of the thermogenic efficiency of shivering were investigated in piglets. Animals were obtained at birth or killed after 5 days at thermoneutrality (34-30 degrees C) or in the cold (24-15 degrees C), to stimulate intense shivering thermogenesis. Fast-twitch-glycolytic (longissimus lumborum) and slow-twitch-oxidative (rhomboid) muscles were prepared for electron microscopic examination and chemical measurements. Muscle-specific changes in energy stores and metabolism were observed after birth, including the switch from glycogen to lipids and variation of the lactate/pyruvate ratio corresponding to the progressive acquisition of the metabolic type of the mature muscles. There was major age-related and/or cold-induced development of the structures involved in excitation-contraction coupling (triadic profiles, +80% in the cold), oxidative metabolism (number of lipid droplets, +81% with age in the cold; number of mitochondria, +29% with age or cold; surface of mitochondrial inner membranes, +18% with age and +32% in the cold) and contraction potential (myofibril volume, +62% with age). In contrast, neither age nor cold affected capillary volume density and capillary-to-fibre ratio. The observed changes reflect the immaturity and remarkable plasticity of piglet skeletal muscle and are likely to underlie its enhanced capacity for shivering thermogenesis after birth.

Endurance training induces muscle-specific changes in mitochondrial function in skinned muscle fibers.

The present study was conducted to investigate the potential role of changes in the apparent K(m) for ADP and in the functional coupling of the creatine (Cr) kinase (CK) system (CK efficiency) in explaining the tighter integration of ATP supply and demand after exercise training. Mitochondrial function was assessed in saponin-skinned fibers from the soleus and the deep red portion of the medial gastrocnemius isolated from trained (T; treadmill running, 5 days/wk, 4 wk) and control (C) female Sprague-Dawley rats. In the soleus, V(max) in the presence of 1 mM ADP was increased by 21% after training (5.9 +/- 0.2 vs. 4.7 +/- 0.4 nmol O(2). min(-1). mg dry wt(-1), P < 0.05). This was accompanied by no change in the K(m) for ADP measured in the absence of Cr (146 +/- 9 vs. 149 +/- 13 microM in T and C, respectively) and in its presence (50 +/- 4 vs. 48 +/- 6 microM in T and C, respectively) and in CK efficiency [K(m) (+Cr)/K(m) (-Cr)]. In contrast, in the red gastrocnemius, training decreased, by 35%, the apparent K(m) for ADP in the absence (83 +/- 5 vs. 129 +/- 9 microM, P < 0.01) of Cr, without affecting V(max) (6.2 +/- 0.4 vs. 6.7 +/- 0.3 nmol O(2). min(-1). mg dry wt(-1) in T and C, respectively) and CK efficiency. These results thus suggest that training induces muscle-specific adaptations of mitochondrial function and that a change in the intrinsic sensitivity of mitochondria to ADP could at least partly explain the tighter integration of ATP and demand commonly observed after training.

Hormone-related, muscle-specific changes in protein metabolism and fiber type profile after faba bean intake.

Male growing Wistar rats were fed, over 15 days, isoenergetic (16.72 +/- 0.49 MJ) and isoproteic (11%) diets containing either lactalbumin or raw Vicia faba L. (Vf) as the sole source of protein. Compared with pair-fed controls (PF), soleus muscles of Vf-fed rats showed increased (P < 0.05) synthesis and breakdown rates. In addition, the soleus of Vf-fed rats displayed a decrease (P < 0.05) in type I and an increase (P < 0.01) in type IIc fibers compared with that of PF animals. On the contrary, extensor digitorum longus muscles of both Vf-fed and PF rats showed an increase (P < 0.01) in type I and a reduction (P < 0.05) in type IIb fibers together with a decrease (P < 0.05) in the cross-sectional area of the latter fibers. Vf-fed rats exhibited a significant decrease in serum insulin (P < 0.05) and thyrotropin (P < 0.01) levels, together with an increase in plasma glucagon (P < 0.05) and 3,5,3'-triiodothyronine (P < 0.01) concentrations, compared with the PF group. Both Vf-fed and PF rats experienced an increase in corticosterone concentrations (P < 0.01 vs. control; P < 0.05 vs. PF). The muscle-specific changes in both protein metabolism and fiber type composition may partly depend on the hormonal changes that were observed after Vf intake.

Developmental and muscle-specific changes in methylation of the myosin light chain LC1f and LC3f promoters during avian myogenesis

The fast alkali myosin light chains LC1f and LC3f are two contractile protein isoforms encoded for by a single gene complex. Expression of these two isoforms is dependent upon differential initiation of transcription at either of two promoters encoding unique 5′ exons for isoform-specific amino termini of these light chains. Studies of protein expression have shown that the two isoforms are first expressed at different stages of development and in the case of the LC3f isoform only in fast twitch muscle fiber types. The molecular mechanisms that regulate the differential transcription of the gene complex are unknown. Experiments reported here demonstrated the direct correlation of isoform protein and mRNA expression with the undermethylation of the DNA in the promoter regions of the gene for each of the expressed myosin light chain isoforms. We find that fast and slow muscles have different patterns of undermethylation of the two promoter regions of the gene. Moreover, changes in methylation of the promoter regions were shown to occur specifically in skeletal muscle tissue, to be developmentally regulated, and to only occur in the LC3f promoter of those muscle groups that express LC3f protein.

The Maintenance of Different Enzyme Activity Levels in Congeneric Fishes Living at Different Depths

To determine the influence of depth-related factors on enzymic activity and, hence, on metabolism, we measured enzymic activities of white skeletal muscle and brain in two teleost fishes of the genus Sebastolobus (Scorpaenidae). These species are genetically close and have similar life histories but differ in their depth distributions. White skeletal muscle of the shallow-living species, S. alascanus, contained approximately twice the activity (measured as units per g wet weight of tissue) of lactate dehydrogenase (LDH), pyruvate kinase (PK), malate dehydrogenase (MDH), creatine phosphokinase (CPK), and citrate synthase (CS) as the white muscle of the deep-living species, S. altivelis. The CS and LDH activities exhibited body-sizerelated scaling; CS activity decreased and LDH activity rose in larger-sized specimens. The interspecific activity differences may derive either from differences in substrate turnover number (e.g., LDH) or from differences in enzyme concentration (e.g., PK). Through these two activity-regulating mechanisms, constant ratios of activity of different enzymes within the muscles of each species are maintained, while approximately twofold interspecific differences are maintained. The interspecific differences in enzymic activity are attributed to the importance of depth-related environmental factors, e.g., food availability, lack of light, and a low-energy physical environment, which may permit reduced levels of metabolism at depth. Correlated with the reduced capacity for muscle metabolism in S. altivelis is a small (9%) reduction in muscle-buffering capacity. Protein and water content did not differ in the muscle tissues of the two species. Unlike muscle, in brain tissue no interspecific differences in enzymic activity (LDH, PK, and CS) were found. This suggests that the differences in enzymic activity noted for muscle tissue reflect muscle-specific changes associated with reduced metabolic and locomotory demands at depth. Comparisons of congeneric species experiencing similar thermal regimes and having similar life history and ecological characteristics, but having different depth distributions, permit sensitive analyses of depth-related biochemical adaptations.