Transversus abdominis muscle function in humans

We used a high-resolution ultrasound to make electrical recordings from the transversus abdominis muscle in humans. The behavior of this muscle was then compared with that of the external oblique and rectus abdominis in six normal subjects in the seated posture. During voluntary efforts such as expiration from functional residual capacity, speaking, expulsive maneuvers, and isovolume "belly-in" maneuvers, the transversus in general contracted together with the external oblique and the rectus abdominis. In contrast, during hyperoxic hypercapnia, all subjects had phasic expiratory activity in the transversus at ventilations between 10 and 18 l/min, well before activity could be recorded from either the external oblique or the rectus abdominis. Similarly, inspiratory elastic loading evoked transversus expiratory activity in all subjects but external oblique activity in only one subject and rectus abdominis activity in only two subjects. We thus conclude that in humans 1) the transversus abdominis is recruited preferentially to the superficial muscle layer of the abdominal wall during breathing and 2) the threshold for abdominal muscle recruitment during expiration is substantially lower than conventionally thought.     

Reduction in hybrid single muscle fiber proportions with resistance training in humans.

The purpose of this investigation was to examine the effects of 12 wk of progressive resistance training (PRT) on single muscle fiber myosin heavy chain (MHC; I, I/IIa, I/IIa/IIx, IIa, IIa/IIx, IIx) isoform proportions in young individuals. Young, untrained men (YM; n = 6) and women (YW; n = 6) (age = 22 +/- 1 and 25 +/- 2 yr for YW and YM, respectively) received pre- and post-PRT muscle biopsies from the right vastus lateralis for single muscle fiber MHC distribution by electrophoretic analysis (192 +/- 5 pre- and 183 +/- 6 post-fibers/subject analyzed; 4,495 fibers total). Data are presented as percentages of the total fibers analyzed per subject. The PRT protocol elicited an increase in the pure MHC IIa (Delta = + 24 and + 27; YW and YM, respectively; P < 0.05) with no change in the pure MHC I distribution. The hybrid MHC distributions decreased I/IIa/IIx (Delta = -2; YM and YW; P < 0.05), IIa/IIx (Delta = -13 and -19 for YM and YW, respectively; P < 0.05), and total hybrid fiber proportion (I/IIa + I/IIa/IIx + IIa/IIx) decreased (Delta = -19 and -30 for YM and YW, respectively; P < 0.05) with the training, as did the MHC IIx distribution (Delta = -2; YW only; P < 0.05). Alterations in the predominance of MHC isoforms within hybrid fibers (decrease in MHC I-dominant I/IIa and nondominant MHC IIa/IIx, increase in MHC IIa-dominant IIa/IIx; P < 0.05) appeared to contribute to the increase in the MHC IIa proportion. Electrophoresis of muscle cross sections revealed an approximately 7% increase (P < 0.05) in MHC IIa proportion in both groups, whereas the MHC IIx decrease by 7.5 and 11.6% post-PRT in YW and YM, respectively. MHC I proportions increase in YM by 4.8% (P < 0.05) post-PRT. These findings further support previous resistance training data in young adults with respect to the increase in the MHC IIa proportions but demonstrate that a majority of the change can be attributed to the decrease in single-fiber hybrid proportions.     

Effect of long-term muscle paralysis on human single fiber mechanics.

This study compared human muscles following long-term reduced neuromuscular activity to those with normal functioning regarding single fiber properties. Biopsies were obtained from the vastus lateralis of 5 individuals with chronic (>3 yr) spinal cord injury (SCI) and 10 able-bodied controls (CTRL). Chemically skinned fibers were tested for active and passive mechanical characteristics and subsequently classified according to myosin heavy chain (MHC) content. SCI individuals had smaller proportions of type I (11 +/- 7 vs. 34 +/- 5%) and IIa fibers (11 +/- 6 vs. 31 +/- 5%), whereas type IIx fibers were more frequent (40 +/- 13 vs. 7 +/- 3%) compared with CTRL subjects (P < 0.05). Cross-sectional area and peak force were similar in both groups for all fiber types. Unloaded shortening velocity of fibers from paralyzed muscles was higher in type IIa, IIa/IIx, and IIx fibers (26, 65, and 47%, respectively; P < 0.01). Consequently, absolute peak power was greater in type IIa (46%; P < 0.05) and IIa/IIx fibers (118%; P < 0.01) of the SCI group, whereas normalized peak power was higher in type IIa/IIx fibers (71%; P < 0.001). Ca(2+) sensitivity and passive fiber characteristics were not different between the two groups in any fiber type. Composite values (average value across all fibers analyzed within each study participant) showed similar results for cross-sectional area and peak force, whereas maximal contraction velocity and fiber power were more than 100% greater in SCI individuals. These data illustrate that contractile performance is preserved or even higher in the remaining fibers of human muscles following reduced neuromuscular activity.     

Temporalis and masseter muscle function during incision in macaques and humans

Most previously published electromyographic (EMG) studies have indicated that the temporalis muscles in humans become almost electrically quiet during incisai biting. These data have led various workers to conclude that these muscles may contribute little to the incisai bite force. The feeding behavior and comparative anatomy of the incisors and temporalis muscles of certain catarrhine primates, however, suggest that the temporalis muscle is an important and powerful contributor to the bite force during incision. One purpose of this study is to analyze the EMG activity of the masseter and temporalis muscles in both humans and macaques with the intention of focusing on the conflict between published EMG data on humans and inferences of muscle function based on the comparative anatomy and behavior of catarrhine primates. The EMG data collected from humans in the present study indicate that, in five of seven subjects, the masseter,anterior temporalis, and posterior temporalis muscles are very active during apple incision (i.e., relative to EMG activity levels during apple and almond mastication). In the other two human subjects the EMG levels of these muscles are lower during incision than during mastication, but in no instance are these muscles ever close to becoming electrically quiet. The EMG data on macaques indicate that, in all six subjects, the masseter, anterior temporalis, and posterior temporalis muscles are very active during incision. These data are in general agreement with inferences on muscle function that have been drawn from the comparative anatomy and behavior of primates, but they do not agree with previous experimental data. The reason for this disagreement is probably due to differences in the experimental procedure. In previous studies subjects simply bit isometrically on their incisors and the resulting EMG pattern was compared to the pattern associated with powerful clenching in centric occlusion. In the present study the subjects incised into actual food objects, and the resulting EMG pattern was compared to the pattern associated with mastication of various foods. It is not surprising that these two procedures result in markedly different EMG patterns, which in turn result in markedly different interpretations of jaw-muscle function. In an attempt to explain the evolution of the postorbital septum in anthropoids, it has been suggested that the anterior temporalis is more active than the masseter during incision (Cachel, 1979). The human and macaque EMG data do not support this hypothesis; during incision, the two muscles show no consistent differences in humans and the masseter appears to be in fact more active than the anterior temporalis in macaques.     

Inspiratory muscle function with restrictive chest wall loading during exercise in normal humans

The effects of selective restriction of rib cage (Res,rc) and abdominal wall (Res,ab) movements on endurance of short-term constant-load heavy exercise and on diaphragmatic function during such exercise were examined in five normal young men. An inelastic surgical corset was used to achieve Res,rc and Res,ab. Subjects exercised on a cycle ergometer at 80% of their maximum power output to exhaustion on three occasions: with Res,rc, with Res,ab, and without restriction of chest wall movements (control). Transdiaphragmatic (Pdi), esophageal, and gastric pressures were measured. Electromyogram of the diaphragm was recorded by an esophageal electrode, and the ratio of the power content of a high-frequency to low-frequency band (H/L ratio) was measured. In addition, maximum Pdi (Pdimax) pre- and immediately postexercise was recorded. Res,rc was associated with a shorter endurance time, a progressive decline of the H/L ratio, and a significant reduction of Pdimax postexercise, whereas no such changes were found with Res,ab. We conclude that diaphragmatic function was well defended with abdominal wall loading, whereas limitation of rib cage expansion reduced diaphragmatic endurance during exercise. The diaphragmatic tension-time index (TTdi) in exercise was always less than the critical value of 0.15 found by Bellemare and Grassino (J. Appl. Physiol. 53: 1190-1195, 1982) when subjects inspired against large resistive loads at normal minute ventilations. We suggest that the higher inspiratory flow rate (P less than 0.05) and breathing frequency (P less than 0.05) account for the occurrence of diaphragmatic fatigue in exercise with Res,rc when the TTdi was 0.06 +/- 0.02.     

Self-organization of muscle cell structure and function

The organization of muscle is the product of functional adaptation over several length scales spanning from the sarcomere to the muscle bundle. One possible strategy for solving this multiscale coupling problem is to physically constrain the muscle cells in microenvironments that potentiate the organization of their intracellular space. We hypothesized that boundary conditions in the extracellular space potentiate the organization of cytoskeletal scaffolds for directed sarcomeregenesis. We developed a quantitative model of how the cytoskeleton of neonatal rat ventricular myocytes organizes with respect to geometric cues in the extracellular matrix. Numerical results and in vitro assays to control myocyte shape indicated that distinct cytoskeletal architectures arise from two temporally-ordered, organizational processes: the interaction between actin fibers, premyofibrils and focal adhesions, as well as cooperative alignment and parallel bundling of nascent myofibrils. Our results suggest that a hierarchy of mechanisms regulate the self-organization of the contractile cytoskeleton and that a positive feedback loop is responsible for initiating the break in symmetry, potentiated by extracellular boundary conditions, is required to polarize the contractile cytoskeleton.     

Muscle fiber size and function in elderly humans: a longitudinal study.

Cross-sectional studies are likely to underestimate age-related changes in skeletal muscle strength and mass. The purpose of this longitudinal study was to assess whole muscle and single muscle fiber alterations in the same cohort of 12 older (mean age: start of study 71.1+/-5.4 yr and end of study 80+/-5.3 yr) volunteers (5 men) evaluated 8.9 yr apart. No significant changes were noted at follow-up in body weight, body mass index, and physical activity. Muscle strength, evaluated using isokinetic dynamometry, and whole muscle specific force of the knee extensors were significantly lower at follow-up. This was accompanied by a significant reduction (5.7%) in cross-sectional area of the total anterior muscle compartment of the thigh as evaluated by computed tomography. Muscle histochemistry showed no significant changes in fiber type distribution or fiber area. Experiments with chemically skinned single muscle fibers (n=411) demonstrated no change in type I fiber size but an increase in IIA fiber diameter. A trend toward an increase in maximal force in both fiber types was observed. Maximum unloaded shortening velocity did not change. In conclusion, single muscle fiber contractile function may be preserved in older humans in the presence of significant alterations at the whole muscle level. This suggests that surviving fibers compensate to partially correct muscle size deficits in an attempt to maintain optimal force-generating capacity.     

Endurance training in humans: aerobic capacity and structure of skeletal muscle

The adaptation of muscle structure, power output, and mass-specific rate of maximal O2 consumption (VO2max/Mb) with endurance training on bicycle ergometers was studied for five male and five female subjects. Biopsies of vastus lateralis muscle and VO2max determinations were made at the start and end of 6 wk of training. The power output maintained on the ergometer daily for 30 min was adjusted to achieve a heart rate exceeding 85% of the maximum for two-thirds of the training session. It is proposed that the observed preferential proliferation of subsarcolemmal vs. interfibrillar mitochondria and the increase in intracellular lipid deposits are two possible mechanisms by which muscle cells adapt to an increased use of fat as a fuel. The relative increase of VO2max/Mb (14%) with training was found to be smaller by more than twofold than the relative increase in maximal maintained power (33%) and the relative change in the volume density of total mitochondria (+40%). However, the calculated VO2 required at an efficiency of 0.25 to produce the observed mass-specific increase in maximal maintained power matched the actual increase in VO2max/Mb (8.0 and 6.5 ml O2 X min-1 X kg-1, respectively). These results indicate that despite disparate relative changes the absolute change in aerobic capacity at the local level (maintained power) can account for the increase in aerobic capacity observed at the general level (VO2max).     

Ontogeny of histochemical fiber types and muscle function in the masseter muscle of miniature swine

In this study of masticatory maturation, the ontogeny of the histochemical fiber type composition of musculus masseter is examined in the omnivorous miniature swine (Sus scrofa). Fiber type characteristics are interpreted by comparison with electromyography (EMG) recorded during feeding behavior. Similar to locomotion studies, the results suggest a correspondence between the composition and arrangement of motor units and their recruitment pattern. Serial sections of masseter muscles from 10 minipigs, ranging from 2 weeks to slightly over 1 year of age, were stained for myosin adenosine triphosphatase (mATPase) activity to distinguish slow-twitch from fast-twitch fibers, and for nicotinamide adenosine dehydrogenase-tetrazolium reductase to assess the aerobic capacity of the same fibers. Although maintaining a uniformly high aerobic capacity throughout ontogeny and in adult animals, a transition is observed in the relative proportions of fast- and slow-twitch fibers. The primarily fast-twitch neonatal pig masseter eventually comprises approximately 25-30% slow-twitch fibers in adults, with a higher predominance of slow fibers in the deep (vs. superficial) and anterior (vs. posterior) regions of the muscle. Furthermore, while individual fibers of adult masseters generally stain for either alkaline- or acid-stable mATPase activity, a substantial proportion of cells in developing animals exhibits the presence of both isozymes. EMG results indicate functional heterogeneity within the masseter of adult pigs. During chewing, when pig chow is replaced by cracked corn, EMG activity in the deep portion of the muscle either decreases or increases slightly. In the superficial portion, however, muscle amplitudes become dramatically higher for corn, surpassing levels generated for chewing the less obdurate chow. These results are consistent with a behavioral transition from neonatal suckling to sustained mastication of foods of more complex textures eaten by adult pigs. The relationship between these fiber type and EMG results for pig masseter corresponds to those pertaining to motor unit recruitment in the extensor muscles of locomotion. Implications of this work for the evolutionary morphology of mastication also are discussed.     

Effect of muscle metaboreflex activation on carotid-cardiac baroreflex function in humans

Whether the activation of metabolically sensitive skeletal muscle afferents (i.e., muscle metaboreflex) influences cardiac baroreflex responsiveness remains incompletely understood. A potential explanation for contrasting findings of previous reports may be related to differences in the magnitude of muscle metaboreflex activation utilized. Therefore, the present study was designed to investigate the influence of graded intensities of muscle metaboreflex activation on cardiac baroreflex function. In eight healthy subjects (24 +/- 1 yr), the graded isolation of the muscle metaboreflex was achieved by post-exercise ischemia (PEI) following moderate- (PEI-M) and high- (PEI-H) intensity isometric handgrip performed at 35% and 45% maximum voluntary contraction, respectively. Beat-to-beat heart rate (HR) and blood pressure were measured continuously. Rapid pulse trains of neck pressure and neck suction (+40 to -80 Torr) were applied to derive carotid baroreflex stimulus-response curves. Mean blood pressure increased significantly from rest during PEI-M (+13 +/- 3 mmHg) and was further augmented during PEI-H (+26 +/- 4 mmHg), indicating graded metaboreflex activation. However, the operating point gain and maximal gain (-0.51 +/- 0.09, -0.48 +/- 0.13, and -0.49 +/- 0.12 beats.min(-1).mmHg(-1) for rest; PEI-M and PEI-H) of the carotid-cardiac baroreflex function curve were unchanged from rest during PEI-M and PEI-H (P > 0.05 vs. rest). Furthermore, the carotid-cardiac baroreflex function curve was progressively reset rightward from rest to PEI-M to PEI-H, with no upward resetting. These findings suggest that the muscle metaboreflex contributes to the resetting of the carotid baroreflex control of HR; however, it would appear not to influence carotid-cardiac baroreflex responsiveness in humans, even with high-intensity activation during PEI.     

Skeletal muscle ATP turnover and muscle fiber conduction velocity are elevated at higher muscle temperatures during maximal power output development in humans

Somatosympathetic reflexes were studied in young hyperinsulinemic, insulin-resistant (Zucker fatty) rats (ZFR) and a related control (Zucker lean) strain (ZLR). Glucose metabolism was characterized by minimal model analysis of intravenous glucose tolerance test data. Seven-week-old ZFR (n=18) and ZLR (n=17) were studied under pentobarbital anesthesia. Mean body weight and plasma glucose and insulin concentration were significantly greater (P<0.05) in ZFR than in ZLR, whereas basal values of mean arterial pressure (MAP) and heart rate (HR) were not significantly different. Increments of MAP (DeltaMAP) and HR (DeltaHR) elicited by electrical stimulation of the sciatic nerve (5-s trains of 100 pulses, 0.5-ms pulse duration, 100- to 400-microA pulse intensity) were significantly higher (ANOVA, P<0.05) in ZFR at each level of stimulus intensity. Regression analysis showed a linear increase in DeltaMAP and DeltaHR with increasing sciatic nerve stimulus intensity. Pressor responses to phenylephrine after ganglionic blockade demonstrated that vascular reactivity to adrenergic stimulation is not increased in ZFR compared with ZLR. Thus this factor does not contribute to enhancement of somatosympathetic reflexes observed in this strain. Insulin sensitivity in ZFR was one-fourth (P<0.05) that in ZLR. These results suggest that stronger sympathetic nervous reactivity in ZFR is associated with a severe insulin-resistant state before the onset of hypertension and support the hypothesis that insulin-mediated stimulation of the sympathetic nervous system is involved in the development of cardiovascular diseases related to alterations of glucose metabolism.     

Effects of lower limb unloading on skeletal muscle mass and function in humans

A model to simulate effects of microgravity on skeletal muscle mass and function in humans has been developed. Unilateral lower limb unloading that allowed ankle, knee, and hip joint mobility was conducted in six healthy men by suspending one lower limb and having the subjects walk on crutches. They performed maximal unilateral concentric or eccentric quadriceps actions at different angular velocities before and after 4 wk of suspension and after 4 days and after 7 wk of uncontrolled recovery. Peak torque (PT) and angle-specific torque (AST) were measured. Muscle cross-sectional area (CSA) and radiological density (RD) of the thigh were assessed by means of computerized tomography. Concentric and eccentric PT and AST across speeds decreased (P less than 0.05) by 22 and 16%, respectively, in response to unloading. At 4 days of recovery PT (-11%) and AST (-7%) were still lower (P less than 0.05) than before. Muscle CSA and RD decreased (P less than 0.05) by 7 and 6%, respectively. After 7 wk of recovery PT, AST, CSA, and RD had returned to normal. The control limb showed no changes over the experimental period except for a 6% decrease (P less than 0.05) in RD. It is suggested that this human model of unloading could serve to simulate effects of microgravity on skeletal muscle mass and function because reductions in muscle mass and strength were of similar magnitude to those produced by bed rest.     

Effects of paralysis with pancuronium on chest wall statics in awake humans

The influence of tonic inspiratory muscle activity on the relaxation characteristics of the chest wall, rib cage (RC), and abdominal wall (ABW) has been investigated in four highly trained subjects. Chest wall shape and volume were estimated with magnetometers. Pleural pressure (Pes) and abdominal pressure were measured with esophageal and gastric balloons, respectively. Subjects were seated reclining 30 degrees from upright, and respiratory muscle weakness was produced by pancuronium bromide until RC inspiratory capacity was decreased to 60% of control. Only minor changes were observed for Konno-Mead relaxation characteristics (RC vs. ABW) between control and paralysis. Similarly, although RC relaxation curves (RC vs. Pes) during paralysis were significantly different from control (P less than 0.05), the changes were small and not consistent. The differences between paralysis-induced changes in resting end-expiratory position of the chest wall and helium-dilution functional residual capacity (FRC) suggested changes in volume of blood within the chest wall. We conclude that 1) although tonic inspiratory activity of chest wall muscles exists, it does not significantly affect the chest wall relaxation characteristics in trained subjects; 2) submaximal paralysis produced by pancuronium bromide is likely to modify either spinal attitude or the distribution of blood between extremities and the thorax; these effects may account for the changes in FRC in other studies.     

Aging, muscle fiber type, and contractile function in sprint-trained athletes.

Biopsy samples were taken from the vastus lateralis of 18- to 84-yr-old male sprinters (n = 91). Fiber-type distribution, cross-sectional area, and myosin heavy chain (MHC) isoform content were identified using ATPase histochemistry and SDS-PAGE. Specific tension and maximum shortening velocity (V(o)) were determined in 144 single skinned fibers from younger (18-33 yr, n = 8) and older (53-77 yr, n = 9) runners. Force-time characteristics of the knee extensors were determined by using isometric contraction. The cross-sectional area of type I fibers was unchanged with age, whereas that of type II fibers was reduced (P < 0.001). With age there was an increased MHC I (P < 0.01) and reduced MHC IIx isoform content (P < 0.05) but no differences in MHC IIa. Specific tension of type I and IIa MHC fibers did not differ between younger and older subjects. V(o) of fibers expressing type I MHC was lower (P < 0.05) in older than in younger subjects, but there was no difference in V(o) of type IIa MHC fibers. An aging-related decline of maximal isometric force (P < 0.001) and normalized rate of force development (P < 0.05) of knee extensors was observed. Normalized rate of force development was positively associated with MHC II (P < 0.05). The sprint-trained athletes experienced the typical aging-related reduction in the size of fast fibers, a shift toward a slower MHC isoform profile, and a lower V(o) of type I MHC fibers, which played a role in the decline in explosive force production. However, the muscle characteristics were preserved at a high level in the oldest runners, underlining the favorable impact of sprint exercise on aging muscle.     

Single soleus muscle fiber function after hindlimb unweighting in adult and aged rats

This investigation compared how hindlimbunweighting (HU) affected the contractile function of single soleusmuscle fibers from 12- and 30-mo-old Fischer 344 Brown Norway F1 Hybridrats. After 1 wk of HU, functional properties of singlepermeabilized fibers were studied, and, subsequently, the fiber typewas established by myosin heavy chain (MHC) analysis. After HU, therelative mass of soleus declined by 12 and 19% and the relative massof the gastrocnemius declined by 15 and 13% in 12- and 30-mo-oldanimals, respectively. In 12-mo-old animals, the peak active force(5.0 ± 0.2 ×10⁴ vs. 3.8 ± 0.2 ×10⁴ N) and thepeak specific tension (92 ± 4 vs. 78 ± 3 kN/m²) were significantlyreduced in the MHC type I fibers by 24 and 15%, respectively. In30-mo-old animals, the peak active force declined by 40% (4.7 ± 0.2 ×10⁴vs. 2.8 ± 0. 3 ×10⁴ N) and the peakspecific tension declined by 30% (79 ± 5 vs. 55 ± 4 kN/m²). The maximal unloadedshortening velocity of the MHC type I fibers increased in 12-mo-oldanimals (from 1.65 ± 0.12 to 2.59 ± 0.26 fiber lengths/s) andin 30-mo-old animals (from 0.90 ± 0.09 to 1.50 ± 0.10 fiberlengths/s) after HU. Collectively, these data suggest thatthe effects of HU on single soleus skeletal muscle fiber function occurin both age groups; however, the single MHC type I fibers from theolder animals show greater changes than do single MHC type I fibersfrom younger animals.