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PubMed2000

Effects of injury proneness and task difficulty on joint kinetic variability.

PURPOSE: The purpose was to investigate the effects of lower extremity overuse injury proneness and landing height on the variability of selected joint moment characteristics. METHODS: Ten subjects from each of two groups (healthy and injury prone) performed 10 landings from a platform (heights: 50, 100, and 200% of maximum vertical jump). Force platform (1000 Hz) and video (200 Hz) information were collected and used to compute ankle, knee, and hip joint moment values during impact (0-100 ms post contact). Moment peak, time to peak, and impulse values were used to determine variability across 10 trials for each subject and height. MANOVAs and univariate follow-up tests were used to assess variability differences between groups and among heights. RESULTS: Results revealed ankle moment variability differences (P < or = 0.05; Tukey) between groups for peak and time to peak variables at the 100% and 50% heights, respectively. The injury-prone group exhibited greater variability for the peak variable, whereas the healthy group showed greater variability for the temporal variable. Groups also differed on the impulse variables (P < or = 0.02; MANOVA), but follow-up tests failed to determine specific joints or directions of differences. Both groups exhibited tendencies for greater variability (P < or = 0.05; Tukey) with increases in height up to 100% MVJ and decreases in variability with further height increases. CONCLUSIONS: Results suggest that groups differed in joint moment variability, possibly indicating a relationship between variability and overuse injuries, although the variables and directions of differences were inconsistent. For some variables, variability increased and then decreased with height increases for both groups, suggesting a range of heights within which the neuromuscular system adapted.

Adult↗
PubMed2006

Effects of hyperbaric exposure with high oxygen concentration on the physical activity of developing rats.

The effects of hyperbaric exposure with high oxygen concentration on the physical activity of developing male rats were investigated. Five-week-old male rats were exposed to an atmospheric pressure of 1.25 with an oxygen concentration of 36.0% for 12 h (7.00-19.00 h) and exercised voluntarily for 12 h (19.00-7.00 h) daily for 8 weeks. The voluntary running activities were compared with those in age-matched rats without hyperbaric exposure. In addition, the properties of the soleus and plantaris muscle fibers and their spinal motoneurons were examined. The voluntary running activities of rats with or without hyperbaric exposure increased during development. However, the mean voluntary running activities were higher in rats with hyperbaric exposure (7,104 m/day) than in those without hyperbaric exposure (4,932 m/day). The oxidative capacities of the soleus and plantaris muscle fibers and their spinal motoneurons increased following hyperbaric exposure. It is suggested that adaptations of neuromuscular units to hyperbaric exposure with high oxygen concentration enhance the metabolism, and thus, the function of neuromuscular units is promoted.

Animals↗
PubMed1982

Sequential neuromuscular changes in rhesus monkeys during the initial adaptation to oral respiration.

Experimental induction of oral respiration in primates altered the neuromuscular use of specific craniofacial muscles. Obstruction of the nasal passage in the rhesus monkey induced changes in the electromyographic discharge (EMG) of both mandibular and facial muscles during the first 6 months of adaptation. Eighteen craniofacial muscles were studied with regard to their type of neuromuscular pattern. The EMG discharge was analyzed in terms of whether it had a rhythmic discharge or a continuous recruitment of motor units. The results of the investigation revealed that certain muscles in the control monkey using nasal respiration could be rhythmically or continuously active, but no significant trend was apparent with either pattern over time. In contrast, a significant number of muscles became rhythmically active within the first month of adaptation to oral respiration in the experimental animals. The rhythmic pattern was evident in key muscles that actively depressed the mandible, protruded the tongue, altered the shape of the tongue, and raised the upper lip. Continuous activity was induced in the first month within the suprahyoid region and tongue but later, by the fifth month, in specific lip and elevator muscles. These results suggested that the neuromuscular system adapted immediately to nasal obstruction but would vary as to (1) which muscles would be important in the initial adaptation, (2) the mode of adaptation, and (3) the time when a particular pattern first began to be used.

Adaptation, Physiological↗
PubMed2005

Effect of rat soleus muscle overload on neuromuscular transmission efficacy during continuous and intermittent activation.

Increased neuromuscular activity is known to provoke morphological and functional adaptations at the neuromuscular synapse. Most of these changes have been documented following endurance exercise training programmes. In this study, the effect of rat soleus muscle overload produced by tenotomy plus voluntary wheel-cage activity on neuromuscular transmission efficacy was investigated. The overload protocol increased miniature endplate potential (MEPP) and endplate potential (EPP) amplitudes by 17 and 19%, respectively (both P < 0.01), and increased MEPP frequency by 86% (P < 0.01). EPP amplitude rundown during continuous trains of activation was attenuated by approximately 10% in the overloaded group (P < 0.01). Also, during intermittent activation, the overload protocol attenuated EPP amplitude rundown, mainly by enhancing EPP amplitude recovery by approximately 10% during the quiescent periods (P < 0.01). Although the present results show that both the degree and direction of adaptation are similar to what has been observed at rat soleus neuromuscular junctions following an endurance training protocol, there are important nuances between the results, suggesting different mechanisms through which these changes may occur.

Adaptation, Physiological↗
PubMed1991

Model-based adaptive closed-loop feedback control of atracurium-induced neuromuscular blockade.

Closed-loop control of atracurium-induced neuromuscular blockade by a model-based adaptive feedback algorithm is described. Mean offsets (+/- s.d.) from setpoints at 50, 70 and 90% neuromuscular blocks using the Relaxograph were 1.1 +/- 1.3, 0.2 +/- 0.7 and 0.1 +/- 0.4%, respectively. Correspondingly, the mean steady-state rates of infusion of atracurium were 0.20 +/- 0.06, 0.25 +/- 0.03 and 0.39 +/- 0.10 mg.kg-1.h-1. The described controller provides reasonable control of atracurium dosing at different degrees of neuromuscular blockade. It gives a solution to the problem of adapting pharmacokinetic and pharmacodynamic data to individuals when using population mean data as starting values for drug therapy.

Algorithms↗
PubMed2000

Adaptive neural network control of cyclic movements using functional neuromuscular stimulation.

In this study, we evaluated the performance of an adaptive feedforward controller and its ability to automatically develop and customize stimulation patterns for use in functional neuromuscular stimulation (FNS) systems. Results from previous experiments using the pattern generator/pattern shaper (PG/PS) controller to generate isometric contractions demonstrated its ability to adjust stimulation patterns to account for recruitment nonlinearities and muscle dynamics. In this study, the PG/PS controller was tested under isotonic conditions. This evaluation required the PG/PS controller to account for muscle length-tension and force-velocity properties as well as limb dynamics. The performance of the adaptive controller was also compared with that of a proportional-derivative (PD) feedback controller. The PG/PS controller is composed of a neural network system that adaptively filters a periodic signal to produce a muscle stimulation pattern for generating cyclic movements. We used computer-simulated models to determine controller parameters for the PG/PS and PD controller that perform well across a variety of musculoskeletal systems. The controllers were then experimentally evaluated on both legs of two subjects with spinal cord injury. Results indicated that the PG/PS controller was able to achieve and maintain better tracking performance than the PD controller. This study indicates that the PG/PS control system may provide an effective mechanism for automatically customizing stimulation patterns for individuals using FNS systems.

Algorithms↗
PubMed2004

Myoelectric manifestations of fatigue during exposure to hypobaric hypoxia for 12 days.

Lack of oxygen, as occurs at high altitude (HA), leads to a number of adaptive processes in muscle, but their precise nature is unclear. To better understand mechanisms of adaptations of the neuromuscular system to HA, we collected surface electromyographic (EMG) signals during a 12-day stay at 5,050 m above sea level (SL). The aim was to investigate the effect of hypobaric hypoxia on muscle-fiber membrane and motor-unit control properties. Surface EMG signals were recorded from the dominant biceps brachii muscle of six subjects at HA and 3 months after their return to SL. Supramaximal electrical stimuli (25 HZ) were delivered and voluntary isometric contractions at 40 and 80% of maximal voluntary torque were performed in 10 experimental sessions at HA and in 3 at SL. Maximal isometric torque was not altered at HA. Surface EMG spectral frequencies at the beginning of the voluntary contractions were greater at HA than SL. The rates of change of spectral frequencies and conduction velocity during the voluntary contractions were significantly larger at HA than SL. No differences in EMG variables were observed in the electrically elicited contractions. The maximal torque and surface EMG variables did not depend on the day of measure at HA. It was concluded that acute exposure to hypobaric hypoxia does not significantly affect the muscle-fiber membrane properties but does impact motor-unit control properties. This provides new insights in the understanding of motor control in extreme conditions of oxygen reduction, with relevance for sport and rehabilitation medicine, and may also explain the pathophysiological adaptations of the neuromuscular system occurring in such disorders as chronic obstructive pulmonary disease.

Acclimatization↗
PubMed2005

Adapting locomotion to different surface compliances: neuromuscular responses and changes in movement dynamics.

Knowledge of how the nervous system deals with surfaces with different physical properties such as compliance that challenge balance during locomotion is of importance as we are constantly faced with these situations every day. The purpose of this study was to examine the control of center of mass (COM) and lower limb dynamics and recovery response modulation of muscle activity during locomotion across an unexpected compliant surface and in particular, scaling behavior across different levels of compliance. Eight young adults walked along a walkway and stepped on an unexpected compliant surface in the middle of the travel path. There were three different levels of surface compliance, and participants experienced either no compliant surface or one of the three compliant surfaces during each trial that were presented in a blocked or random fashion. Whole body kinematics were collected along with surface electromyography (EMG) of selected bilateral lower limb and trunk muscles. The recovery response to the first compliant-surface trial demonstrated muscle onset latencies between 97 and 175 ms, and activity was modulated while on the compliant surface. Vertical COM trajectory was not preserved after contact with the compliant surface: peak vertical COM, while on the compliant surface was lower than when on stable ground. Perturbed-limb knee flexion after toe-off increased with increased surface compliance, which enabled toe clearance with the ground to be similar to control trials. The results suggest that stepping off of a compliant surface is actively modulated by the CNS and is geared toward maintaining dynamic stability.

Adaptation, Physiological↗
PubMed1986

[Morphometric characteristics of neuromuscular spindles in hypertrophied skeletal muscle].

Skeletal muscle hypertrophy in young male rats was found to be accompanied by adaptive changes in neuromuscular spindles. The changes consisted in connective capsule thickening, increased diameter of NMS and intrafusal muscle fibers, expanded afferent and efferent nerve terminals, increased microcirculatory bed capacity. The quantitative and qualitative shifts observed in NMS structure are morphologically equivalent to the rise in their functional potential, which forms the basis for the functional changes in conditions of increasing skeletal muscle hypertrophy.

Animals↗
PubMed1996

Response of the neuromuscular unit to spaceflight: what has been learned from the rat model.

Despite the inherent limitations placed on spaceflight investigations, much has been learned about the adaptations of the neuromuscular system to weightlessness from studies of rats flown for relatively short periods (approximately 4-22 days). Below is a summary of the major effects of spaceflight observed in muscles of rats that are not in their rapid growth stage: 1. Skeletal muscles atrophy rapidly during spaceflight; significant atrophy is observed as early as after 4 days of flight. 2. The atrophic response appears to be related to the primary function of the muscle. In the hindlimb, the relative amount of atrophy can be characterized as slow extensors > fast extensors > fast flexors. This pattern of relative atrophy does not appear to be occurring in the forelimb; however, not enough data are available to draw any definitive conclusions at this time. 3. Both slow and fast fibers atrophy during spaceflight, with the largest fibers within an individual muscle generally showing the greatest atrophic response. Interestingly, the amount of fiber atrophy appears to reach a plateau after about 14 days of flight. 4. Adaptations have been observed in the concentration and content of all muscle proteins pools, with the protein pools in slow muscles the most affected. 5. Some slow and fast fibers in predominantly slow and fast muscles show shifts in their histochemical and biochemical properties, toward those observed in a "faster" phenotype. 6. Some fibers, presumably expressing slow MHC isoforms before flight, begin to express fast MHC isoforms during flight. 7. The oxidative capacity of the muscles or fibers is relatively unaffected by spaceflight, particularly in the slow muscles. Any change in whole-body fatigability associated with spaceflight most likely reflects the loss in muscle and fiber mass. 8. The glycolytic capacity of the muscles and muscle fibers is enhanced after spaceflight. This metabolic adaptation seems to be related to the shift in the contractile proteins towards "faster" isoforms. 9. The vascularity of muscles appears to be maintained after flight, based, at least, on histological observations of capillarity. 10. The force capabilities of the muscles and fibers appear to decrease in parallel with the decreases in size, i.e., the specific tension is not significantly affected after flight. 11. Changes in the speed-related properties of the slow muscles are consistent with the adaptations in the myosin molecule, i.e., the slow muscles and some fibers in the slow muscles become "faster." 12. Some muscle fiber and neuromuscular junction damage has been observed after flight, particularly in the slow muscles. The extent of damage may be related to the amount of time that the muscles are allowed to reload before removal, i.e., in general, shorter intervals result in less fiber damage. 13. Adaptations in the motor (ventral horn) and sensory (spinal ganglia) neurons have been quite variable, but this may be related to the amount of time that the muscles are allowed to reload before removal. Morphological adaptations after relatively short periods of reloading may reflect a decrease in the activation of the neural elements during flight.

Adaptation, Physiological↗
PubMed1988

Daily hormonal and neuromuscular responses to intensive strength training in 1 week.

Daily adaptive responses in the neuromuscular and endocrine systems to a 1-week very intensive strength training period with two training sessions per day were investigated in eight elite weight lifters. The morning and the afternoon sessions resulted in acute decreases (P less than 0.05-0.01) in maximal isometric strength and in the maximal neural activation (iEMG) of the leg extensor muscles, but the basic levels remained unaltered during the entire training period. Significant (P less than 0.05-0.01) acute increases in serum total and free testosterone levels were found during the afternoon sessions. During the 1-week training period, serum total and free testosterone concentrations decreased gradually (P less than 0.05-0.001) as observed in the basic morning values before the sessions, but after 1 day of rest serum total and free testosterone reached (P less than 0.01 and 0.05) the pretraining level. The sessions resulted also in acute changes (P less than 0.05-0.01) in serum cortisol and somatotropin concentrations, but the basic morning levels did not change during the training period. The present findings suggest that during a short period of intense strength training the changes especially in serum testosterone concentrations indicate the magnitude of physiologic stress of training. The acute changes in serum hormone concentrations during a period of a few days do not, however, necessarily directly imply the changes in performance capacity. A longer period of follow-up lasting a few weeks is probably needed if an individual trainability status of a strength athlete is to be evaluated on the basis of the hormone determinations.

Adult↗
PubMedDate not supplied

Neuromuscular properties and functional aspects of taped ankles.

We used electromyographic and goniometric methods to test 40 subjects to describe the neuromuscular and biomechanical adaptation of the ankle with respect to application of two different adhesive tapes and to exercises. The neuromuscular responses to inversion injury simulation, together with the mechanical displacements of the joint complex, were analyzed before and after controlled athletic exercises. The proprioceptive amplification ratio was calculated on the basis of the integrated reflex electromyographic results and on the maximum inversion amplitude. Relevant stability gains were achieved immediately after applying tape. There was reduced tape stability after athletic exercise for one of the two tape materials tested. No further loosening was detected, even after prolonged wearing of tape (24 hours). Compared with the unprotected ankle, the taped ankle had a significant increase in the proprioceptive amplification ratio. Both fatigue and mechanical loosening may be responsible for the significant reduction in this ratio immediately after exercise. After the 24-hour interval, the ratio was increased, which could be explained by physiologic neuromuscular regeneration and mechanical restabilization of the tape itself. The sensitivity of the proprioceptive amplification ratio, both to external stabilization and to internal fatigue, supports its potential value to quantify functional joint stability.

Adolescent↗
PubMed2001

Chronic extrinsic denervation after small bowel transplantation in rat jejunum: Effects and adaptation in nitrergic and non-nitrergic neuromuscular inhibitory mechanisms.

BACKGROUND: Extrinsic denervation of the transplanted small bowel could play a substantial role in motor dysfunction of the transplanted gut. We attempted to determine the effect of chronic extrinsic denervation on intestinal contractility. METHODS: Jejunal longitudinal muscle strips were obtained from rats 1 week and 8 weeks after (1) syngeneic small bowel transplantation, (2) ischemia/reperfusion, or (3) gut transection/reanastomosis. Nonoperated rats (naive controls) and sham-operated rats (sham controls), 1 week after celiotomy/gut manipulation, served as controls. We evaluated the effects of exogenous nitric oxide, increasing doses of cholinergic and adrenergic agonists, and electrical field stimulation (EFS) in the presence or absence of N(G)-monomethyl-l-arginine, methylene blue, tetraethylammonium, or tetrodotoxin. RESULTS: Spontaneous contractile activity (_chi +/- SEM), when compared with the naive controls (11.3 +/- 2.0 g.5 min/mg), was increased in all 4 groups at 1 week (15.9 +/- 10 to 19.4 +/- 2 g.5 min/mg; P < or =.03 each) but not at 8 weeks postoperatively. The inhibition of contractile activity by nitric oxide was increased in small bowel transplantation in naive controls at 8 weeks to 80% +/- 10% versus 50% +/- 7% (P <.02). EFS induced an inhibition of contractile activity that was tetraethylammonium- and tetrodotoxin-sensitive but N(G)-monomethyl-l-arginine- and methylene blue-insensitive; the maximal EFS-induced inhibition was increased at 1 week and 8 weeks but only in the small bowel transplantation groups to 103% +/- 5% and 95% +/- 7%, respectively, versus 72% +/- 8% in naive controls (P </=.05). CONCLUSIONS: [corrected] Increased inhibition of contractile function after small bowel transplantation lasts at least 8 weeks and is mediated by changes in the enteric neuromuscular unit caused by extrinsic denervation.

Adaptation, Physiological↗
PubMed2000

[Ultrastructural characteristics of rat neuromuscular junctions after physical load].

It is interesting to ascertain the adaptive reaction of rat neuromuscular junctions (NMJ) of muscle fibers of different types to a chronic physical load. We examined ultrastructural changes in NMJ following both static load (pre- and postnatal ontogenesis of Wistar rats till a 2 month age took place under a constant rotation on the centrifuge at hypergravity conditions 2G), and after three kinds of dynamic loads (1/run on treadmill with a speed 35 m/min for 6 wks, 10-60 min/day; 2/swimmings, each 10 hrs/day for 10 days; 3/strength exercises on a vertical treadmill with load for 6 wks). Differences in NMJ reaction of muscle fibers of the same type to various loads were established. A low secretory activity of axonal terminals of type I muscle fibers of m. soleus was shown after the static load. The dynamic load (run) is accompanied with a high secretory activity of axonal terminals in m. soleus type I muscle fibers and of some axonal terminals of m. quadriceps femoris IIB type muscle fibers after strength exercises; the secretory activity of axonal terminals of m. quadriceps femoris IIA and IIB types muscle fibers is expressed in a lesser degree after swimming. The NMJ ultrastructure remodelling (terminal renewal) of type I muscle fibers of the 2 month old control rats increases after static and dynamic (run) loads. Some correlations between different kinds of physical load, muscle fiber type and the degree of NMJ ultrastructure transformation have been shown.

Animals↗
PubMed1992

Adaptations in coactivation after isometric resistance training.

Twenty sedentary male university students were randomly assigned to an experimental or a control group. The experimental group trained the knee extensors of one leg by producing 30 isometric extension maximal voluntary contractions (MVC) per day, three times per week for 8 wk. After 8 wk of training, extensor MVC in the trained leg increased 32.8% (P less than 0.05), but there was no change in vastus lateralis maximal integrated electromyographic activity (IEMGmax). The most important finding was that the degree of hamstring coactivation during extension MVC decreased by approximately 20% (P less than 0.05) after the 1st wk of training. Less pronounced adaptations occurred in the untrained leg: extension MVC force increased 16.2% (P less than 0.05), hamstring coactivity decreased 13% (P less than 0.05) after 2 wk of training, and vastus lateralis IEMGmax was unchanged. The same measures in legs of the control group were not changed during the study. There were no changes in flexion MVC, biceps femoris IEMGmax, or the degree of quadriceps coactivity during flexion MVC in either leg of the control or experimental group. A reduction in hamstring coactivity in the trained and untrained legs indicates that these muscles provide less opposing force to the contracting quadriceps. We conclude that this small but significant decrease in hamstring coactivation that occurs during the early stages of training is a nonhypertrophic adaptation of the neuromuscular system in response to static resistance training of this type.

Adaptation, Physiological↗