Neural adaptations to resistance training - Implications for movement control

It has long been believed that resistance training is accompanied by changes within the nervous system that play an important role in the development of strength. Many elements of the nervous system exhibit the potential for adaptation in response to resistance training, including supraspinal centres, descending neural tracts, spinal circuitry and the motor end plate connections between motoneurons and muscle fibres. Yet the specific sites of adaptation along the neuraxis have seldom been identified experimentally, and much of the evidence for neural adaptations following resistance training remains indirect. As a consequence of this current lack of knowledge, there exists uncertainty regarding the manner in which resistance training impacts upon the control and execution of functional movements. We aim to demonstrate that resistance training is likely to cause adaptations to many neural elements that are involved in the control of movement, and is therefore likely to affect movement execution during a wide range of tasks. We review a small number of experiments that provide evidence that resistance training affects the way in which muscles that have been engaged during training are recruited during related movement tasks. The concepts addressed in this article represent an important new approach to research on the effects of resistance training. They are also of considerable practical importance, since most individuals perform resistance training in the expectation that it will enhance their performance in-related functional tasks.

Neural influences on sprint running - Training adaptations and acute responses

Performance in sprint exercise is determined by the ability to accelerate, the magnitude of maximal velocity and the ability to maintain velocity against the onset of fatigue. These factors are strongly influenced by metabolic and anthropometric components. Improved temporal sequencing of muscle activation and/or improved fast twitch fibre recruitment may contribute to superior sprint performance. Speed of impulse transmission along the motor axon may also have implications on sprint performance. Nerve conduction velocity (NCV) has been shown to increase in response to a period of sprint training. However, it is difficult to determine if increased NCV is likely to contribute to improved sprint performance. An increase in motoneuron excitability, as measured by the Hoffman reflex (H-reflex), has been reported to produce a more powerful muscular contraction, hence maximising motoneuron excitability would be expected to benefit sprint performance. Motoneuron excitability can be raised acutely by an appropriate stimulus with obvious implications for sprint performance. However, at rest reflex has been reported to be lower in athletes trained for explosive events compared with endurance-trained athletes. This may be caused by the relatively high, fast twitch fibre percentage and the consequent high activation thresholds of such motor units in power-trained populations. In contrast, stretch reflexes appear to be enhanced in sprint athletes possibly because of increased muscle spindle sensitivity as a result of sprint training. With muscle in a contracted state, however, there is evidence to suggest greater reflex potentiation among both sprint and resistance-trained populations compared with controls. Again this may be indicative of the predominant types of motor units in these populations, but may also mean an enhanced reflex contribution to force production during running in sprint-trained athletes. Fatigue of neural origin both during and following sprint exercise has implications with respect to optimising training frequency and volume. Research suggests athletes are unable to maintain maximal firing frequencies for the full duration of, for example, a 100m sprint. Fatigue after a single training session may also have a neural manifestation with some athletes unable to voluntarily fully activate muscle or experiencing stretch reflex inhibition after heavy training. This may occur in conjunction with muscle damage. Research investigating the neural influences on sprint performance is limited. Further longitudinal research is necessary to improve our understanding of neural factors that contribute to training-induced improvements in sprint performance.

Reversal of cardiovascular remodelling with candesart

Cardiovascular remodelling, defined as ventricular and vascular hypertrophy together with fibrosis, characterises hypertension following inhibition of the production of the endogenous vasodilator, nitric oxide (NO). This study has determined whether the cardiovascular remodelling following chronic NO synthase inhibition can e reversed by administration of the selective angiotensin II AT(1)-receptor antagonist, candesartan. Male Wistar rats were treated with L-nitroarginine methyl ester (L-NAME, 400 mg/l in drinking water) for eight weeks and with candesartan cilexetil (2 mg/kg/day by oral gavage) for the last four weeks. L-NAME-treated rats became hypertensive with systolic blood pressure increasing from 110 +/- 4 mmHg (control) to 170 +/- 10 mmHg. Rats developed left ventricular hypertrophy (control 1.70 +/- 0.06; L-NAME 2.10 +/- 0.04 mg/kg body wt) with markedly increased deposition of perivascular and interstitial collagen. Candesartan returned blood pressure, left ventricular weights and collagen deposition to control values. Echo cardiographic assessment showed concentric hypertrophy with an increased fractional shortening; this was reversed by candesartan treatment. Heart failure was not evident. In the isolated Langendorff heart, diastolic stiffness increased in L-NAME-treated rats while the rate of increase in pressure (+dP/dt) increased after eight weeks only; candesartan reduced collagen deposition and normalised +dP/dt. In isolated left ventricular papillary muscles, the potency (negative log EC50) of noradrenaline as a positive inotropic compound was unchanged, (control 6.56 +/- 0.14); maximal increase in force before ectopic beats was reduced from 5.0 +/- 0.4 mN to 2.0 +/- 0.2 mN. Noradrenaline potency as a vasoconstrictor in thoracic aortic rings was unchanged, but maximal contraction was markedly reduced from 25.2 +/- 2.0 mN to 3.0 +/- 0.3 mN; this was partially reversed by candesartan treatment. Thus, chronic inhibition of NO production with L-NAME induces hypertension, hypertrophy and fibrosis with increased toxicity and significant decreases in vascular responses to noradrenaline. These changes were at least partially reversible by treatment with candesartan, implying a significant role of AT(1)-receptors in L-NAME-induced cardiovascular changes.

Reversal of cardiac and renal fibrosis by pirfenidone and spironolactone in streptozotocin-diabetic rats

1 Fibrosis leads to chronic impairment of cardiac and renal function and thus reversal of existing fibrosis may improve function and survival. This project has determined whether pirfenidone, a new antifibrotic compound, and spironolactone, an aldosterone antagonist, reverse both deposition of the major extracellular matrix proteins, collagen and fibronectin, and functional changes in the streptozotocin(STZ)-diabetic rat. 2 Streptozotocin (65 mg kg(-1) i.v.)-treated rats given pirfenidone (5-methyl-1-phenyl-2-[1H]-pyridone; approximately-200 mg kg(-1) day(-1) as 0.2-2g l(-1) drinking water) or spironolactone (50 mg kg(-1) day(-1) s.c.) for 4 weeks starting 4 weeks after STZ showed no attenuation of the increased blood glucose concentrations and increased food and water intakes which characterize diabetes in this model. 3 STZ-treatment increased perivascular and interstitial collagen deposition in the left ventricle and kidney, and surrounding the aorta. Cardiac, renal and plasma fibronectin concentrations increased in STZ-diabetic rats. Passive diastolic stiffness increased in isolated hearts from STZ-diabetic rats. Both pirfenidone and spironolactone treatment attenuated these increases without normalizing the decreased + dP/dt(max) of STZ-diabetic hearts. 4 Left ventricular papillary muscles from STZ-treated rats showed decreased maximal positive inotropic responses to noradrenaline, EMD 57033 (calcium sensitizer) and calcium chloride; this was not reversed by pirfenidone or spironolactone treatment. STZ-treatment transiently decreased GFR and urine flow rates in isolated perfused kidneys; pirfenidone but not spironolactone prevented the return to control values. 5 Thus, short-term pirfenidone and spironolactone treatment reversed cardiac and renal fibrosis and attenuated the increased diastolic stiffness without normalizing cardiac contractility or renal function in STZ-diabetic rats.

Effect of chronic morphine treatment on alpha(2)-adrenoceptor mediated autoinhibition of transmitter release from sympathetic varicosities of the mouse vas deferens

1 The effect of chronic morphine treatment (CMT) on sympathetic innervation of the mouse vas deferens and on alpha (2)-adrenoceptor mediated autoinhibition has been examined using intracellular recording of excitatory junction potentials (EJPs) and histochemistry. 2 In chronically saline treated (CST) preparations. morphine (1 muM) and the alpha (2)-adrenoceptor agonist (clonidine, 1 muM) decreased the mean amplitude of EJPs evoked with 0.03 Hz stimulation by 81+/-8% (n=16) and 92+/-6% (n=7) respectively. In CMT preparations, morphine (1 muM) and clonidine (1 muM) decreased mean EJP amplitude by 68+/-8% (n = 7) and 79+/-8% (n = 7) respectively. 3 When stimulating the sympathetic axons at 0.03 Hz. the mean EJP amplitude recorded from smooth muscles acutely withdrawn from CMT was four times greater than for CST smooth muscles (40.7+/-3.8 mV, n = 7 compared with 9.9+/-0.3 mV, n = 7). 4 Part of the increase in mean EJP amplitude following CMT was produced by a 31% increase in the density of sympathetic axons and varicosities innervating the smooth muscle. 5 Results from the present study indicate that the effectiveness of alpha (2)-adrenocrptor mediated autoinhibition is only slightly reduced in CMT preparations. Most of the cross tolerance which develops between morphine, clonidine and alpha (2)-adrenoceptor mediated autoinhibition occurs as a consequence of increased efficacy of neuromuscular transmission which is produced by an increase in the probability of transmitter release and an increase in the density of sympathetic innervation.

Maintaining muscle mass during extended disuse: Aestivating frogs as a model species

Prolonged muscle disuse in vertebrates can lead to a pathological change resulting in muscle wasting and a loss of muscle strength. In this paper, we review muscle disuse atrophy in the vertebrates and examine the factors that influence the magnitude of the atrophic response during extended periods of inactivity, both artificially imposed (e.g. limb immobilisation) and naturally occurring, such as the quiescence associated with dormancy (e.g. hibernation and aestivation). The severity of muscle atrophy is positively correlated with mass-specific metabolic rate, and the metabolic depression that occurs during dormancy would appear to have a protective role, reducing or preventing muscle atrophy despite periods of inactivity lasting 6-9 months. In the light of these findings, the role of reactive oxygen species and antioxidants during muscle disuse is emphasised.

Squat, stoop, or something in between?

This article reviews the empirical and theoretical bases for recommendations regarding lifting technique. Lifting from postures involving extreme lumbar vertebral flexion, (approximately 60degrees of lumbar flexion, characterised by absence of electromyographical activity in erector spinae) has the potential to contribute to damage to ligaments and intervertebral discs, especially if combined with lateral flexion or rotation. The only appropriate recommendation regarding posture of the lumbar spine during lifting is to avoid postures involving extreme lumbar vertebral flexion (and rotation and lateral flexion). There is no empirical basis for avoiding postures involving moderate lumbar vertebral flexion, and no justification for advocating lifting from a full squat posture. Further, lifting from semi-squat postures, involving a moderate range of flexion at both knees and trunk, allows a pattern of interjoint coordination which appears to be functional in reducing muscular effort. Lifting training is generally ineffective, and there is unlikely to be a single best technique which is appropriate in all situations. Consequently, it may be preferable to provide education in general lifting guidelines and assist lifters to discover individually appropriate postures and patterns of movement. The article concludes by presenting recommendations for lifting technique which are justified by current knowledge.

Quadriceps concentric and eccentric exercise 1: Changes in contractile and electrical activity following eccentric and concentric exercise

The purpose of this study was to determine whether or not losses of strength or endurance following eccentric and concentric exercise are associated with reduced excitation. The effects of eccentric and concentric work on maximal voluntary isometric contraction (MVC) and surface electromyogram (EMG) of the quadriceps were studied in 10 healthy male subjects following bench-stepping for 20 min with a constant leading leg. Prior to stepping and at 0, 0.25, 0.50, 0.75, 1, 3. 24 and 48 h afterwards the subjects performed a 30 s leg extension MVC with each leg during which the isometric force and the root mean square voltage of the EMG were recorded. In the eccentrically exercised muscles (ECC), MVC0-3 (force during the first 3 s of contraction) fen immediately after the bench-stepping exercise to 88 +/- 2% (mean SE) of the pre-exercise value and remained significantly lower than the concentrically exercised muscles (p < 0.05). The muscle weakness in the ECC could not be attributed to central fatigue as surface EMG amplitude at MVC0-3 increased during the recovery period. Muscle weakness after eccentric exercise appears to be due to contractile failure, which is not associated with a reduction in excitation as assessed by surface EMG. Muscular fatigue over 30 s did not change in the two muscle groups after exercise (p = 0.79), indicating that the ECC were weaker but not more fatiguable after exercise.

Deep and superficial fibers of the lumbar multifidus muscle are differentially active during voluntary arm movements

AB Study Design. A cross-sectional study was conducted. Objective. To determine the activity of the deep and superficial fibers of the lumbar multifidus during voluntary movement of the arm. Summary of Background Data. The multifidus contributes to stability of the lumbar spine. Because the deep and superficial parts of the multifidus are near the center of lumbar joint rotation, the superficial fibers are well suited to control spine orientation, and the deep fibers to control intervertebral movement. However, there currently are limited in vivo data to support this distinction. Methods. Electromyographic activity was recorded in both the deep and superficial multifidus, transversus abdominis, erector spinae, and deltoid using selective intramuscular electrodes and surface electrodes during single and repetitive arm movements. The latency of electromyographic onset in each muscle during single movements and the pattern of electromyographic activity during repetitive movements were compared between muscles. Results. With single arm movements, the onset of electromyography in the erector spinae and superficial multifidus relative to the deltoid was dependent on the direction of movement, but the onset in the deep multifidus and transversus abdominis was not. With repetitive arm movements, peaks in superficial multifidus and erector spinae electromyography occurred only during flexion for most subjects, whereas peaks in deep multifidus electromyography occurred during movement in both directions. Conclusions. The deep and superficial fibers of the multifidus are differentially active during single and repetitive movements of the arm. The data from this study support the hypothesis that the superficial multifidus contributes to the control of spine orientation, and that the deep multifidus has a role in controlling intersegmental motion. (C) 2002 Lippincott Williams & Wilkins, Inc.

Role of peripheral afference during acquisition of a complex coordination task

It has long been supposed that the interference observed in certain patterns of coordination is mediated, at least in part, by peripheral afference from the moving limbs. We manipulated the level of afferent input, arising from movement of the opposite limb, during the acquisition of a complex coordination task. Participants learned to generate flexion and extension movements of the right wrist, of 75degrees amplitude, that were a quarter cycle out of phase with a 1-Hz sinusoidal visual reference signal. On separate trials, the left wrist either was at rest, or was moved passively by a torque motor through 50degrees, 75degrees or 100degrees, in synchrony with the reference signal. Five acquisition sessions were conducted on successive days. A retention session was conducted I week later. Performance was initially superior when the opposite limb was moved passively than when it was static. The amplitude and frequency of active movement were lower in the static condition than in the driven conditions and the variation in the relative phase relation across trials was greater than in the driven conditions. In addition, the variability of amplitude, frequency and the relative phase relation during each trial was greater when the opposite limb was static than when driven. Similar effects were expressed in electromyograms. The most marked and consistent differences in the accuracy and consistency of performance (defined in terms of relative phase) were between the static condition and the condition in which the left wrist was moved through 50degrees. These outcomes were exhibited most prominently during initial exposure to the task. Increases in task performance during the acquisition period, as assessed by a number of kinematic variables, were generally well described by power functions. In addition, the recruitment of extensor carpi radialis (ECR), and the degree of co-contraction of flexor carpi radialis and ECR, decreased during acquisition. Our results indicate that, in an appropriate task context, afferent feedback from the opposite limb, even when out of phase with the focal movement, may have a positive influence upon the stability of coordination.

The embryonic development of the bodywall and nervous system of the cestode flatworm Hymenolepis diminuta

Cestodes (tapeworms) are a derived, parasitic clade of the phylum Platyhelminthes (flatworms). The cestode body wall represents an adaptation to its endoparasitic lifestyle. The epidermis forms a nonciliated syncytium, and both muscular and nervous system are reduced. Morphological differences between cestodes and free-living flatworms become apparent already during early embryogenesis. Cestodes have a complex life cycle that begins with an infectious larva, called the oncosphere. In regard to cell number, cestode oncospheres are among the simplest multicellular organisms, containing in the order of 50-100 cells. As part of our continuing effort to analyze embryonic development in flatworms, we describe here the staining pattern obtained with acTub in embryos and larvae of the cestode Hymenolepis diminuta and, briefly, the monogenean Neoheterocotyle rhinobatidis. In addition, we labeled the embryonic musculature of Hymenolepis with phalloidin. In Hymenolepis embryos, two different cell types that we interpret as neurons and epidermal gland cells express acTub. There exist only two neurons that develop close to the midline at the anterior pole of the embryo. The axons of these two neurons project posteriorly into the center of the oncosphere, where they innervate the complex of muscles that is attached to the booklets. In addition to neurons, acTub labels a small and invariant set of epidermal gland cells that develop at superficial positions, anteriorly adjacent to the neurons, in the dorsal midline, and around the posteriorly located hooklets. During late stages of embryogenesis they spread and form a complete covering of the embryo. We discuss these data in the broader context of platyhelminth embryology.

Preservation of three-dimensional capillary structure in frog muscle during aestivation

In mammals, prolonged immobilization of the limbs can result in a loss of capillary tortuosity, resulting in skeletal muscle haemorrhaging if rapid remobilization is permitted. In this study, we examined the effect of 4 months' immobilization on semimembranosus capillary structure in the Green-striped burrowing frog, Cyclorana alboguttata. C alboguttata routinely aestivates as part of a physiological strategy to avoid desiccation in semi-arid environments and, in this capacity, the hindlimbs of C alboguttata are immobilized in a cocoon for months at a time. We found that 4 months' aestivation had no effect on three-dimensional capillary structure in the semimembranosus muscle and that capillary tortuosity is preserved in immobilized C. alboguttata. The preservation of capillary structure in the hindlimb muscles of C alboguttata in part accounts for their remarkable ability to emerge with a fully competent locomotor system after prolonged immobilization.

Relationship between cranio-cervical flexion range of motion and pressure change during the cranio-cervical flexion test

The purpose of this study was to quantify the sagittal angular displacement of the head (cranio-cervical flexion) for the five incremental stages of the cranio-cervical flexion test (CCFT). Range of cranio-cervical flexion during the CCFT was measured using a digital imaging method in 20 healthy volunteer subjects. The intra- and inter-rater reliability of the digital imaging technique for the assessment of this movement were also examined. The results of this study demonstrated a linear relationship between the incremental pressure targets of the CCFT and the percentages of full range cranio-cervical flexion range of motion (ROM) measured in the supine lying position of the test using a digital imaging technique. A mean of 22.9% full range cranio-cervical flexion was used to reach the first pressure target of the CCFT followed by linear increments up to 76.6% for the last stage of the test. An increasing amount of cranio-cervical flexion ROM was used to achieve the five successive stages of the CCFT reflecting an increasing contractile demand on the deep cervical flexor muscles. Excellent inter-rater (ICC = 0.994) and intra-rater reliability (ICC = 0.988-0.998) were demonstrated for the angular measurements using this digital imaging technique. (C) 2003 Elsevier Science Ltd. All rights reserved.

Myoelectric manifestations of sternocleidomastoid and anterior scalene muscle fatigue in chronic neck pain patients

Objective: This study compares myoelectric manifestations of fatigue of the sternocleidomastoid (SCM) and anterior scalene (AS) muscles between 10 chronic neck pain subjects and 10 normal matched controls. Methods: Surface electromyography (sEMG) signals were recorded from the sternal bead of SCM and AS muscles bilaterally during submaximal isometric cervical flexion contractions at 25 and 50% of the maximum voluntary contraction (MVC). The mean frequency, average rectified value and conduction velocity of the sEMG signal were calculated to quantify myoelectric manifestations of muscle fatigue. Results: For both the SCM and AS muscles, the Mann-Whitney U test indicated that the initial value and slope of the mean frequency in neck pain patients were greater than in healthy subjects (P < 0.05). This was significant both at 25 and 50% of MVC. Conclusions: These results suggest: (a) a predominance of type-II fibres in the neck pain patients and/or (b) greater fatigability of the superficial cervical flexors in neck pain patients. These results are in agreement with previous muscle biopsy studies in subjects with neck pain, which identified transformation of slow-twitch type-I fibres to fast-twitch type-IIB fibres, as well as the clinical observation of reduced endurance in the cervical flexors in neck pain patients. (C) 2003 Elsevier Science Ireland Ltd. All rights reserved.