Contralateral Effects of Unilateral Strength and Skill Training: Modified Delphi Consensus to Establish Key Aspects of Cross-Education

Background

Cross-education refers to increased motor output (i.e., force generation, skill) of the opposite, untrained limb following a period of unilateral exercise training. Despite extensive research, several aspects of the transfer phenomenon remain controversial.

Methods

A modified two-round Delphi online survey was conducted among international experts to reach consensus on terminology, methodology, mechanisms of action, and translational potential of cross-education, and to provide a framework for future research.

Results

Through purposive sampling of the literature, we identified 56 noted experts in the field, of whom 32 completed the survey, and reached consensus (75% threshold) on 17 out of 27 items.

Conclusion

Our consensus-based recommendations for future studies are that (1) the term ‘cross-education’ should be adopted to refer to the transfer phenomenon, also specifying if transfer of strength or skill is meant; (2) functional magnetic resonance imaging, short-interval intracortical inhibition and interhemispheric inhibition appear to be promising tools to study the mechanisms of transfer; (3) strategies which maximize cross-education, such as high-intensity training, eccentric contractions, and mirror illusion, seem worth being included in the intervention plan; (4) study protocols should be designed to include at least 13–18 sessions or 4–6 weeks to produce functionally meaningful transfer of strength, and (5) cross-education could be considered as an adjuvant treatment particularly for unilateral orthopedic conditions and sports injuries. Additionally, a clear gap in views emerged between the research field and the purely clinical field.

The present consensus statement clarifies relevant aspects of cross-education including neurophysiological, neuroanatomical, and methodological characteristics of the transfer phenomenon, and provides guidance on how to improve the quality and usability of future cross-education studies.

Electronic supplementary material

The online version of this article (10.1007/s40279-020-01377-7) contains supplementary material, which is available to authorized users.

Prevalence of motor impairment in residents of New South Wales, Australia aged 55 years and over: cross-sectional survey of the 45 and Up cohort

BACKGROUND

The population prevalence of many diseases is known. However, little is known of the population prevalence of motor impairments.

METHODS

The aim of this study was to determine the point prevalence of specific motor impairments (weakness, fatigue, contracture, impaired balance and impaired coordination) in the population aged 55 years and older resident in New South Wales, Australia in 2018. 55,210 members of the 45 and Up cohort were invited to participate in a follow-up survey that included questions on motor impairment. Responses were received from 20,141 people (36%). Calibrated estimates of prevalence of specific motor impairments, and of having at least one motor impairment, were obtained using survey weights based on the known multivariate distributions of age, gender and geographical location (28 regions) in the population.

RESULTS

More than one-third of adults aged over 55 residing in New South Wales have difficulty using their hands, arms or legs. The prevalence of each motor impairment (muscle weakness, fatigue, contracture, impaired balance or impaired coordination) in this population is between 4 and 12%. The prevalence of at least one of these impairments is 21%. The prevalence of at least one impairment in people aged 85 and over is 42%. Women consistently had more difficulty using hands, arms and legs, and more motor impairment, than men. Difficulty using hands, arms and legs and the prevalence of all motor impairments, especially poor balance, greatly increased with age.

CONCLUSION

The prevalence of specific motor impairments in older Australian adults is high - comparable to that of the most prevalent diseases. There may be merit in considering motor impairment as a significant public health problem in its own right.

Differential activation of the human costal and crural diaphragm during voluntary and involuntary breaths

Simultaneous electromyographic recordings from the human costal and crural diaphragm during voluntary augmented breathing and involuntary rebreathing show that the increase in inspiratory crural diaphragm activity was ~60% of the increase in costal diaphragm activity. However costal to crural diaphragm activation did not differ between the two tasks. The dissociation in the amplitude of activation of the costal and crural diaphragm becomes apparent only as the drive to breathe increases above tidal breathing.

Unlike voluntary contractions, stimulated contractions of a hand muscle do not reduce voluntary activation or motoneuronal excitability

Voluntary force declines during sustained, maximal voluntary contractions (MVC) due to changes in muscle and central nervous system properties. Central fatigue, an exercise-induced reduction in voluntary activation, is influenced by multiple processes. Some may occur independently of descending voluntary drive. To differentiate the effects associated with voluntary drive from other central and peripheral influences, we measured voluntary activation and motoneuron excitability following fatiguing contractions produced voluntarily or by electrical stimulation. On two separate days, participants performed either a 2-min MVC of adductor pollicis muscle or received 2-min continuous supramaximal electrical stimulation of the ulnar nerve. In study 1 (n = 14), the superimposed twitch elicited by ulnar nerve stimulation during brief MVCs was increased, and, hence, voluntary activation was reduced, up to 240 s after the 2-min MVC [-20 ± 12% (SD), P = 0.002] but not the 2-min stimulated contraction (-4 ± 7%), despite large reductions in MVC force (voluntary, -54 ± 18%; stimulated, -46 ± 16%). In study 2 (n = 12), F-waves recorded from the adductor pollicis were reduced in area for 150 s following the 2-min MVC (-21 ± 16%, P = 0.007) but not after the stimulated contraction (5 ± 27%). Therefore, voluntary activation and motoneuron excitability decreased only when descending voluntary drive was present during the fatiguing task. The findings do not exclude a cortical or brain stem contribution to the reduced voluntary activation but suggest that neither sensory feedback from the fatigued muscle nor repetitive activation of motoneurons underlie the changes, whereas they are consistent with motoneuronal inhibition by released factors linked to voluntary drive.NEW & NOTEWORTHY We demonstrate that reductions in voluntary activation and motoneuron excitability following 2-min isometric maximal contractions in humans occur only when fatigue is produced through voluntary contractions and not through electrically stimulated contractions. This is contrary to studies that suggest that changes in the superimposed twitch and therefore voluntary activation are explained by changes in peripheral factors alone. Thus, the interpolated twitch technique remains a viable tool to assess voluntary activation and central fatigue.

Increased diaphragm motor unit discharge frequencies during quiet breathing in people with chronic tetraplegia

KEY POINTS

Respiratory muscle strength is compromised in people with tetraplegia, which may be compensated for by an increase in neural drive to the diaphragm. We found that the discharge frequencies of diaphragm motor units are higher in people with chronic tetraplegia compared with able-bodied people during quiet breathing. Furthermore, we found that the area of single motor unit potentials was increased in people with tetraplegia. These results suggest an increased motoneurone output to the diaphragm and remodelling of diaphragm motor units to maintain ventilation in tetraplegia.

ABSTRACT

People with tetraplegia have reduced inspiratory muscle strength, ∼40% of able-bodied individuals. Paralysed or partially paralysed respiratory muscles as a result of tetraplegia compromise lung function, increase the incidence of respiratory infections and can cause dyspnoea. We hypothesised that reduced inspiratory muscle strength in tetraplegia may increase neural drive to the inspiratory muscles to maintain ventilation. We recorded the discharge properties of single motor units from the diaphragm in participants with chronic tetraplegia (8 males, 42-78 years, C3-C6 injury, AIS A-C) and able-bodied control participants (6 males matched for age and body mass index). In each group, 117 and 166 single motor units, respectively, were discriminated from recordings in the costal diaphragm using a monopolar electrode. A linear mixed-effects model analysis showed higher peak discharge frequencies of motor units during quiet breathing in tetraplegia (17.8 ± 4.9 Hz; mean ± SD) compared with controls (12.4 ± 2.2 Hz) (P < 0.001). There were no differences in tidal volume, inspiratory time or mean air flow between groups. Motor unit potentials in tetraplegia, compared with controls, were larger in amplitude (1.1 ± 0.7 mV and 0.5 ± 0.3 mV, respectively, P = 0.007) and area (1.83 ± 1.49 µV ms and 0.69 ± 0.52 µV ms, respectively, P = 0.003). The findings indicate that diaphragm motor unit remodelling is likely to have occurred in people with chronic tetraplegia and that there is an increase in diaphragm motor unit discharge rates during quiet breathing. These neural changes ensure that ventilation is maintained in people with chronic tetraplegia.

Abdominal Functional Electrical Stimulation to Augment Respiratory Function in Spinal Cord Injury

Background: Functional electrical stimulation (FES) is the application of electrical pulses to a nerve to achieve a functional muscle contraction. Surface electrical stimulation of the nerves that innervate the abdominal muscles, termed abdominal FES, can cause the abdominal muscles to contract, even when paralysed after spinal cord injury. As the abdominal muscles are the major expiratory muscles, and commonly partially or completely paralysed in tetraplegia, abdominal FES offers a promising method of improving respiratory function for this patient group. Objective: The aim of the article is to provide readers with a better understanding of how abdominal FES can be used to improve the health of the spinal cord-injured population. Methods: A narrative review of the abdominal FES literature was performed. Results: Abdominal FES can achieve an immediate effective cough in patients with tetraplegia, while the repeated application over 6 weeks of abdominal FES can improve unassisted respiratory function. Ventilator duration and tracheostomy cannulation time can also be reduced with repeated abdominal FES. Conclusion: Abdominal FES is a noninvasive method to achieve functional improvements in cough and respiratory function in acute and chronically injured people with tetraplegia. Potential practical outcomes of this include reduced ventilation duration, assisted tracheostomy decannulation, and a reduction in respiratory complications. All of these outcomes can contribute to reduced morbidity and mortality, improved quality of life, and significant potential cost savings for local health care providers.

Passive changes in muscle length

This review, the first in a series of minireviews on the passive mechanical properties of skeletal muscles, seeks to summarize what is known about the muscle deformations that allow relaxed muscles to lengthen and shorten. Most obviously, when a muscle lengthens, muscle fascicles elongate, but this is not the only mechanism by which muscles change their length. In pennate muscles, elongation of muscle fascicles is accompanied by changes in pennation and changes in fascicle curvature, both of which may contribute to changes in muscle length. The contributions of these mechanisms to change in muscle length are usually small under passive conditions. In very pennate muscles with long aponeuroses, fascicle shear could contribute substantially to changes in muscle length. Tendons experience moderate axial strains even under passive loads, and, because tendons are often much longer than muscle fibers, even moderate tendon strains may contribute substantially to changes in muscle length. Data obtained with new imaging techniques suggest that muscle fascicle and aponeurosis strains are highly nonuniform, but this is yet to be confirmed. The development, validation, and interpretation of continuum muscle models informed by rigorous measurements of muscle architecture and material properties should provide further insights into the mechanisms that allow relaxed muscles to lengthen and shorten.

Big data vs accurate data in health research: Large-scale physical activity monitoring, smartphones, wearable devices and risk of unconscious bias

Fundamental to the advancement of scientific knowledge is unbiased, accurate and validated measurement techniques. Recent United Nations and landmark Nature publications highlight the global uptake of mobile technology and the staggering potential for big data to encourage people to be physically active and to influence health policy. However, concerns exist about inconsistencies in smartphone health apps. Big data has many benefits, but noisy data may lead to wrong conclusions. In reaction to the increasing availability of low quality data; we call for a rigorous debate into the validity of substituting big data for accurate data in health research. We evaluated the step counting accuracy of a smartphone app previously used by 717,527 people from 111 countries. Our new data (from 48 participants; aged 21-59 years; body mass index 17.7-33.5 kg/m²) revealed significant (15-66%) undercounting by Apple phones. In contrast to the generally positive performances of wearable devices for stereotypical treadmill like walking, we observed extraordinarily large (0-200% of steps taken) error ranges for both Android and Apple phones. Unconscious bias (developers' perceptions of usual behaviour) may be embedded into many unvalidated smartphone apps. Consumer-grade wearable devices appear unsuitable to detect steps in people with slow, short or non-stereotypical gait patterns. Specifically, there is a risk of systematically undercounting the steps by obese people, females or people from different ethnic groups resulting in biases when reporting associations between physical inactivity and obesity. More research is required to develop smartphone apps suitable for all people of the heterogeneous global population.

Effects of acute isometric resistance exercise on cervicomedullary motor evoked potentials

Cervicomedullary motor evoked potentials (CMEPs) in relaxed biceps brachii have been reported to facilitate after acute isometric exercise of the elbow flexors. This facilitation, which reflects either enhanced corticospinal transmission or increased motoneurone excitability, has only been documented in the limb posture used during exercise. In Experiment 1, we tested if these spinal changes "transfer" to a second posture. Fourteen individuals completed 12 sets of high-force isometric contractions of the elbow flexors with the forearm pronated. Before and after exercise, biceps CMEPs were acquired with the forearm either pronated or supinated. CMEPs in pronation and supination were facilitated after exercise, indicating transfer (57.5 ± 55.5% and 53.9 ± 54.9%, respectively; mean ± SD). In Experiment 2, we examined if exercise posture influences the effect that exercise has on CMEPs. A different sample of 14 individuals performed isometric exercise in 2 sessions. In one, exercise was performed in supination. In the other, exercise was performed in pronation. Exercise intensity and volume were the same as in Experiment 1, as were participant characteristics. CMEPs were unchanged after exercise in supination (13.6 ± 31.2%) and pronation (7.7 ± 41.5%). The absence of an effect differs from the finding of Experiment 1. Thus, effects of acute isometric resistance exercise on corticospinal transmission and/or motoneurone excitability are not as consistent as previously thought. When exercise induces this spinal change, the effect is not specific to the posture used for exercise. However, the change does not always occur, and the reasons for this remain unknown.

Body ownership and a new proprioceptive role for muscle spindles

Knowledge of which body parts belong to us is referred to as the sense of body ownership. There is increasing evidence that this important aspect of human proprioception is highly malleable. Research into ownership of individual body parts was stimulated by Botvinick and Cohen's rubber-hand illusion (Nature 391,1998, 756), which demonstrated that an artificial body part can be incorporated in one's body representation and can cause real body parts to be sensed erroneously. Here, we review key studies that have advanced our understanding of the sense of body ownership, including the important role played by multisensory integration and spatiotemporal congruence of sensory signals. We also discuss our recent discovery that body ownership can be induced in response to movement stimuli by signals from a single class of sensory receptor, namely muscle spindles.

Recovery of central and peripheral neuromuscular fatigue after exercise

Sustained physical exercise leads to a reduced capacity to produce voluntary force that typically outlasts the exercise bout. This “fatigue” can be due both to impaired muscle function, termed “peripheral fatigue,” and a reduction in the capacity of the central nervous system to activate muscles, termed “central fatigue.” In this review we consider the factors that determine the recovery of voluntary force generating capacity after various types of exercise. After brief, high-intensity exercise there is typically a rapid restitution of force that is due to recovery of central fatigue (typically within 2 min) and aspects of peripheral fatigue associated with excitation-contraction coupling and reperfusion of muscles (typically within 3–5 min). Complete recovery of muscle function may be incomplete for some hours, however, due to prolonged impairment in intracellular Ca2+ release or sensitivity. After low-intensity exercise of long duration, voluntary force typically shows rapid, partial, recovery within the first few minutes, due largely to recovery of the central, neural component. However, the ability to voluntarily activate muscles may not recover completely within 30 min after exercise. Recovery of peripheral fatigue contributes comparatively little to the fast initial force restitution and is typically incomplete for at least 20–30 min. Work remains to identify what factors underlie the prolonged central fatigue that usually accompanies long-duration single joint and locomotor exercise and to document how the time course of neuromuscular recovery is affected by exercise intensity and duration in locomotor exercise. Such information could be useful to enhance rehabilitation and sports performance.

Use of a physiological profile to document motor impairment in ageing and in clinical groups

Ageing decreases exercise performance and is frequently accompanied by reductions in cognitive performance. Deterioration in the physiological capacity to stand, locomote and exercise can manifest itself as falling over and represents a significant deterioration in sensorimotor control. In the elderly, falling leads to serious morbidity and mortality with major societal costs. Measurement of a suite of physiological capacities that are required for successful motor performance (including vision, muscle strength, proprioception and balance) has been used to produce a physiological profile assessment (PPA) which has been tracked over the age spectrum and in different diseases (e.g. multiple sclerosis, Parkinson's disease). As well as measures of specific physiological capacities, the PPA generates an overall 'score' which quantitatively measures an individual's cumulative risk of falling. The present review collates data from the PPA (and the physiological capacities it measures) as well as its use in strategies to reduce falls in the elderly and those with different diseases. We emphasise that (i) motor impairment arises via reductions in a wide range of sensorimotor abilities; (ii) the PPA approach not only gives a snapshot of the physiological capacity of an individual, but it also gives insight into the deficits among groups of individuals with particular diseases; and (iii) deficits in seemingly restricted and disparate physiological domains (e.g. vision, strength, cognition) are funnelled into impairments in tasks requiring upright balance. Motor impairments become more prevalent with ageing but careful physiological measurement and appropriate interventions offer a way to maximise health across the lifespan.

Changes in the length and three-dimensional orientation of muscle fascicles and aponeuroses with passive length changes in human gastrocnemius muscles

The mechanisms by which skeletal muscles lengthen and shorten are potentially complex. When the relaxed human gastrocnemius muscle is at its shortest in vivo lengths it falls slack (i.e. it does not exert any passive tension). It has been hypothesised that when the muscle is passively lengthened, slack is progressively taken up, first in some muscle fascicles then in others. Two-dimensional imaging methods suggest that, once the slack is taken up, changes in muscle length are mediated primarily by changes in the lengths of the tendinous components of the muscle. The aims of this study were to test the hypothesis that there is progressive engagement of relaxed muscle fascicles, and to quantify changes in the length and three-dimensional orientation of muscle fascicles and tendinous structures during passive changes in muscle length. Ultrasound imaging was used to determine the location, in an ultrasound image plane, of the proximal and distal ends of muscle fascicles at 14 sites in the human gastrocnemius muscle as the ankle was rotated passively through its full range. A three-dimensional motion analysis system recorded the location and orientation of the ultrasound image plane and the leg. These data were used to generate dynamic three-dimensional reconstructions of the architecture of the muscle fascicles and aponeuroses. There was considerable variability in the measured muscle lengths at which the slack was taken up in individual muscle fascicles. However, that variability was not much greater than the error associated with the measurement procedure. An analysis of these data which took into account the possible correlations between errors showed that, contrary to our earlier hypothesis, muscle fascicles are not progressively engaged during passive lengthening of the human gastrocnemius. Instead, the slack is taken up nearly simultaneously in all muscle fascicles. Once the muscle is lengthened sufficiently to take up the slack, about half of the subsequent increase in muscle length is due to elongation of the tendinous structures and half is due to elongation of muscle fascicles, at least over the range of muscle-tendon lengths that was investigated (up to ∼60 or 70% of the range of in vivo lengths). Changes in the alignment of muscle fascicles and flattening of aponeuroses contribute little to the total change in muscle length.

Measuring anisotropic muscle stiffness properties using elastography

Physiological and pathological changes to the anisotropic mechanical properties of skeletal muscle are still largely unknown, with only a few studies quantifying changes in vivo. This study used the noninvasive MR elastography (MRE) technique, in combination with diffusion tensor imaging (DTI), to measure shear modulus anisotropy in the human skeletal muscle in the lower leg. Shear modulus measurements parallel and perpendicular to the fibre direction were made in 10 healthy subjects in the medial gastrocnemius, soleus and tibialis anterior muscles. The results showed significant differences in the medial gastrocnemius (μ‖ = 0.86 ± 0.15 kPa; μ⊥ = 0.66 ± 0.19 kPa, P < 0.001), soleus (μ‖ = 0.83 ± 0.22 kPa; μ⊥ = 0.65 ± 0.13 kPa, P < 0.001) and the tibialis anterior (μ‖ = 0.78 ± 0.24 kPa; μ⊥ = 0.66 ± 0.16 kPa, P = 0.03) muscles, where the shear modulus measured in the direction parallel is greater than that measured in the direction perpendicular to the muscle fibres. No significant differences were measured across muscle groups. This study provides the first direct estimates of the anisotropic shear modulus in the triceps surae muscle group, and shows that the technique may be useful for the probing of mechanical anisotropy changes caused by disease, aging and injury.

Twitch interpolation: superimposed twitches decline progressively during a tetanic contraction of human adductor pollicis

The assessment of voluntary activation of human muscles usually depends on measurement of the size of the twitch produced by an interpolated nerve or cortical stimulus. In many forms of fatiguing exercise the superimposed twitch increases and thus voluntary activation appears to decline. This is termed 'central' fatigue. Recent studies on isolated mouse muscle suggest that a peripheral mechanism related to intracellular calcium sensitivity increases interpolated twitches. To test whether this problem developed with human voluntary contractions we delivered maximal tetanic stimulation to the ulnar nerve (≥60 s at physiological motoneuronal frequencies, 30 and 15 Hz). During the tetani (at 30 Hz) in which the force declined by 42%, the absolute size of the twitches evoked by interpolated stimuli (delivered regularly or only in the last second of the tetanus) diminished progressively to less than 1%. With stimulation at 30 Hz, there was also a marked reduction in size and area of the interpolated compound muscle action potential (M wave). With a 15 Hz tetanus, a progressive decline in the interpolated twitch force also occurred (to ∼10%) but did so before the area of the interpolated M wave diminished. These results indicate that the increase in interpolated twitch size predicted from the mouse studies does not occur. Diminution in superimposed twitches occurred whether or not the M wave indicated marked impairment at sarcolemmal/t-tubular levels. Consequently, the increase in superimposed twitch, which is used to denote central fatigue in human fatiguing exercise, is likely to reflect low volitional drive to high-threshold motor units, which stop firing or are discharging at low frequencies.

Measuring changes in muscle stiffness after eccentric exercise using elastography

Muscle stiffness has been reported to increase following eccentric muscle exercise, but to date only indirect methods have been used to measure it. This study aimed to use Magnetic Resonance Elastography (MRE), a noninvasive imaging technique, to assess the time-course of passive elasticity changes in the medial gastrocnemius and soleus muscles before and after a bout of eccentric exercise. Shear storage modulus (G') and loss modulus (G'') measurements were made in eight healthy subjects for both muscles in vivo before, one hour after, 48 hours after and 1 week after eccentric exercise. The results show a 21% increase in medial gastrocnemius storage modulus following eccentric exercise with a peak occurring ~48 hours after exercise (before exercise 1.15 ± 0.23 kPa, 48 hours after 1.38 ± 0.27 kPa). No significant changes in soleus muscle storage modulus were measured for the exercise protocol used in this study, and no significant changes in loss modulus were observed. This study provides the first direct measurements in skeletal muscle before and after eccentric exercise damage and suggests that MRE can be used to detect the time course of changes to muscle properties.

Dynamic changes in the perceived posture of the hand during ischaemic anaesthesia of the arm

Contorted 'phantom' limbs often form when sensory inputs are removed, but the neural mechanisms underlying their formation are poorly understood. We tracked the evolution of an experimental phantom hand during ischaemic anaesthesia of the arm. In the first study subjects showed the perceived posture of their hand and fingers using a model hand. Surprisingly, if the wrist and fingers were held straight before and during anaesthesia, the final phantom hand was bent at the wrist and fingers, but if the wrist and fingers were flexed before and during anaesthesia, the final phantom was extended at wrist and fingers. Hence, no 'default' posture existed for the phantom hand. The final perceived posture may depend on the initial and evolving sensory input during the block rather than the final sensory input (which should not differ for the two postures). In the second study subjects selected templates to indicate the perceived size of their hand. Perceived hand size increased by 34 ± 4% (mean ± 95% CI) during the block. Sensory changes were monitored. In all subjects, impairment of large-fibre cutaneous sensation began distally with von Frey thresholds increasing before cold detection thresholds (Aδ fibres) increased. Some C fibres subserving heat pain still conducted at the end of cuff inflation. These data suggest that changes in both perceived hand size and perceived position of the finger joints develop early when large-fibre cutaneous sensation is beginning to degrade. Hence it is unlikely that block of small-fibre afferents is critical for phantom formation in an ischaemic block.

Movement of the human upper airway during inspiration with and without inspiratory resistive loading

The electromyographic (EMG) activity of human upper airway muscles, particularly the genioglossus, has been widely measured, but the relationship between EMG activity and physical movement of the airway muscles remains unclear. We aimed to measure the motion of the soft tissues surrounding the airway during normal and loaded inspiration on the basis of the hypothesis that this motion would be affected by the addition of resistance to breathing during inspiration. Tagged MR imaging of seven healthy subjects was performed in a 3-T scanner. Tagged 8.6-mm-spaced grids were used, and complementary spatial modulation of magnetization images were acquired beginning ∼200 ms before inspiratory airflow. Deformation of tag line intersections was measured. The genioglossus moved anteriorly during normal and loaded inspiration, with less movement during loaded inspiration. The motion of tissues at the anterior border of the upper airway was nonuniform, with larger motions inferiorly. At the level of the soft palate, the lateral dimension of the airway decreased significantly during loaded inspiration (-0.15 ± 0.09 and -0.48 ± 0.09 mm during unloaded and loaded inspiration, respectively, P < 0.05). When resistance to inspiratory flow was added, genioglossus motion and lateral dimensions of the airway at the level of the soft palate decreased. Our results suggest that genioglossus motion begins early to dilate the airway prior to airflow and that inspiratory loading reduces the anterior motion of the genioglossus and increases the collapse of the lateral airway walls at the level of the soft palate.

Voluntary and involuntary ventilation do not alter the human inspiratory muscle loading reflex

The reflex mechanism of the short-latency inhibitory reflex to transient loading of human inspiratory muscles is unresolved. Muscle afferents mediate this reflex, but they may act via pontomedullary inspiratory centers, other bulbar networks, or spinal circuits. We hypothesized that altered chemical drive to breathe would alter the initial inhibitory reflex if the neural pathways involve inspiratory medullary centers. Inspiration was transiently loaded in 11 subjects during spontaneous hypercapnic hyperpnea and matched voluntary hyperventilation. Electromyographic activity was recorded bilaterally from scalene muscles with surface electrodes. The latencies of the initial inhibitory response (IR) onset (32 ± 0.7 and 38 ± 1 ms for spontaneous and voluntary conditions respectively, P < 0.001) and subsequent excitatory response (ER) onset (80 ± 2.9 and 78 ± 2.6 ms, respectively, P = 0.46) and the normalized sizes of IR (65 ± 2 and 67 ± 3%, respectively, P = 0.50) and ER (51 ± 8 and 69 ± 6%, respectively, P = 0.005) were measured. Mean end-tidal Pco2 was 43 ± 1.5 Torr with dead space ventilation and was 14 ± 0.6 Torr with matched voluntary hyperventilation ( P < 0.001). A mean minute volume >30 liters was achieved in both conditions. The absence of significant difference in the size of the IR suggested that the IR reflex arc does not transit the brain stem inspiratory centers and that the reflex may be integrated at a spinal level. In voluntary hyperventilation, an initial excitation occurred more frequently and, consequently, the IR onset latency was significantly longer. The size of the later ER was also greater during voluntary hyperventilation, which is consistent with it being mediated via longer, presumably cortical, pathways, which are influenced by voluntary drive.

Probing the corticospinal link between the motor cortex and motoneurones: some neglected aspects of human motor cortical function

This review considers the operation of the corticospinal system in primates. There is a relatively widespread cortical area containing corticospinal outputs to a single muscle and thus a motoneurone pool receives corticospinal input from a wide region of the cortex. In addition, corticospinal cells themselves have divergent intraspinal branches which innervate more than one motoneuronal pool but the synergistic couplings involving the many hand muscles are likely to be more diverse than can be accommodated simply by fixed patterns of corticospinal divergence. Many studies using transcranial magnetic stimulation of the human motor cortex have highlighted the capacity of the cortex to modify its apparent excitability in response to altered afferent inputs, training and various pathologies. Studies using cortical stimulation at 'very low' intensities which elicit only short-latency suppression of the discharge of motor units have revealed that the rapidly conducting corticospinal axons (stimulated at higher intensities) drive motoneurones in normal voluntary contractions. There are also major non-linearities generated at a spinal level in the relation between corticospinal output and the output from the motoneurone pool. For example, recent studies have revealed that the efficacy of the human corticospinal connection with motoneurones undergoes activity-dependent changes which influence the size of voluntary contractions. Hence, corticospinal drives must be sculpted continuously to compensate for the changing functional efficacy of the descending systems which activate the motoneurones. This highlights the need for proprioceptive monitoring of movements to ensure their accurate execution.

Limits to the Control of the Human Thumb and Fingers in Flexion and Extension

In humans, hand performance has evolved from a crude multidigit grasp to skilled individuated finger movements. However, control of the fingers is not completely independent. Although musculotendinous factors can limit independent movements, constraints in supraspinal control are more important. Most previous studies examined either flexion or extension of the digits. We studied differences in voluntary force production by the five digits, in both flexion and extension tasks. Eleven healthy subjects were instructed either to maximally flex or extend their digits, in all single- and multidigit combinations. They received visual feedback of total force produced by “instructed” digits and had to ignore “noninstructed” digits. Despite attempts to maximally flex or extend instructed digits, subjects rarely generated their “maximal” force, resulting in a “force deficit,” and produced forces with noninstructed digits (“enslavement”). Subjects performed differently in flexion and extension tasks. Enslavement was greater in extension than in flexion tasks ( P = 0.019), whereas the force deficit in multidigit tasks was smaller in extension ( P = 0.035). The difference between flexion and extension in the relationships between the enslavement and force deficit suggests a difference in balance of spillover of neural drive to agonists acting on neighboring digits and focal neural drive to antagonist muscles. An increase in drive to antagonists would lead to more individualized movements. The pattern of force production matches the daily use of the digits. These results reveal a neural control system that preferentially lifts fingers together by extension but allows an individual digit to flex so that the finger pads can explore and grasp.

Passive mechanical properties of gastrocnemius in people with multiple sclerosis

BACKGROUND

There is evidence to suggest that contractile properties of muscles in people with multiple sclerosis change as a consequence of demyelination in central nervous system. However, passive properties of muscles in people with multiple sclerosis have not been previously investigated. The purpose of this study was to characterise passive mechanical properties of gastrocnemius in people with multiple sclerosis and to compare these properties with those of gastrocnemius in neurologically normal people.

METHODS

Ten people with multiple sclerosis having signs and symptoms of weakness in the legs (Disease step 1-3) and 10 age- and sex-matched healthy people participated in the study. Ultrasound images of muscle fascicles of medial gastrocnemius as well as passive ankle torque and ankle angle data were obtained simultaneously as the ankle was rotated through its full range with the knee in a range of positions. Analysis of ultrasound images and passive ankle torque-angle relations allowed us to derive the slack lengths and maximal strains of whole muscle-tendon units, muscle fascicles and tendons. Paired-samples t-tests were used to compare these variables in the two groups.

RESULT

There was no difference between subjects with multiple sclerosis and healthy controls in the mean slack lengths and mean maximal strains of the whole muscle-tendon units or of their fascicles or tendons.

INTERPRETATIONS

These data suggests that typically, in people with multiple sclerosis who have impaired lower limbs but are still ambulatory, the passive mechanical properties of the gastrocnemius muscles are normal.

Noninvasive Stimulation of Human Corticospinal Axons Innervating Leg Muscles

These studies investigated whether a single electrical stimulus over the thoracic spine activates corticospinal axons projecting to human leg muscles. Transcranial magnetic stimulation of the motor cortex and electrical stimulation over the thoracic spine were paired at seven interstimulus intervals, and surface electromyographic responses were recorded from rectus femoris, tibialis anterior, and soleus. The interstimulus intervals (ISIs) were set so that the first descending volley evoked by cortical stimulation had not arrived at (positive ISIs), was at the same level as (0 ISI) or had passed (negative ISIs) the site of activation of descending axons by the thoracic stimulation at the moment of its delivery. Compared with the responses to motor cortical stimulation alone, responses to paired stimuli were larger at negative ISIs but reduced at positive ISIs in all three leg muscles. This depression of responses at positive ISIs is consistent with an occlusive interaction in which an antidromic volley evoked by the thoracic stimulation collides with descending volleys evoked by cortical stimulation. The cortical and spinal stimuli activate some of the same corticospinal axons. Thus it is possible to examine the excitability of lower limb motoneuron pools to corticospinal inputs without the confounding effects of changes occurring within the motor cortex.

Movement of the tongue during normal breathing in awake healthy humans

Electromyographic (EMG) activity of the airway muscles suggest that genioglossus is the primary upper airway dilator muscle. However, EMG data do not necessarily translate into tissue motion and most imaging modalities are limited to assessment of the surfaces of the upper airway. In this study, we hypothesized that genioglossus moves rhythmically during the respiratory cycle and that the motion within is inhomogeneous. A 'tagged' magnetic resonance imaging technique was used to characterize respiratory-related tissue motions around the human upper airway in quiet breathing. Motion of airway tissues at different segments of the eupnoeic respiratory cycle was imaged in six adult subjects by triggering the scanner at the end of inspiration. Displacements of the 'tags' were analysed using the harmonic phase method (HARP). Respiratory timing was monitored by a band around the upper abdomen. The genioglossus moved during the respiratory cycle. During expiration, the genioglossus moved posteriorly and during inspiration, it moved anteriorly. The degree of motion varied between subjects. The maximal anteroposterior movement of a point tracked on the genioglossus was 1.02 +/- 0.54 mm (mean +/- s.d.). The genioglossus moved over the geniohyoid muscle, with minimal movement in other muscles surrounding the airway at the level of the soft palate. Local deformation of the tongue was analysed using two-dimensional strain maps. Across the respiratory cycle, positive strains within genioglossus reached peaks of 17.5 +/- 9.3% and negative strains reached peaks of -16.3 +/- 9.3% relative to end inspiration. The patterns of strains were consistent with elongation and compression within a constant volume structure. Hence, these data suggest that even during respiration, the tongue behaves as a muscular hydrostat.

Local subcutaneous and muscle pain impairs detection of passive movements at the human thumb

Activity in both muscle spindle endings and cutaneous stretch receptors contributes to the sensation of joint movement. The present experiments assessed whether muscle pain and subcutaneous pain distort proprioception in humans. The ability to detect the direction of passive movements at the interphalangeal joint of the thumb was measured when pain was induced experimentally in four sites: the flexor pollicis longus (FPL), the subcutaneous tissue overlying this muscle, the flexor carpi radialis (FCR) muscle and the subcutaneous tissue distal to the metacarpophalangeal joint of thumb. Tests were conducted when pain was at a similar subjective intensity. There was no significant difference in the ability to detect flexion or extension under any painful or non-painful condition. The detection of movement was significantly impaired when pain was induced in the FPL muscle, but pain in the FCR, a nearby muscle that does not act on the thumb, had no effect. Subcutaneous pain also significantly impaired movement detection when initiated in skin overlying the thumb, but not in skin overlying the FPL muscle in the forearm. These findings suggest that while both muscle and skin pain can disturb the detection of the direction of movement, the impairment is site-specific and involves regions and tissues that have a proprioceptive role at the joint. Also, pain induced in FPL did not significantly increase the perceived size of the thumb. Proprioceptive mechanisms signalling perceived body size are less disturbed by a relevant muscle nociceptive input than those subserving movement detection. The results highlight the complex relationship between nociceptive inputs and their influence on proprioception and motor control.

Passive mechanical properties of human gastrocnemius muscle tendon units, muscle fascicles and tendons in vivo

This study provides the first in vivo measures of the passive length-tension properties of relaxed human muscle fascicles and their tendons. A new method was used to derive passive length-tension properties of human gastrocnemius muscle-tendon units from measures of ankle stiffness obtained at a range of knee angles. Passive length-tension curves of the muscle-tendon unit were then combined with ultrasonographic measures of muscle fascicle length and pennation to determine passive length-tension curves of the muscle fascicles and tendons. Mean slack lengths of the fascicles, tendons and whole muscle-tendon units were 3.3+/-0.5 cm, 39.5+/-1.6 cm and 42.3+/-1.5 cm, respectively (means +/- s.d., N=6). On average, the muscle-tendon units were slack (i.e. their passive tension was zero) over the shortest 2.3+/-1.2 cm of their range. With combined changes of knee and ankle angles, the maximal increase in length of the gastrocnemius muscle-tendon unit above slack length was 6.7+/-1.9 cm, of which 52.4+/-11.7% was due to elongation of the tendon. Muscle fascicles and tendons underwent strains of 86.4+/-26.8% and 9.2+/-4.1%, respectively, across the physiological range of lengths. We conclude that the relaxed human gastrocnemius muscle-tendon unit falls slack over about one-quarter of its in vivo length and that muscle fascicle strains are much greater than tendon strains. Nonetheless, because the tendons are much longer than the muscle fascicles, tendons contribute more than half of the total compliance of the muscle-tendon unit.

Gender differences in brain activity evoked by muscle and cutaneous pain: a retrospective study of single-trial fMRI data

Gender greatly influences pain processing. Not only do females display greater pain sensitivity, many chronic pain conditions affect females more than males. Although gender-based differences in pain sensitivity may be related to cultural and social factors, animal studies also reveal gender differences in pain sensitivity, suggesting that physiological factors may contribute to differences in the processing of pain in males and females. It has been recently reported that noxious cutaneous heat stimuli evoke gender-based differences in activity in some brain regions. Given that most chronic pain conditions, including those with gender bias are of "deep" origin (e.g. arising in muscle, joints or viscera), we investigated whether gender differences also exist in the central processing of muscle pain. In 24 healthy adults we used functional magnetic resonance imaging (fMRI) to measure signal intensity changes during muscle and cutaneous pain induced by intramuscular and subcutaneous injections of hypertonic saline, respectively. In addition to activating the "pain neuromatrix", i.e. cingulate, insular, somatosensory and cerebellar cortices, both muscle pain and cutaneous pain evoked gender-based differences in the mid-cingulate cortex, dorsolateral prefrontal cortex, hippocampus and cerebellar cortex. These differences may reflect differences in emotional processing of noxious information in men and women and may underlie the gender bias that exists in many chronic pain conditions.

Group III and IV muscle afferents differentially affect the motor cortex and motoneurones in humans

The influence of group III and IV muscle afferents on human motor pathways is poorly understood. We used experimental muscle pain to investigate their effects at cortical and spinal levels. In two studies, electromyographic (EMG) responses in elbow flexors and extensors to stimulation of the motor cortex (MEPs) and corticospinal tract (CMEPs) were evoked before, during, and after infusion of hypertonic saline into biceps brachii to evoke deep pain. In study 1, MEPs and CMEPs were evoked in relaxed muscles and during contractions to a constant elbow flexion force. In study 2, responses were evoked during elbow flexion and extension to a constant level of biceps or triceps brachii EMG, respectively. During pain, the size of CMEPs in relaxed biceps and triceps increased (by approximately 47% and approximately 56%, respectively; P < 0.05). MEPs did not change with pain, but relative to CMEPs, they decreased in biceps (by approximately 34%) and triceps (by approximately 43%; P < 0.05). During flexion with constant force, ongoing background EMG and MEPs decreased for biceps during pain (by approximately 14% and 15%; P < 0.05). During flexion with a constant EMG level, CMEPs in biceps and triceps increased during pain (by approximately 30% and approximately 26%, respectively; P < 0.05) and relative to CMEPs, MEPs decreased for both muscles (by approximately 20% and approximately 17%; P < 0.05). For extension, CMEPs in triceps increased during pain (by approximately 22%) whereas MEPs decreased (by approximately 15%; P < 0.05). Activity in group III and IV muscle afferents produced by hypertonic saline facilitates motoneurones innervating elbow flexor and extensor muscles but depresses motor cortical cells projecting to these muscles.

Thumb and finger forces produced by motor units in the long flexor of the human thumb

The uncommonly good proprioceptive performance of the long flexor of the thumb, flexor pollicis longus (FPL), may add significantly to human manual dexterity. We investigated the forces produced by FPL single motor units during a weak static grip involving all digits by spike-triggered averaging from single motor units, and by averaging from twitches produced by intramuscular stimulation. Nine adult subjects were studied. The forces produced at each digit were used to assess how forces produced in FPL are distributed to the fingers. Most FPL motor units produced very low forces on the thumb and were positively correlated with the muscle force at recruitment. Activity in FPL motor units commonly loaded the index finger (42/55 units), but less commonly the other fingers (P < 0.001). On average, these motor units produced small but significant loading forces on the index finger ( approximately 5.3% of their force on the thumb) with the same time-to-peak force as the thumb ( approximately 50 ms), but had no significant effect on other fingers. However, intramuscular stimulation within FPL did not produce significant forces in any finger. Coherence at 2-10 Hz between the thumb and index finger force was twice that for the other finger forces and the coherence to the non-index fingers was not altered when the index finger did not participate in the grasp. These results indicate that, within the long-term coordinated forces of all digits during grasping, FPL motor units generate forces highly focused on the thumb with minimal peripheral transfer to the fingers and that there is a small but inflexible neural coupling to the flexors of the index finger.

Impairment of human proprioception by high-frequency cutaneous vibration

These experiments assessed whether the impairment in proprioceptive acuity in the hand during 'interfering' cutaneous stimulation could be caused by inputs from Pacinian corpuscles. The ability to detect passive movements at the proximal interphalangeal joint of the index finger was measured when vibrotactile stimuli were applied to the adjacent middle finger and thenar eminence at frequencies and amplitudes that favour activation of rapidly adapting cutaneous afferents. Inputs from Pacinian corpuscles are favoured with high-frequency vibration (300 Hz), while those from Meissner corpuscles are favoured by lower frequencies (30 Hz). Detection of movement was significantly impaired when 300 Hz (20 microm peak-to-peak amplitude) complex vibration or 300 Hz (50 microm) sinusoidal vibration was applied to the middle finger and thenar eminence. In contrast, detection of movements was not altered by low-frequency sinusoidal vibration at 30 Hz with an amplitude of 50 microm or with a larger amplitude matched in subjective intensity to the 300 Hz sinusoidal stimulus. Thus it is unlikely that the impairment in detection was due to attention being diverted by vibration of an adjacent digit. In addition, an increase in amplitude of 300 Hz vibration led to a greater impairment of movement detection, so that the impairment was graded with the input. The time taken to nominate the direction of applied movement also increased during 300 Hz but not during 30 Hz sinusoidal vibration. These findings suggest that stimuli which preferentially activate Pacinian, but not Meissner corpuscles, impair proprioceptive acuity in a movement detection task.

Muscle fatigue changes cutaneous suppression of propriospinal drive to human upper limb muscles

Some voluntary drive reaches human upper limb muscles via cervical propriospinal premotoneurones. Stimulation of the superficial radial nerve can inhibit these premotoneurones selectively and the resultant suppression of voluntary drive to motoneurones changes on-going electromyographic (EMG) activity. We investigated whether muscle fatigue changes this cutaneous-induced suppression of propriospinal drive to motoneurones of upper limb muscles. EMG was recorded from the extensors and flexors of the wrist and elbow. In the first study (n = 10 subjects), single stimuli (2 x perception threshold; 2PT) to the superficial radial nerve were delivered during contraction of the wrist extensors, before and after sustained fatiguing contractions of wrist extensors. In the second study (n = 10), similar stimuli were applied during elbow extension, before and during fatigue of elbow extensors. In the final study (n = 10), trains of three stimuli (2PT) were delivered during contractions of wrist extensors, before and while they were fatigued. With fatigue of either the wrist or elbow extensors, EMG suppression to single cutaneous stimuli increased significantly (by approximately 75%) for the fatigued muscle (P < 0.05). Conversely, in the other muscles, which were coactivated but not principally involved in the task, inhibition decreased or facilitation increased. Trains of stimuli produced greater suppression of on-going wrist extensor EMG than single stimuli and this difference persisted with fatigue. A control study of the H reflex in extensor carpi radialis showed that the mechanism responsible for the altered EMG suppression in fatigue was not at a motoneurone level. The findings suggest that the proportion of descending drive mediated via the disynaptic propriospinal pathway or the excitability of inhibitory interneurones projecting to propriospinal neurones increases substantially to fatigued muscles, but decreases to other active muscles. This pattern of changes may maintain coordination during multimuscle movements when one group of muscles is fatigued.

Theta burst stimulation does not reliably depress all regions of the human motor cortex

OBJECTIVE

The excitability of the human motor cortex projecting to hand muscles can be reduced by theta burst transcranial magnetic repetitive stimulation (TBS). This study compared the magnitude and variability of changes evoked by TBS for a distal and proximal arm muscle.

METHODS

Eight subjects participated in three studies. In each study, electromyographic responses (MEPs) to single-pulse transcranial magnetic stimulation assessed cortical excitability before and after 40s of TBS. In the first two studies, TBS (intensity, 80% active motor threshold) was delivered to the optimal locations for biceps or first dorsal interosseous (FDI). In the final study, weaker intensity TBS was delivered over the biceps representation.

RESULTS

TBS targeting biceps produced highly variable results among subjects. For the group, MEPs were not significantly depressed. Repeat studies in individual subjects highlighted the variability of responses. For FDI, MEPs were significantly depressed 5min after TBS and remained depressed for >30min (p<0.05). No significant changes in biceps MEPs occurred with weaker TBS.

CONCLUSIONS

The magnitude and reliability of TBS depends on the region of the cortex targeted.

SIGNIFICANCE

Results obtained for the hand should not be considered indicative of changes that will occur in other regions of the motor cortex or the brain.

Discrete changes in cortical activation during experimentally induced referred muscle pain: a single-trial fMRI study

Noxious stimulation of skeletal muscle evokes pain that is often referred into distal areas. Despite referred pain being of significant clinical importance, the brain regions responsible for the perception of referred pain remain unexplored. The aim of this investigation is to define these regions using functional magnetic resonance imaging. We induced muscle pain by hypertonic saline injections (0.5 ml) into the tibialis anterior (TA) or flexor carpi radialis (FCR) muscle. TA injections evoked pain that was referred to the ankle/foot in 10/17 subjects, whereas FCR injections evoked pain that was projected into the wrist/hand in 6/12 subjects. Regional brain responses were statistically tested by convolving the temporal profile of the subjective pain intensity rating with the hemodynamic response function. For all subjects, signal increased in the region of primary somatosensory cortex (SI), which represents the leg or arm, that is, the area corresponding to the injection site. However, for those subjects who reported referred pain, signal intensity increases also occurred in the SI region representing the foot or hand. Interestingly, differential signal changes also occurred in anterior cingulate, cerebellar, and insular cortices. This is the first study to provide evidence of cortical differentiation in the processing of primary and referred pain.

Decreased input to the motor cortex increases motor cortical excitability

OBJECTIVE

To investigate whether a short-duration reduction of input to the motor cortex affects excitability in the hand region of the motor cortex.

METHODS

Subjects (n=10) received sets of transcranial magnetic stimulation of the motor cortex (TMS) and peripheral ulnar nerve stimulation. Stimuli were delivered before and after 20 min of inactivity of the test hand. The evoked compound muscle action potentials were recorded in two relaxed intrinsic hand muscles using surface EMG.

RESULTS

Motor evoked potential size (MEP; expressed relative to the maximal M-wave) increased by approximately 30-40 in both hand muscles (P=0.012) following inactivity. The enlarged MEP was not associated with changes in F-wave size, a marker of motoneurone excitability, or changes in intracortical inhibition and facilitation measured with paired-pulse TMS.

CONCLUSIONS

MEP growth most likely reflects an increase in motor cortical excitability. The increased excitability appears to be more associated with reduced voluntary drive to and from the motor cortex rather than reduced afferent input from the periphery.

SIGNIFICANCE

These results have important implications for any investigation of motor cortical excitability in relaxed subjects. The outcome of an experimental intervention is the net result of the intervention itself and alterations in cortical excitability produced by the subjects' inactivity.

Output of Human Motoneuron Pools to Corticospinal Inputs During Voluntary Contractions

This study investigated transmission of corticospinal output through motoneurons over a wide range of voluntary contraction strengths in humans. During voluntary contraction of biceps brachii, motor evoked potentials (MEPs) to transcranial magnetic stimulation of the motor cortex grow up to about 50% maximal force and then decrease. To determine whether the decrease reflects events at a cortical or spinal level, responses to stimulation of the cortex and corticospinal tract (cervicomedullary motor evoked potentials, CMEPs) as well as maximal M-waves (Mmax) were recorded during strong contractions at 50 to 100% maximum. In biceps and brachioradialis, MEPs and CMEPs (normalized to Mmax) evoked by strong stimuli decreased during strong elbow flexions. Responses were largest during contractions at 75% maximum and both potentials decreased by about 25% Mmax during maximal efforts ( P < 0.001). Reductions were smaller with weaker stimuli, but again similar for MEPs and CMEPs. Thus the reduction in MEPs during strong voluntary contractions can be accounted for by reduced responsiveness of the motoneuron pool to stimulation. During strong contractions of the first dorsal interosseous, a muscle that increases voluntary force largely by frequency modulation, MEPs declined more than in either elbow flexor muscle (35% Mmax, P < 0.001). This suggests that motoneuron firing rates are important determinants of evoked output from the motoneuron pool. However, motor cortical output does not appear to be limited at high contraction strengths.

Effect of eccentric exercise on position sense at the human forearm in different postures

This is a study of the ability of blindfolded human subjects to match the position of their forearms before and after eccentric exercise. The hypothesis tested was that the sense of effort contributed to forearm position sense. The fall in force after the exercise was predicted to alter the relationship between effort and force and thereby induce position errors. In the arms-in-front posture, subjects had their unsupported reference arm set to one of two angles from the horizontal, 30 or 60°, and they matched its position by voluntary placement of their other arm. Matching errors were compared with a task where the arms were counterweighted, so could be moved in the vertical plane with minimal effort, and where the arms were moved in the horizontal plane. In these latter two tasks, the intention was to test whether removal of an effort sensation from holding the arm against gravity influenced matching performance. It was found that, although absolute errors for counterweighted and horizontal matching were no larger than for unsupported matching, their standard deviations, 6.1 and 6.8°, respectively, were significantly greater than for unsupported matching (4.6°), indicating more erratic matching. The eccentric exercise led, the next day, to a fall in maximum voluntary muscle torque of ≥15%. This was accompanied by a significant increase in matching errors for the unsupported matching task from 2.7 ± 0.5 to 0.8 ± 0.7° but not for counterweighted (1.4 ± 0.2 to −0.2°± 1.1°) or horizontal matching (−1.3 ± 0.7° to −1.8 ± 0.7°). This, it is postulated, is because the reduced voluntary torque after exercise was accompanied by a greater effort required to support the arms, leading to larger matching errors. However, effort is only able to provide positional information for unsupported matching where gravity plays a role. In gravity-neutral tasks like counterweighted or horizontal matching, a change in the effort-force relationship after exercise leaves matching accuracy unaffected.

Length-dependent changes in voluntary activation, maximum voluntary torque and twitch responses after eccentric damage in humans

To assess the contribution of central and peripheral factors to changes in maximum voluntary force and its length dependence after eccentric muscle damage, voluntary and twitch torque were measured across a wide angular range, along with voluntary activation using twitch interpolation. Isometric torque from both maximum voluntary contractions (MVCs) and paired twitches to motor nerve stimulation were measured from 60 to 150 deg elbow flexion in 10 deg increments in eight subjects. Optimal angles were determined by curve fitting. Each subject then performed eccentric contractions until voluntary torque had decreased by approximately 40%. Measurements were repeated at 2 h, 1 day and 8 days post-exercise to follow acute and longer-term changes. Before exercise, the optimal angle was in the mid-range (93+/-10 deg; mean+/-s.d.) for MVCs, and at a more extended elbow angle for the twitch (106+/-6 deg, P < 0.05). Voluntary activation was generally high (> 94%) but depended on elbow angle, with activation being approximately 4% lower at the most flexed compared to the most extended angle. Two hours after exercise, MVCs decreased 40%, while twitch torque declined 70%. All subjects showed a shift in optimal angle to longer muscle lengths for MVCs (17+/-16 deg at 2 h, 14+/-7 deg at day 1, P < 0.05). This shift contributed minimally (approximately 3%) to the reduction in torque at 90 deg, as the torque-angle relation was relatively flat around the optimum. The twitch showed a smaller shift (approximately 4 deg) to longer lengths which was not statistically significant. Voluntary activation was significantly impaired in the early stages after exercise (2 h and day 1, P < 0.05), particularly at short muscle lengths. By 8 days after exercise, the optimal angle had returned to pre-exercise values, but MVC, twitch torque and voluntary activation had not fully recovered. Eccentric exercise causes a short-term shift in the optimal angle for MVCs and produces a length-dependent impairment in voluntary activation. Therefore, it appears that both central and peripheral factors limit muscle performance following eccentric damage, with limits to voluntary drive being especially important at short lengths.

Distinct forebrain activity patterns during deep versus superficial pain

All pain is unpleasant, but different perceptual and emotional qualities are characteristic of pain originating in different structures. Pain of superficial (cutaneous) origin usually is sharp and restricted, whereas pain of deep origin (muscle/viscera) generally is dull and diffuse. Despite the differences it has been suggested previously that all pain is mediated by an invariant set ("neuromatrix") of brain structures. However, we report here, using functional magnetic resonance imaging (fMRI), that striking regional differences in brain activation patterns were the rule. Signal differences were found in regions implicated in emotion (perigenual cingulate cortex), stimulus localization and intensity (somatosensory cortex) and motor control (motor cortex, cingulate motor area). Further, most fMRI signal changes matched perceived changes in pain intensity. These findings clearly indicate that distinct neural activity patterns in distinct sets of brain structures are evoked by pain originating from different tissues of the body. Further, we suggest that these differences underlie the different perceptual and emotional reactions evoked by deep versus superficial pain.

Neural sites involved in the sustained increase in muscle sympathetic nerve activity induced by inspiratory capacity apnea: a fMRI study

A maximal inspiratory breath hold (inspiratory capacity apnea) against a closed glottis evokes a large and sustained increase in muscle sympathetic nerve activity (MSNA). Because of its dependence on a high intrathoracic pressure, it has been suggested that this maneuver causes unloading of the low-pressure baroreceptors, known to increase MSNA. To determine the central origins of this sympathoexcitation, we used functional magnetic resonance imaging to define the loci and time course of activation of different brain areas. We hypothesized that, as previously shown for the Valsalvsa maneuver, discrete but widespread regions of the brain would be involved. In 15 healthy human subjects, a series of 90 gradient echo echo-planar image sets was collected during three consecutive 40-s inspiratory capacity apneas using a 3-T scanner. Global signal intensity changes were calculated and subsequently removed by using a detrending technique, which eliminates the global signal component from each voxel's signal intensity change. Whole brain correlations between changes in signal intensity and the known pattern of MSNA during the maneuver were performed on a voxel-by-voxel basis, and significant changes were determined by using a random-effects analysis procedure ( P < 0.01, uncorrected). Significant signal increases emerged in multiple areas, including the rostral lateral medulla, cerebellar nuclei, anterior insula, dorsomedial hypothalamus, anterior cingulate, and lateral prefrontal cortexes. Decreases in signal intensity occurred in the dorsomedial and caudal lateral medulla, cerebellar cortex, hippocampus, and posterior cingulate cortex. Given that many of these sites have roles in cardiovascular control, the sustained increase in MSNA during an inspiratory capacity apnea is likely to originate from a distributed set of discrete areas.