Changes in neural modulation and motor control during voluntary movement of older individuals

BACKGROUND: Changes in the modulation of soleus alpha motoneuron excitability, as assessed by H reflexes, and temporal sequencing of the soleus and tibialis anterior muscles during voluntary ankle dorsiflexions and plantar flexions of young (24.7 +/- 11.5; n = 13) and older (68.7 +/- 5.4; n = 13) subjects were assessed to determine potential neural mechanisms that might contribute to motor control changes associated with aging. METHODS: A repetitive stimulation (5 Hz) soleus H-reflex testing protocol and surface electromyography (EMG) were used to assess the latencies of soleus H-reflex changes in relation to tibialis anterior and soleus EMG activations of standing subjects during voluntary ankle dorsiflexions and plantar flexions at self-selected speeds. The pattern and latency of H-reflex changes in relation to EMG activity were compared between young and old subjects. RESULTS: There were no differences in the relative amount of antagonist muscle (soleus) inhibition during voluntary ankle dorsiflexions between young and old subjects (26.4% and 27.2% decrease from resting H-reflex values, respectively). Older subjects, however, required additional time to achieve these levels of inhibition. Delays in the activation of soleus H reflexes during the plantar flexion task were also observed in older subjects. Older subjects also had considerable intra- and intersubject variability in muscle temporal sequencing patterns during ankle plantar flexions. CONCLUSIONS: Although older subjects exhibited similar relative levels of alpha motoneuron inhibition and excitation during voluntary movements, this modulation was delayed when compared to younger subjects. Temporal sequencing of distal muscle activations also appears to undergo change with aging.     

Enteral absorption of insulin in rats from mucoadhesive chitosan-coated liposomes

Whole muscle contractile characteristics and fatigue resistance were studied in male patients with chronic heart failure (n = 6) and in healthy control subjects (n = 6). Maximum voluntary isometric strength in the major muscle groups of leg (plantar flexors and knee extensors) and arm (elbow extensors and elbow flexors), was found to be similar for both groups of subjects. However, a faster isometric twitch time course was observed in the plantar flexor and knee extensor muscles of heart failure chronic patients. The poor resistance to fatigue in the knee extensors of chronic heart failure patients was confirmed in the present study, but using twitch interpolation this was shown not to be due to poor activation. The plantar flexors of chronic heart failure patients also showed a tendency to be less resistant to fatigue, even when the muscle was activated by direct electrical stimulation. The present study shows that independent of muscle strength, patients with chronic heart failure may possess muscles that are faster to contract and less resistant to fatigue. However, it seems this increased fatigability is not due to poor muscle activation.     

Intracortical inhibition of lower limb motor-evoked potentials after paired transcranial magnetic stimulation

The aim of the present study was to determine the characteristics of intracortical inhibition in the motor cortex areas representing lower limb muscles using paired transcranial magnetic (TMS) and transcranial electrical stimulation (TES) in healthy subjects. In the first paradigm (n=8), paired magnetic stimuli were delivered through a double cone coil and motor evoked potentials (MEPs) were recorded from quadriceps (Q) and tibialis anterior (TA) muscles during relaxation. The conditioning stimulus strength was 5% of the maximum stimulator output below the threshold MEP evoked during weak voluntary contraction of TA (33+/-5%). The test stimulus (67+/-2%) was 10% of the stimulator output above the MEP threshold in the relaxed TA. Interstimulus intervals (ISIs) from 1-15 ms were examined. Conditioned TA MEPs were significantly suppressed (P<0.01) at ISIs of less than 5 ms (relative amplitude from 20-50% of the control). TA MEPs tended to be only slightly facilitated at 9-ms and 10-ms ISIs. The degree of MEP suppression was not different between right and left TA muscles despite the significant difference in size of the control responses (P<0.001). Also, conditioned MEPs were not significantly different between Q and TA. The time course of TA MEP suppression, using electrical test stimuli, was similar to that found using TMS. In the second paradigm (n=2), the suppression of TA MEPs at 2, 3, and 4 ms ISIs was examined at three conditioning intensities with the test stimulation kept constant. For the pooled 2- to 4-ms ISI data, relative amplitudes were 34+/-6%, 61+/-5%, and 98+/-9% for conditioning intensities of 0.95, 0.90, and 0.85x active threshold, respectively (P<0.01). In conclusion, the suppression of lower limb MEPs following paired TMS showed similar characteristics to the intracortical inhibition previously described for the hand motor area.     

Large index-fingertip forces are produced by subject-independent patterns of muscle excitation

Are fingertip forces produced by subject-independent patterns of muscle excitation? If so, understanding the mechanical basis underlying these muscle coordination strategies would greatly assist surgeons in evaluating options for restoring grasping. With the finger in neutral ad- abduction and flexed 45 degrees at the MCP and PIP, and 10 degrees at DIP joints, eight subjects attempted to produce maximal voluntary forces in four orthogonal directions perpendicular to the distal phalanx (palmar, dorsal, lateral and medial) and in one direction collinear with it (distal). Forces were directed within 4.7 +/- 2.2 degrees (mean +/- S.D.) of target and their magnitudes clustered into three distinct levels (p < 0.05; post hoc pairwise RMANOVA). Palmar (27.9 +/- 4.1 N), distal (24.3 +/- 8.3 N) and medial (22.9 +/- 7.8 N) forces were highest, lateral (14.7 +/- 4.8 N) was intermediate, and dorsal (7.5 +/- 1.5 N) was lowest. Normalized fine-wire EMGs from all seven muscles revealed distinct muscle excitation groups for palmar, dorsal and distal forces (p < 0.05; post hoc pairwise RMANOVA). Palmar force used flexors, extensors and dorsal interosseous; dorsal force used all muscles; distal force used all muscles except for extensors; medial and lateral forces used all muscles including significant co-excitation of interossei. The excitation strategies predicted to achieve maximal force by a 3-D computer model (four pinjoints, inextensible tendons, extensor mechanism and isometric force models for all seven muscles) reproduced the observed use of extensors and absence of palmar interosseous to produce palmar force (to regulate net joint flexion torques), the absence of extensors for distal force, and the use of intrinsics (strong MCP flexors) for dorsal force. The model could not predict the interossei co-excitation seen for medial and lateral forces, which may be a strategy to prevent MCP joint damage. The model predicts distal force to be most sensitive to dorsal interosseous strength, and palmar and distal forces to be very sensitive to MCP and PIP flexor moment arms, and dorsal force to be sensitive to the moment arm of and the tension allocation to the PIP extensor tendon of the extensor mechanism.     

Electrophysiologic evaluation of denervated muscles in incomplete paraplegia using macro electromyography

OBJECTIVE: To evaluate denervated muscles in persons with incomplete paraplegia due to thoracolumbar spinal injury (TLSI) using macro electromyography in determining indications for functional electrical stimulation (FES). DESIGN: A randomized clinical trial and a criterion standard. SETTING: A department of orthopedic surgery in a university hospital. PATIENTS AND OTHER PARTICIPANTS: Eighteen patients with incomplete paraplegia, including 11 with TSLI, and 50 healthy adults. INTERVENTION: Area and amplitude of macro motor unit potential (macro MUP) were measured at the tibialis anterior, the vastus lateralis, and the vastus medialis. The normal limits of macro MUP parameters were defined based on values from healthy subjects. Abnormal denervated muscles were detected by macro EMG and conventional EMG in paralytic patients. The correlation between macro MUP parameter values and muscle forces of the tibialis anterior and quadriceps femoris induced by electrical stimulation was analyzed. MAIN OUTCOME MEASURES: The number of abnormal muscles, parameter values, and muscle force induced by electrical stimulation. RESULTS: Abnormal muscles were found only in the TLSI patients and 13 abnormal muscles were detected by macro EMG only. The abnormal muscles defined by macro EMG showed insufficient contraction induced by electrical stimulation. The increase of parameter value negatively correlated with the muscle force (tibialis anterior area r=-.797, amplitude r=-.866; quadriceps area r=-.866, amplitude r=-.893; p < .001). CONCLUSIONS: These results suggest that macro EMG is useful in detecting denervated muscles, in determining indications for FES, and in predicting FES effects before implantation of electrodes.     

Studies on the corticospinal control of human walking. I. Responses to focal transcranial magnetic stimulation of the motor cortex

Experiments were done to determine the extent to which the corticospinal tract is linked with the segmental motor circuits controlling ankle flexors and extensors during human walking compared with voluntary motor tasks requiring attention to the level of motor activity. The motor cortex was activated transcranially using a focal magnetic stimulation coil. For each subject, the entire input-output (I-O) curve [i.e., the integral of the motor evoked-potential (MEP) versus stimulus strength] was measured during a prescribed tonic voluntary contraction of either the tibialis anterior (TA) or the soleus. Similarly, I-O curves were measured in the early part of the swing phase, or in the early part of the stance phase of walking. The I-O data points were fitted by the Boltzmann sigmoidal function, which accounted for >/=80% of total data variance. There was no statistically significant difference between the I-O curves of the TA measured during voluntary ankle dorsiflexion or during the swing phase of walking, at matched levels of background electromyographic (EMG) activity. Additionally, there was no significant difference in the relation between the coefficient of variation and the amplitude of the MEPs measured in each task, respectively. In comparison, during the stance phase of walking the soleus MEPs were reduced on average by 26% compared with their size during voluntary ankle plantarflexion. Furthermore, during stance the MEPs in the inactive TA were enhanced relative to their size during voluntary ankle plantarflexion and in four of six subjects the TA MEPs were larger than those of the soleus. Finally, stimulation of the motor cortex at various phases of the step cycle did not reset the cycle. The time of the next step occurred at the expected moment, as determined from the phase-resetting curve. One interpretation of this result is that the motor cortex may not be part of the central neural system involved in timing the motor bursts during the step cycle. We suggest that during walking the corticospinal tract is more closely linked with the segmental motor circuits controlling the flexor, TA, than it is with those controlling the extensor, soleus. However, during voluntary tasks requiring attention to the level of motor activity, it is equally linked with the segmental motor circuits of ankle flexors or extensors.     

Skeletal muscle fatigue and endurance in young and old men and women

The effects of increasing age on skeletal muscle fatigue and endurance were assessed in 22 healthy young (14 men and 8 women; mean age, 28 +/- 6 years) and 16 healthy old (8 men and 8 women; mean age, 73 +/- 3 years) individuals. All subjects performed 100 repeated maximum dynamic knee extensions at 90 degrees.s-1 (1.57 rad.s-1) using an isokinetic dynamometer (Cybex II). Peak torque was recorded during every contraction, and for each individual the maximal voluntary contraction (MVC), the fatigue rate, the endurance level, and the relative reduction in muscle force were determined. MVC and endurance level were significantly lower in old men and women, but there was no discernible difference in relative muscle force reduction and fatigue rate between young and old individuals. We conclude that thigh muscles of older individuals are weaker than those of younger individuals, but relative to their strength, older individuals have similar properties as younger individuals with respect to muscle fatigue and endurance.     

Essential dynamics from NMR clusters: dynamic properties of the Myb DNA-binding domain and a hinge-bending enhancing variant

We investigated factors affecting maximal voluntary torque and the assessment of the level of voluntary drive in the elbow flexor muscles. First, the effective compliance of the system was tested by using single, paired, and trains of four stimuli to measure voluntary activation. At high voluntary torques the responses to all these stimuli were identical, suggesting that single stimuli are adequate for estimating voluntary drive. Second, the contribution of torque from synergist elbow flexor muscles was assessed. In attempted maximal voluntary contractions (MVCs), the voluntary activation of brachioradialis (median 91.5%, range 68.9-100%) was lower than for biceps brachii (median 99.1%, range 78.5-100%; P < 0.01). This suggests extra torque may be generated by brachioradialis during elbow flexion, beyond the torque where biceps brachii is maximally activated. Finally, lengthening of the elbow flexors occurred during MVCs, due to slight shoulder movements. This would allow force to increase independently of an increase in voluntary drive.     

Effects of experimental muscle pain on muscle activity and co-ordination during static and dynamic motor function

The relation between muscle pain, muscle activity, and muscle co-ordination is still controversial. The present human study investigates the influence of experimental muscle pain on resting, static, and dynamic muscle activity. In the resting and static experiments, the electromyography (EMG) activity and the contraction force of m. tibialis anterior were assessed before and after injection of 0.5 ml hypertonic saline (5%) into the same muscle. In the dynamic experiment, injections of 0.5 ml hypertonic saline (5%) were performed into either m. tibialis anterior (TA) or m. gastrocnemius (GA) and the muscle activity and co-ordination were investigated during gait on a treadmill by EMG recordings from m. TA and m. GA. At rest no evidence of EMG hyperactivity was found during muscle pain. The maximal voluntary contraction (MVC) during muscle pain was significantly lower than the control condition (P < 0.05). During a static contraction at 80% of the pre-pain MVC muscle pain caused a significant reduction in endurance time (P < 0.043). During dynamic contractions, muscle pain resulted in a significant decrease of the EMG activity in the muscle, agonistic to the painful muscle (P < 0.05), and a significant increase of the EMG activity of the muscle, antagonistic to the painful muscle (P < 0.05). Muscle pain seems to cause a general protection of painful muscles during both static and dynamic contractions. The increased EMG activity of the muscle antagonistic to the painful muscle is probably a functional adaptation of muscle co-ordination in order to limit movements. Modulation of muscle activity by muscle pain could be controlled via inhibition of muscles agonistic to the movement and/or excitation of muscles antagonistic to the movement. The present results are in accordance with the pain-adaptation model (Lund, J.P., Stohler, C.S. and Widmer, C.G. In: H. Vaer?y and H. Merskey (Eds.), Progress in Fibromyalgia and Myofascial Pain. Elsevier, Amsterdam, 1993, pp. 311-327.) which predicts increased activity of antagonistic muscle and decreased activity of agonistic muscle during experimental and clinical muscle pain.     

Evidence for a striatal NMDA receptor modulation of nigral glutamate release. A dual probe microdialysis study in the awake freely moving rat

Insulin-like growth factor I (IGF-I) peptide levels have been shown to increase in overloaded skeletal muscles (G. R. Adams and F. Haddad. J. Appl. Physiol. 81: 2509-2516, 1996). In that study, the increase in IGF-I was found to precede measurable increases in muscle protein and was correlated with an increase in muscle DNA content. The present study was undertaken to test the hypothesis that direct IGF-I infusion would result in an increase in muscle DNA as well as in various measurements of muscle size. Either 0.9% saline or nonsystemic doses of IGF-I were infused directly into a non-weight-bearing muscle of rats, the tibialis anterior (TA), via a fenestrated catheter attached to a subcutaneous miniosmotic pump. Saline infusion had no effect on the mass, protein content, or DNA content of TA muscles. Local IGF-I infusion had no effect on body or heart weight. The absolute weight of the infused TA muscles was approximately 9% greater (P < 0.05) than that of the contralateral TA muscles. IGF-I infusion resulted in significant increases in the total protein and DNA content of TA muscles (P < 0.05). As a result of these coordinated changes, the DNA-to-protein ratio of the hypertrophied TA was similar to that of the contralateral muscles. These results suggest that IGF-I may be acting to directly stimulate processes such as protein synthesis and satellite cell proliferation, which result in skeletal muscle hypertrophy.     

Electrical stimulation of human tibialis anterior: (A) contractile properties are stable over a range of submaximal voltages; (B) high- and low-frequency fatigue are inducible and reliably assessable at submaximal voltages

OBJECTIVES: To investigate the validity and reliability of submaximal voltage stimulation for assessing the 'fresh' contractile properties of human tibialis anterior muscle (TA) and the efficacy of such stimulation in inducing and assessing high- and low-frequency fatigue. INTERVENTIONS: (A) Contractile properties of fresh TA were assessed in six normal volunteers using multifrequency stimulation trains (comprising 2 seconds at each of 10, 20 and 50 Hz, arranged contiguously) over a range of submaximal voltages. (B) On three separate occasions, fatigue was induced in the TA of 10 normal volunteers by means of a 3-minute unbroken sequence of the described multifrequency stimulation trains, delivered at a 'standardized' submaximal voltage. This fatiguing protocol was preceded by discrete multifrequency stimulation trains, at the same standardized voltage, but followed by discrete multifrequency trains delivered over a range of submaximal voltages (which included the standardized voltage). OUTCOME MEASURES: In experiment A the 10:50 Hz and 20:50 Hz force ratios were analysed for between-voltages variability using coefficients of variation (CVs), and for trends using Friedman tests and post-hoc Wilcoxon tests. In experiment B low-frequency fatigue was detected using 10:50 Hz and 20:50 Hz force ratios derived from the discrete multifrequency trains. High-frequency fatigue was calculated from the decline in high-frequency force which occurred during the fatiguing protocol itself. Each parameter was assessed for between-days repeatability using CVs. RESULTS: In experiment A the 'fresh' 10:50 Hz force ratio was clearly unreliable at voltages which generated <10% of maximal voluntary contractile force (MVC) (CV< or =29.7%), but was reasonably reliable at voltages which generated 20-30% of MVC (CV < or = 11.5%; p = 0.847). The 'fresh' 20:50 Hz force ratio was,in contrast, extremely reliable throughout the tested voltage range (CV< or =5.8%; p = 0.636) in fresh muscle. In experiment B paired t-tests indicated that the fatiguing protocol induced significant high-frequency fatigue (p <0.0037) and low-frequency fatigue (p <0.0008 for 'fresh' versus 'fatigued' 10:50 Hz force ratio; p <0.0001 for 'fresh' versus 'fatigued' 20:50 Hz force ratio). In muscle thus fatigued, the 20:50 Hz force ratio was extremely reliable in the 20-33% of MVC range (CV < or =7.3%; p = 0.847). Between-days repeatability was poor for the 10:50 Hz force ratio in both fresh and fatigued muscle (CV < or =23.8 and 44.4% respectively), but was highly acceptable for both voluntary and stimulated fatigue indices and for the 20:50 Hz force ratio, the latter in both fresh and fatigued muscle. CONCLUSIONS: These results confirm the validity and reliability of submaximal voltages in assessing contractile properties (including low-frequency fatiguability) and inducing fatigue of human TA.     

Fatigue effects on the bilateral deficit are speed dependent

PURPOSE: The bilateral deficit is characterized by a decrease in maximum voluntary contraction (MVC) force during bilateral activation of homologous muscles compared with sum of the forces produced during unilateral MVC of the muscles. A proposed relationship between the bilateral deficit and the ability to activate high threshold motor units was investigated. The effects of muscle fatigue, induced using two contraction speeds, on the size of the bilateral deficit were measured. Based on previously published findings, it was expected that fast velocity isokinetic contractions compared with slow velocity isokinetic contractions would produce a larger bilateral deficit. It was hypothesized that following a unilateral fatigue protocol, the size of the bilateral deficit at the fast and slow velocities would be comparable. METHODS: The bilateral deficit was measured for isokinetic knee extension in 20 men (age:25 +/- 3 yr) before and after a fatigue protocol performed at 30 and 150 degreesxs(-1). RESULTS: The size of the bilateral deficit was initially the same at both velocities. The fatigue protocol at each contraction speed significantly decreased the maximum voluntary knee extension moment. The size of the bilateral deficit was not influenced by the 150 degrees fatigue protocol (pretest: -14.0% post-test: -12.5%; P > 0.05). However, the size of the bilateral deficit increased following the 30 degreesxs(-1) fatigue protocol (pretest: -13.7%; post-test: -21.9%; P < 0.05). CONCLUSIONS: The failure of the size of the bilateral deficit to be comparable at 30 and 150 degreesxs(-1) following fatigue is contrary to previous published reports that suggested reduced activation of high threshold motor unit is the primary mechanism underlying the bilateral deficit.     

Brief periods of occlusion and reperfusion increase skeletal muscle force output in humans

Prolonged periods of ischemia/reperfusion are known to deleteriously affect skeletal muscle performance. However, in animal models, brief bouts of both skeletal and cardiac muscle ischemia/reperfusion have been shown to decrease skeletal muscle injury and increase skeletal muscle force output, a phenomenon termed "preconditioning". Because there are transient periods of ischemia/reperfusion during isometric and concentric muscle contractions, the purpose of this study was to examine how short duration forearm occlusion/reperfusion prior to exercise, influenced isometric skeletal muscle force output in humans. Eleven subjects (6 men and 5 women, mean age 25 +/- 1 years) participated in this study. Using a Biodex multijoint ergometer, a protocol of isolated, isometric forearm wrist flexions was utilized to measure muscle force output in two separate trials. In the first trial, 15 isometric maximal voluntary contractions (MVCs) of the wrist flexors were performed in 20 intervals interspersed with 10 s of rest. In the second trial, forearm occlusion was induced (2 min at 200 mmHg by blood pressure cuff occlusion, with 10 s of hyperemia) prior to exercise. Following cuff occlusion, an identical exercise protocol was followed, i.e. 15 isometric wrist flexor MVCs performed in 20 intervals interspersed with 10 s of rest. The total force output over 15 MVCs was greater following intermittent cuff occlusion (no occlusion 2619 +/- 320 ft.lbs vs cuff occlusion 2986 +/- 195 ft.lbs; p < 0.05). The mean force output per MVC also increased during exercise following intermittent cuff occlusion (no occlusion 174 +/- 21 ft.lbs vs cuff occlusion 199 +/- 13 ft.lbs; p < 0.05). In a second set of experiments, we found a 3 to 4 fold hyperemic blood flow following cuff occlusion. These data suggest that brief periods of cuff occlusion/reperfusion may increase repetitive MVC force output by skeletal muscle. Although further study is needed to fully understand the effects of occlusion/reperfusion on skeletal muscle force output, we hypothesize that, in part, this putative effects is secondary to the hyperemic blood flow which follows cuff occlusion.     

Cocontraction and phasic activity during GAIT in children with cerebral palsy

Simultaneous activity of agonist and antagonistic muscles during a task is known as cocontraction. The primary aim of the present study was to use a cocontraction index (CI) to quantify differences in EMG activity between a group of CP and control children at two different walking speeds. The secondary aim was to compare the amount of time the muscles were activated ("on" thresholds) between the groups. Seventeen subjects volunteered for the study. One group consisted of 9 (7M, 2F) children with CP (age 12.7 +/- 2.8 years, mean +/- SD). The second group consisted of 8 able-bodied controls (7M, 1F). The discontinuous submaximal treadmill walking protocol had two 4min stages at 0% gradient. Speeds selected were 3 km.h-1 and 90% of the pre-determined fastest walking speed (FWS). Two sites of CI were measured from the EMGs of tibialis anterior and soleus (leg) and vastus lateralis and hamstrings (thigh). Significantly (p < 0.05) higher CI values were noted for the CP subjects compared to the controls, irrespective of speed or cocontraction site and there was a significant (p < 0.05) increase in CI values with increased walking speed for both CP and control subjects. Phasic analyses for 5% max EMG and 10% max EMG "on" thresholds demonstrated significant (p < 0.05) main effects for group (CP subjects had a longer time period of muscle activation than controls) and speed (muscles were active longer at 90% FWS than 3 km.h-1). The precise mechanisms by which cocontraction contributes toward abnormal gait and wasted mechanical energy require further research incorporating both electromyographic and kinematic analysis.     

Alternate activity in the synergistic muscles during prolonged low-level contractions

The purpose of this study was to investigate the functional interrelationship between synergistic muscle activities during low-level fatiguing contractions. Six human subjects performed static and dynamic contractions at an ankle joint angle of 110 degrees plantar flexion and within the range of 90-110 degrees (anatomic position = 90 degrees) under constant load (10% maximal voluntary contraction) for 210 min. Surface electromyogram records from lateral gastrocnemius (LG), medial gastrocnemius (MG), and soleus (Sol) muscles showed high and silent activities alternately in the three muscles and a complementary and alternate activity between muscles in the time course. In the second half of all exercise times, the number of changes in activity increased significantly (P < 0.05) in each muscle. The ratios of active to silent periods of electromyogram activity were significantly higher (P < 0.05) in MG (4.5 +/- 2.2) and Sol (4.3 +/- 2.8) than in the LG (0.4 +/- 0.1), but no significant differences were observed between MG and Sol. These results suggest that the relative activation of synergistic motor pools are not constant during a low-level fatiguing task.     

Altered rhodopsin regeneration in diabetic mice caused by acid conditions within the rod photoreceptors

BACKGROUND: We sought to describe changes in spirometric variables and lung volume subdivisions in healthy subjects and patients with chronic obstructive pulmonary disease (COPD) during moderate acute hypobaric hypoxia as occurs during air travel. We further questioned whether changes in lung function may associate with reduced maximum ventilation or worsened arterial blood gases. METHODS: Ambulatory patients with COPD and healthy adults comprised the study populations (n = 27). We obtained baseline measurements of spirometry, lung volumes and arterial blood gases from each subject at sea level and repeated measurements during altitude exposure to 8000 ft (2438 m) above sea level in a man-rated hypobaric chamber. RESULTS: Six COPD patients and three healthy subjects had declines in FVC during altitude exposure greater than the 95% confidence interval (CI) for expected within day variability (p < 0.05). Average forced vital capacity (FVC) declined by 0.123 +/- 0.254 L (mean +/- SD; 95% CI = -0.255, -0.020; p < 0.05) for all subjects combined. The magnitude of decline in FVC did not differ between groups (p > 0.05) and correlated with increasing residual volume (r = -0.455; <0.05). Change in maximum voluntary ventilation (MVV) in the COPD patients equaled -1.244 +/- 4.797 L x min(-1) (95% CI = -3.71, 1.22; p = 0.301). Decline in maximum voluntary ventilation (MVV) in the COPD patients correlated with decreased FVC (r = 0.630) and increased RV (r = -0.546; p < 0.05). Changes in spirometric variables for patients and controls did not explain significant variability in the arterial blood gas variables PaO2, PaCO2 or pH at altitude. CONCLUSIONS: We observed a decline in forced vital capacity in some COPD patients and normal subjects greater than expected for within day variability. Spirometric changes correlated with changes in reduced maximum voluntary ventilation in the patients but not with changes in resting arterial blood gases.     

Amino-terminal identity of co-existent amyloid and non-amyloid immunoglobulin kappa light chain deposits. A human disease to study alterations of protein conformation

The sizes of the motor-evoked potentials (MEPs) and the durations of the silent periods after transcranial magnetic stimulation were examined in biceps brachii, brachioradialis and adductor pollicis in human subjects. Stimuli of a wide range of intensities were given during voluntary contractions producing 0-75% of maximal force (maximal voluntary contraction, MVC). In adductor pollicis, MEPs increased in size with stimulus intensity and with weak voluntary contractions (5% MVC), but did not grow larger with stronger contractions. In the elbow flexors, MEPs grew little with stimulus intensity, but increased in size with contractions of up to 50% of maximal. In contrast, the duration of the silent period showed similar changes in the three muscles. In each muscle it increased with stimulus intensity but was unaffected by changes in contraction strength. Comparison of the responses evoked in biceps brachii by focal stimulation over the contralateral motor cortex with those evoked by stimulation with a round magnetic coil over the vertex suggests an excitatory contribution from the ipsilateral cortex during strong voluntary contractions.     

Ground reaction forces and EMG activity with ankle bracing during inversion stress

PURPOSE: The purpose of this investigation was to evaluate the effects of external ankle support on ground reaction forces and myoelectrical activity of selected lower extremity muscles during dynamic inversion stress. METHODS: Twenty-four healthy males performed five trials of a lateral dynamic movement at a rate between 80-90% of their maximal speed under three ankle brace conditions (no brace--control, Aircast Sport-Stirrup, Active Ankle). Ground reaction forces along the mediolateral axis and EMG activity of the peroneus longus, tibialis anterior, and medial gastrocnemius were simultaneously recorded during force plate contact. RESULTS: Ankle bracing did not affect peak impact force (P > 0.05), maximum loading force (P > 0.05), or peak propulsion force (P > 0.05) in the lateral direction compared with the control condition. Ankle bracing reduced the EMG activity of the peroneus longus during peak impact force compared with the control condition (P < 0.05), although no differences were noted between the two braces. Furthermore, peroneous longus activity during maximum loading force and peak propulsion remained unaffected (P < 0.05). Ankle bracing did not affect the EMG activity of the tibialis anterior and medial gastrocnemius at the point of peak impact force, maximum loading force (P > 0.05), and peak propulsion force (P > 0.05). CONCLUSIONS: These data suggest that ankle bracing may not affect the forces experienced at the foot and ankle, but helps reduce the strain placed on the peroneus longus during peak impact force. Furthermore, ankle bracing does not alter the function of the tibialis anterior and medial gastrocnemius during dynamic inversion stress.     

Differential respiratory muscle recruitment induced by clonidine in awake goats

The purpose of this study was to test the hypothesis that dysrhythmic breathing induced by the alpha2-agonist clonidine is accompanied by differential recruitment of respiratory muscles. In adult goats (n = 14) electromyographic (EMG) measurements were made from inspiratory muscles (diaphragm and parasternal intercostal) and expiratory muscles [triangularis sterni (TS) and transversus abdominis (Abd)]. EMG of the thyroarytenoid (TA) muscle was used as an index of upper airway (glottal) patency. Peak EMG activities of all spinal inspiratory and expiratory muscles were augmented by central and peripheral chemoreceptor stimuli. Phasic TA was apparent in the postinspiratory phase of the breathing cycle under normoxic conditions. During dysrhythmic breathing episodes induced by clonidine, TS and Abd activities were attenuated or abolished, whereas diaphragm and parasternal intercostal activities were unchanged. There was no tonic activation of TS or Abd EMG during apneas; however, TA activity became tonic throughout the apnea. We conclude that 1) alpha2-adrenoceptor stimulation results in differential recruitment of respiratory muscles during respiratory dysrhythmias and 2) apneas are accompanied by active glottic closure in the awake goat.     

Resistance exercise prevents plantar flexor deconditioning during bed rest

Because resistance exercise (REX) and unloading induce opposing neuromuscular adaptations, we tested the efficacy of REX against the effects of 14 d of bed rest unloading (BRU) on the plantar flexor muscle group. Sixteen men were randomly assigned to no exercise (NOE, N = 8) or REX (N = 8). REX performed 5 sets x 6-10 repetitions to failure of constant resistance concentric/eccentric plantar flexion every other day during BRU. One-repetition maximum (1RM) strength was tested on the training device. The angle-specific torque-velocity relationship across 5 velocities (0, 0.52, 1.05, 1.75, and 2.97 rad.s-1) and the full range-of-motion power-velocity relationship were assessed on a dynamometer. Torque-position analyses identified strength changes at shortened, neutral, and stretched muscle lengths. Concentric and eccentric contractile work were measured across ten repetitions at 1.05 rad.s-1. Maximal neural activation was measured by surface electromyography (EMG). 1RM decreased 9% in NOE and improved 11% in REX (P < 0.05). Concentric (0.52 and 1.05 rad.s-1), eccentric (0.52 and 2.97 rad.s-1), and isometric angle-specific torques decreased (P < 0.05) in NOE, averaging 18%, 17%, and 13%, respectively. Power dropped (P < 0.05) in NOE at three eccentric (21%) and two concentric (14%) velocities. REX protected angle-specific torque and average power at all velocities. Concentric and eccentric strength decreased at stretched (16%) and neutral (17%) muscle lengths (P < 0.05) in NOE while REX maintained or improved strength at all joint positions. Concentric (15%) and eccentric (11%) contractile work fell in NOE (P < 0.05) but not in REX. Maximal plantar flexor EMG did not change in either group. In summary, constant resistance concentric/eccentric REX completely prevented plantar flexor performance deconditioning induced by BRU. The reported benefits of REX should prove useful in prescribing exercise for astronauts in microgravity and for patients susceptible to functional decline during bed- or chair-bound hospital stays.