Alexander V. Nowicky

Corticomotor excitability contributes to neuromuscular fatigue following marathon running in man

It is unknown whether changes in corticomotor excitability follow prolonged exercise in healthy humans. Furthermore, the role of supraspinal fatigue in decrements of force production and voluntary activation following prolonged exercise has not been established. This study investigated peripheral and central fatigue after a marathon (42.2 km) on a treadmill. Isometric ankle dorsiflexion force and electromyographic responses of the tibialis anterior in response to magnetic stimulation of the peroneal nerve (PNMS) and the motor cortex (TMS) were measured before, immediately after, 4 and 24 h post‐marathon (MAR) in nine volunteers (mean ±s.d. completion time, 208 ± 22 min). Maximal voluntary contraction decreased by 18 ± 7% immediately after MAR (P= 0.009) and remained significantly decreased after 4 h. The amplitude of the evoked response to TMS, but not to PNMS, was depressed immediately post‐MAR by 57 ± 25% (P= 0.04). Potentiated resting twitch force was reduced in response to both TMS and PNMS post‐MAR (71 ± 8 and 35 ± 2% decrease, P= 0.035 and 0.037, respectively), and voluntary activation was reduced to 61.9 ± 18% immediately post‐MAR (P < 0.05). All measures had returned to baseline values after 24 h. These results suggest that fatigue was attributable to both a disturbance of the contractile apparatus within the muscle and submaximal output from the motor cortex.

Do oarsmen have asymmetries in the strength of their back and leg muscles

The aim of this study was to establish whether asymmetry of the strength of the leg and trunk musculature is more prominent in rowers than in controls. Nineteen oarsmen and 20 male controls matched for age, height and body mass performed a series of isokinetic and isometric strength tests on an isokinetic dynamometer. These strength tests focused on the trunk and leg muscles. Comparisons of strength were made between and within groups for right and left symmetry patterns, hamstring :quadriceps ratios, and trunk flexor and extensor ratios. The results revealed no left and right asymmetries in either the knee extensor or flexor strength parameters (including both isometric and isokinetic measures). Knee extensor strength was significantly greater in the rowing population, but knee flexor strength was similar between the two groups. No difference was seen between the groups for the hamstring:quadriceps strength ratio. In the rowing population, stroke side had no influence on leg strength. No differences were observed in the isometric strength of the trunk flexors and extensors between groups, although EMG activity was significantly higher in the rowing population. Patterns of asymmetry of muscle activity were observed between the left and right erector spinae muscles during extension, which was significantly related to rowing side ( P < 0.01). These observations could be related to the high incidence of low back pain in oarsmen.

Somatotopy of Perceptual Threshold to Cutaneous Electrical Stimulation in Man

Neurological testing tools for measuring and monitoring somatosensory function lack resolution and are often dependent on the clinician testing. In this study we have measured perceptual threshold (PT) to electrical stimulation of the skin and compared it with two‐point discriminative ability (TPDA) in 12 control subjects. Tests were made on both sides of the body at American Spinal Injury Association (ASIA) key points on seven spinal dermatomes (C3 (neck), C4 (shoulder), C5 (upper arm), C6 (thumb), T8 (abdomen), L3 (knee), L5 (foot)) and in the mandibular (chin) and maxillary (cheek) fields of the trigeminal (V) nerve. Electrical stimulation (0.5 ms pulse width; 3 Hz) was applied via a self‐adhesive cathode and an anode strapped to the wrist or ankle. The stimulus intensity was adjusted and PT was recorded as the lowest current at which the subject reported sensation. Sites were tested in random order. Indices for both TPDA and PT differed according to the dermatome tested but there was no correlation between TPDA and PT for any dermatome. There was good correlation between results from equivalent dermatomes on left and right sides for both PT and TPDA. Women frequently had lower mean (± s.e.) PTs and better TPDA than men; differences were significant (P < 0.05) for PT on the knee (women, 1.31 ± 0.15 mA; men, 2.05 ± 0.26 mA) and the foot (women, 2.90 ± 0.19 mA; men, 4.13 ± 0.28 mA) and for TPDA on the thumb (women, 3.8 ± 0.2 mm; men, 7.8 ± 1.3 mm) and the knee (women, 17.8 ± 1.6 mm; men, 27.1 ± 4.0 mm). Four subjects repeated the experiment on another day and the results correlated well with the first test for PT (r2, 0.62) and TPDA (r2, 0.48). PT differs between dermatomes in a predictable way but does not relate to TPDA. PT is easy to measure and may be a useful assessment tool with which to monitor recovery or deterioration in neuropathies, neurotrauma or after surgery.

Corticospinal facilitation studied during voluntary contraction of human abdominal muscles

Transcranial magnetic stimulation (TMS) of the human motor cortex was used to study facilitation of motor‐evoked potentials (MEPs) in the rectus abdominis (RA) muscle, a trunk flexor, during voluntary activation. MEPs could be produced in the relaxed RA muscles of all six normal subjects studied. The MEPs had short latencies (18‐22 ms) which are consistent with other studies suggesting a fast corticospinal input to the trunk muscles. Marked facilitation was observed in the MEPs when subjects were asked to produce graded levels of voluntary contractions. The two tasks used to produce voluntary contractions were a forced expiration during a breath‐holding task (FEBH) and bilateral trunk flexion (BTF). Maximal voluntary EMG activity during the BTF task produced around 4.2 times more integrated EMG than during the FEBH task. Similarly the MEP amplitude at MVC was 2.3 times greater during BTF than FEBH. The pattern of MEP facilitation with increasing voluntary EMG was not linear and a maximal MEP amplitude was observed at a level of voluntary contraction around 30% MVC in both tasks. There were some subtle differences in the pattern of facilitation in the two tasks. When TMS was applied to the right cortex only, MEPs were seen in both left and right RA muscles suggesting some ipsilateral corticospinal innervation. The latency of the right (ipsilateral) response was approximately 2 ms longer than the left. Comparison with studies in hand and leg muscles suggests that the facilitation pattern in RA may reflect a substantial degree of corticospinal innervation.

Diaphragm and intercostal surface EMG and muscle performance after acute inspiratory muscle loading

We examined the effect of an acute bout of submaximal non-fatiguing inspiratory loading (IL) on maximal inspiratory pressure (MIP), and on the activation of the diaphragm (DI) and intercostals (IC) using surface electromyography (sEMG). After baseline measurements, 12 healthy subjects performed two sets of 30 inspiratory efforts at a load equivalent to 40% of their initial MIP. MIP and maximal DI and IC sEMG activity were recorded after the first and second set of IL, and 15 min after task cessation. After IL, MIP reached (+/-S.E.M.) 111+/-4% (P=0.032) of baseline values, and during MIP, DI and IC root mean square (RMS) sEMG amplitude increased significantly above baseline (143+/-21%, P=0.039 and 137+/-33%, P=0.016, respectively). The significant increase in MIP and RMS amplitude after IL suggests that MIP efforts were initially submaximal, and that prior loading enabled full activation. The changes in DI and IC RMS amplitude may also reflect an improvement in the synergy between them during these maximal efforts.

Mapping the cortical representation of the lumbar paravertebral muscles

OBJECTIVE The aim of this study was to map the cortical representation of the lumbar spine paravertebral (LP) muscles in healthy subjects. METHODS Transcranial magnetic stimulation (TMS) was employed to map the cortical representations of the LP muscles at two sites. Stimuli were applied to points on a grid representing scalp positions. The amplitude of motor evoked potentials (n=6) was averaged for each position. RESULTS The optimal site for evoking responses in the contralateral LP muscles was situated 1cm anterior and 4 cm lateral to the vertex. Ipsilateral responses were evoked from sites lateral to the optimal site for evoking contralateral responses. Contralateral responses were also obtained from areas anterior to the optimal site. CONCLUSIONS Maps of these muscles can be produced. The results suggest discrete contra- and ipsilateral cortical projections. Anterior sites at which excitation can be evoked may indicate projections arising in the SMA are involved. SIGNIFICANCE This study provides normative data regarding the cortical representation of the paravertebral muscles and provides a technique for evaluating cortical motor plasticity in patients presenting with spinal pathologies.

Segmental recording of cortical motor evoked potentials from thoracic paravertebral myotomes in complete spinal cord injury.

Study Design. A study of thoracic paravertebral muscle motor-evoked potentials using transcranial magnetic stimulation in spinal cord injury patients and control participants. Objectives. To develop a method to study the level and density of corticospinal lesion s in thoracic spinal cord injury. Summary of Background Data. Cervical and lumbar spinal cord injury, unlike thoracic spinal cord injury, can be quantified by recording muscle motor-evoked potentials from limb muscles. For thoracic spinal cord injury, the use of paravertebral muscles is limited by complex innervation patterns and the greater difficulty in obtaining muscle motor-evoked potentials. Methods. In 10 patients with complete midthoracic spinal cord injury (T4–T7) and 10 age-matched control participants, muscle motor-evoked potentials were recorded from all thoracic paravertebral muscles using transcranial magnetic stimulation with a double-cone stimulating coil over the vertex. Results. In control participants, muscle motor-evoked potential responses evoked in all myotomes had progressively increasing latency in a rostrocaudal direction. Threshold was comparable in all segments. The duration of muscle motor-evoked potentials was unrelated to the spinal level. In spinal cord injury, responses were elicited in all segments above a lesion and in a varying range of segments below the lesion. In comparison with control participants, threshold was lower above and higher below the lesion (P < 0.001) in patients with spinal cord injury. Latency was longer than normal both above and below the lesion (P < 0.001). Duration was not significantly different from that in control participants at any level. Conclusions. Paravertebral muscle motor-evoked potentials can be elicited below the level of a complete spinal cord injury. Possible reasons for this include the multisegmental innervation of these muscles and the long muscle fiber conduction. Stretch reflex activation elicited by contraction of muscles above the lesion is thought to be an unlikely mechanism because of the latency of the response. Although the presence or absence of muscle motor-evoked potentials does not appear to be a sensitive indicator of the level of thoracic spinal cord injury lesion, analysis of muscle motor-evoked potentials reveals abnormal patterns that may assist in defining lesions. Finally, lower threshold above the lesion suggests corticospinal hyperexcitability of this pathway as a result of central plasticity after spinal cord injury.

Activation of back muscles during voluntary abduction of the contralateral arm in humans.

Study Design. Motor-evoked responses to transcranial magnetic stimulation of the motor cortex were recorded from erector spinae and deltoid muscles while the arm was abducted voluntarily in 10 normal subjects. Objective. To understand the neuronal substrate for the activation of the contralateral erector spinae muscle when the opposite arm is abducted. Background Data. When a standing individual abducts an arm, the center of gravity is altered; to avoid falling, trunk muscles become activated on the contralateral side. Methods. Surface EMG activity was recorded from the right deltoid and left and right erector spinae muscles. Subjects maintained abduction of their right arm to 90° at five different levels of isometric force in standing and lying postures. Transcranial magnetic stimulation was delivered to the motor cortex, producing motor-evoked responses in the three muscles during arm abduction and while relaxed. Results. EMG activity in the left erector spinae increased with the force of right arm abduction in both postures. EMG levels in right erector spinae showed no consistent change with right arm abduction force. As arm abduction force was increased, motor-evoked responses were facilitated in deltoid and the left erector spinae but not the right erector spinae in both postures. The pattern of motor-evoked potential facilitation with arm abduction force tended to plateau between 50% and 70% maximum voluntary contraction in the deltoid, whereas it continued to climb more linearly in the left erector spinae. Conclusions. Facilitation of erector spinae with arm abduction remains evident in the lying posture when spinal postural stabilization mechanisms are presumably reduced. Similar facilitation profiles have been seen previously with changing voluntary activation of erector spinae in a trunk extension task, supporting the notion that during arm abduction the drive to the contralateral erector spinae has a corticospinal origin.

Voluntary motor function in patients with chronic fatigue syndrome

INTRODUCTION The pathogenesis of chronic fatigue syndrome (CFS) remains unknown. In particular, little is known of the involvement of the motor cortex and corticospinal system. METHODS Transcranial magnetic stimulation (TMS) was used to assess corticospinal function in terms of latency and threshold of motor-evoked potentials (MEPs) in thenar muscles. Reaction times and speed of movement were assessed using button presses in response to auditory tones. RESULTS Patients had higher (P<.05) self-assessed indices of fatigue (7/10) than for pain (5/10), anxiety (4/10) or depression (3/10). Mean (+/-S.E.M.) simple reaction times (SRTs) were longer (P<.05) in the patients (275+/-19 ms) than in the controls (219+/-9 ms); choice reaction times (CRTs) were not significantly longer in the patients. Movement times, once a reaction task had been initiated, were longer (P<.05) in the patients in both SRTs (patients, 248+/-13 ms; controls, 174+/-9 ms) and CRTs (patients, 269+/-13 ms; controls, 206+/-12 ms). There was no difference (P>.05) in threshold or latency of MEPs in hand muscles between the patients (threshold, 54.5+/-2.2% maximum stimulator output [% MSO]; latency 22+/-0.3 ms) and controls (threshold 54.6+/-3.6% MSO; latency 22.9+/-0.5 ms). Regression analysis showed no correlation (P>.05) of SRTs with either threshold for MEPs or fatigue index. CONCLUSION Corticospinal conduction times and excitability were within the normal range despite a slower performance time for motor tasks and an increased feeling of fatigue. This suggests that the feeling of fatigue and the slowness of movement seen in CFS are manifest outside the corticospinal system.

Hemispheric lateralisation and immune function: A systematic review of human research☆

Past studies examined relationships between hemispheric lateralisation (HL) and immune system functioning. However, there has been no up-dated systematic review of this research area. This article reviews relevant published studies, evaluates study quality and effect sizes. Eleven studies were selected: three revealing a relationship between weaker left hemisphere function and poorer immune function, three describing a relationship between weaker right hemisphere function and stronger immune functioning, and five describing both relationships. Mean effect-size of the studies was r=0.536 (range 0.280-0.866). Collectively, studies point at left-HL and stronger immunity relationships. Limitations, mechanisms and clinical implications are discussed.