Ordering and excitations in the field-induced magnetic phase of Cs3Cr2Br9.

Field-induced magnetic order has been investigated in detail in the interacting spin 3/2 dimer system Cs3Cr2Br9. Elastic and inelastic neutron scattering measurements were performed up to H=6 T, well above the critical field H(c1) approximately 1.5 T. The ordering displays incommensurabilities and a large hysteresis before a commensurate structure is reached. This structure is fully determined. Surprisingly, the lowest excitation branch never closes. Above H(c1), the gap increases slowly with the field. An analysis in terms of projected pseudospins is given.

Dzyaloshinski-Moriya interaction in the 2D spin gap system SrCu(2)(BO(3))(2).

The Dzyaloshinski-Moriya interaction partially lifts the magnetic frustration of the spin-1/2 oxide SrCu(2)(BO(3))(2). It explains the fine structure of the excited triplet state and its unusual magnetic field dependence, as observed in previous ESR and new neutron inelastic scattering experiments. We claim that it is mainly responsible for the dispersion. We propose also a new mechanism for the observed ESR transitions forbidden by standard selection rules, which relies on an instantaneous Dzyaloshinski-Moriya interaction induced by spin-phonon couplings.

Charge ordering and spin dynamics in NaV(2)O(5).

We report high-resolution neutron inelastic scattering experiments on the spin excitations of NaV(2)O(5). Below T(c), two branches with distinct energy gaps are identified. From the dispersion and intensity of the spin excitation modes, we deduce the precise zigzag charge distribution on the ladder rungs and the corresponding charge order: Delta(c) approximately 0.6. We argue that the spin gaps observed in the low-T phase of this compound are primarily due to the charge transfer.

Practicing a maximal performance task: a cooperative strategy for muscle activity.

The effect of practice on predicting elbow flexion movement time was studied. Participants (N = 18) performed 400 elbow flexion trials to a target in the horizontal plane. The trials were distributed equally over four sessions. The goal was to decrease the movement time (MT) for the same degree of accuracy. The electromyographic (EMG) activity of the biceps and triceps brachii was monitored with standard Beckman Ag/AgCl surface electrodes. The EMG measures formed two variable sets within one prediction equation. One variable set was composed of the onset of muscle activity relative to the start of movement (motor time) and the duration of muscle activity. The other variable set consisted of the mean amplitude value of the entire burst and of the first 30 ms (Q30) of activity. As the maximal speed of limb movement increased, the duration of muscle activity (motor time and EMG duration) decreased, and the magnitude of muscle activity (MAV and Q30) increased. Most of the change in the duration of muscle activity occurred in Session 1, while the magnitude of muscle activity continued to increase until Session 3. Multiple regression analysis revealed a cooperative strategy between the magnitude and duration of muscle activity. Early in learning, participants adjusted the magnitude of muscle activity to increase limb movement speed. As practice continued, alterations in the duration of muscle activity became more important, while the magnitude changes were less involved. Late in learning, both dimensions of muscle activity were used to decrease MT. We suggest that the interplay between the magnitude and duration of muscle activity may be due to: (a) cognitive factors related to the division of attention in a motor skill, (b) an increase in the frequency of motor unit firing that affects both dimensions of muscle activity, or (c) some combination of (a) and (b).

ESR investigation on the breather mode and the spinon-breather dynamical crossover in Cu benzoate.

A "breather excitation" is observed directly by electron spin resonance in the quantum spin chain Cu benzoate, in which an unexpected field-induced gap has recently been found. The nonlinear field dependence of the resonance field agrees well with the formula based on a quantum sine-Gordon model. The power-law temperature dependence of the linewidth is observed in the gapless spinon regime while the width decreases exponentially for the gapped breather regime. In the intermediate range, a distinct anomaly is found, which is the manifestation of "the spinon-breather dynamical crossover."

Pelvic restraint effect on lumbar gluteal and hamstring muscle electromyographic activation.

OBJECTIVE: To evaluate the influence of pelvic restraint on electromyographic activation (neural drive) of the lumbar extensor, gluteal, and hamstring muscles during submaximal dynamic lumbar extension in an upright seated position. DESIGN: Randomized, balanced, crossover trial comparing the electromyographic activation of the lumbar, gluteal, and hamstring muscles during dynamic lumbar extension exercise with and without pelvic restraint. SETTING: Research laboratory at a private research university. SUBJECTS: Twelve apparently healthy men (ages 18 to 50 yrs). INTERVENTION: Dynamic lumbar extension exercises. MAIN OUTCOME MEASURE: Electromyographic recordings. RESULTS: Pelvic restraint did not influence the relative activation of the lumbar, gluteal, or hamstring muscle groups (p< or =.05) during submaximal dynamic lumbar extension exercise. The relative activation of the gluteal muscles was significantly lower than that of the lumbar extensor and hamstring muscle groups. CONCLUSIONS: Pelvic restraint is not a requirement to promote neural drive to the lumbar musculature during submaximal dynamic lumbar extension exercise performed in an upright seated position. Submaximal lumbar extension exercise reduces the relative contribution of the gluteal muscles compared with a maximal voluntary isometric contraction.

Effect of manual segmental vibration on neuromuscular excitability.

OBJECTIVE: To measure the effects of manual segmental vibration (MSV) on motoneuron pool excitability. METHODS: Seven healthy subjects were tested under five conditions: pretest, upholding static knee position (USKP), MSV, posttest 1 and posttest 2. Each test lasted 2 min, and a 2-min rest was given between pretest, USKP and MSV, whereas the last two tests were consecutive. Cutaneous pressure and vibration frequency were stabilized through training periods. The effects of MSV on motoneuron excitability were investigated by measuring the H-reflex peak-to-peak amplitude. MEASURES: The H-reflex and M response were taken from the medial gastrocnemius electromyogram and elicited every 7 sec by a 1 ms square wave pulses to the tibial nerve. Fifteen trials were recorded for each condition. Stimulation intensity was set at 10% (+/- 2.5%) of Mmax stimulus. RESULTS: Repeated-measures analysis of variance comparing conditions revealed no significant differences in M response [F(4,24) = 2.37, p > .05, beta = .35]. H-reflex comparisons revealed significant differences [F(4,24) = 30.39, p < .05, beta = .01] between conditions. Duncan post hoc analysis showed that MSV produced the greatest inhibition (decreases 95%) and USKP also elicited a significant inhibition (decreases 41%). CONCLUSIONS: The inhibition may be the result of the following mechanisms: the reflex inhibition, the implication of cutaneous afferences and the higher control mechanism. Because MSV produces a reduction in neuromuscular excitability, it could also play a role in predisposing the system for changes to occur and therefore be used as an adjunct for other therapeutic applications.

Practice effects on the timing and magnitude of antagonist activity during ballistic elbow flexion to a target.

This study examined changes in antagonist timing and magnitude in response to ballistic elbow flexion practice. Seventeen men performed 400 ballistic elbow flexion trials to a target in the horizontal plane over 4 days of testing. A potentiometer and microswitch system at the elbow axis of rotation of a manipulandum recorded angular displacement and movement onset. Surface electrodes (Beckman Ag/AgCl) monitored the triceps brachii lateral head, and the electromyographic (EMG) signals were bandpassed between 20 and 300 Hz. The antagonist EMG burst was divided in two: early low-level activity (ANT1), and the large portion of the burst which occurs near target achievement (ANT2). Movement time decreased from 178 ms on the first test day to 136 ms on the last session. As practice improved the speed of limb movement, onset of the first component (ANT1) remained unchanged, while the second component (ANT2) started earlier. The magnitude of both portions of the antagonist burst increased from the first to last test day, but the change in ANT2 relative to ANT1 was more pronounced. These findings are used to explain discrepant observations in the literature for the temporal measure.

Effects of repetitive dynamic contractions upon electromechanical delay.

The effect of repeated maximal effort isotonic contractions on electromechanical delay was studied. Over 4 days, 17 male subjects performed 400 rapid elbow flexion trials. The kinematics and surface electromyographic (EMG) activity of the biceps brachii of these subjects were recorded. The period from the onset of the EMG until the beginning of movement was defined as the electromechanical delay. The period from the beginning of movement until the end of the EMG was defined as the second component of the contraction. Over the 4 day period there was an increase in the speed of limb movement. The mean power frequency and the duration of the EMG during the electromechanical delay did not change, while the root-mean-square amplitude increased. The duration of the EMG during the second component of the contraction remained stable. The mean power frequency and the root-mean-square amplitude of the EMG during the second component of the contraction increased with the speed of limb movement. We conclude that the faster contractions were a result of changes in motor unit recruitment during the second component of the contraction, rather than in the electromechanical delay.

Investigation of quadriceps femoris function through electrical stimulation.

OBJECTIVE: Patellofemoral dysfunctions are associated with problems in the knee extensor mechanism, including specific atrophy of the distal fibers of the vastus medialis, also known as the vastus medialis oblique. Rehabilitation of the vastus medialis in patellofemoral dysfunctions has been shown to be ineffective. This limitation had stemmed from the lack of understanding of the vastus medialis' specific functions. The purpose of this project was to investigate the role of four portions of the quadriceps femoris using surface electrical stimulation. DESIGN AND SETTING: Single-group and single-test (repeated measures on muscle portions) design. All tests were performed in a university laboratory. SUBJECTS: Eight healthy university students received electrical stimulation. MEASUREMENTS: Subjects were seated with the knee at 90 degrees of flexion and the leg pushing against a strain gauge that measured the force exerted. Electrical stimulation was administered on the vastus lateralis, rectus femoris, and proximal and distal fibers of the vastus medialis to quantify the torque produced by the knee extensors. The electrical stimulation maximum intensity was adjusted to achieve specific fiber recruitment and the highest isolated contraction. Force was measured with a strain gauge placed on the anterior aspect of the distal end of the tibia. Force was recorded in two levels: force applied before the onset of electrical stimulation and force produced by the stimulation. Subjects also performed two maximum isometric knee extensions before and after electrical stimulation for normalization. RESULTS: Electrical stimulation elicited mean torques of 6.31 per thousand, 14.0 per thousand, 20.2 per thousand, and 28.0 per thousand of maximum isometric voluntary contractions of the distal and proximal fibers of the vastus medialis, the vastus lateralis, and rectus femoris, respectively. CONCLUSIONS: The distal fibers of the vastus medialis do not contribute significantly to knee extension.

Pain effect on monosynaptic and polysynaptic reflex inhibition.

OBJECTIVE: Determine the electrophysiological effects of anterior knee pain using monosynaptic and polysynaptic reflexes. DESIGN: A pretest/posttest control group experimental design with repeated measures. SETTING: All tests were performed in a university laboratory. PARTICIPANTS: Six subjects suffering from patello-femoral dysfunction (PFD) composed the experimental group. These patients were diagnosed and referred by medical and chiropractic doctors. Six healthy subjects were recruited for the control group. INTERVENTION: Pain relief by cold application (10 minutes) on the knee. MAIN OUTCOME MEASURES: Monosynaptic reflex: H reflex of the superficial portions of the quadriceps femoris. Polysynaptic reflexes: nociceptive flexion response (NFR) of the long head of the biceps femoris. RESULTS: Cold application, producing significant pain relief (p < 0.05), did not change H-reflex amplitude (p > 0.05) for both groups and did not show modulation of the NFR. However, the PFD group showed significantly lower pain (p < 0.05) and NFR thresholds (p < 0.05) than the control group. Finally, nociceptive stimulation was applied at the sural nerve during an isometric extension of the knee (20% of maximum electromyographic contraction). After this stimulation, uni-articular extensors, especially the distal fibers of the vastus medialis, showed a longer and stronger inhibition than the bi-articular rectus femoris. CONCLUSIONS: Pain episodes do not appear to affect monosynaptic responses. NFR threshold differences found between PFD and control group could be explained by the increased excitability of the nervous system in subjects experimenting painful bouts. Functional implications of these findings are discussed.

Normal and paralyzed muscle force and fatigability induced by electrical stimulation.

OBJECTIVE: The aim of this article is to review the basic research concerned with force and fatigue induced by electrical stimulation. Specific reports on human and animal models are covered. Articles dealing with specific afflictions of the neuromuscular system are also reviewed. DATA SOURCES: The main sources for article retrieval were the classic bibliographic data bases in the exercise, rehabilitation and physiological science fields such as Index Medicus and Medline. Indexing terms used included human and animal in conjunction with electrical stimulation, force and fatigue. Language and time constraints limited the search to French and English texts of this century. Proceedings of different electrical stimulation conferences were also covered. STUDY SELECTION: All authors had to approve the selection of the articles to be included in this review. The narrow scope of the review made it possible to include most of the articles investigated. Redundancy of information was the only rejection criterion. DATA EXTRACTION: Initially, the first author collated the articles and extracted the relevant data. This collation was then verified with respect to the original information and approved by all authors. DATA SYNTHESIS: Possible sites of the fatigue mechanisms may include the central nervous system, the peripheral nervous system, the neuromuscular junction, and the muscle excitation-contraction coupling. Even though not unanimous, data gathered from normal muscles seem to support that the major contributor to fatigue is the muscle fiber and not the nervous system or the neuromuscular junction. Other work involving paralyzed muscles suggest that electrically induced exercise alters the contractile properties of the muscle more specifically. CONCLUSIONS: It appears that muscle intracellular processes dominate the force decrease associated with muscular fatigue. Accordingly, when dealing with a problem involving loss of force and hyperfatigability, the metabolic aspects and muscle physiology should be prime considerations in choosing a therapeutic approach. The nervous and/or neuromuscular implications are, however, not to be discarded.

Quadriceps femoris muscle activity in patellofemoral pain syndrome.

To elucidate and attempt to dissociate the two mechanisms, neuromuscular and mechanical, underlying patellofemoral pain syndrome, 18 subjects, divided into two groups based on a diagnosis of patellofemoral pain syndrome and the knee Q angle, were studied. The control group was asymptomatic and exhibited a normal Q angle (mean, 8.25 degrees), whereas the other group, diagnosed as patellofemoral pain syndrome patients, reported knee pain and had an above-normal Q angle (mean, 21.05 degrees). All subjects were tested for isometric maximum knee extension at 90 degrees, 30 degrees, and 15 degrees of knee flexion while they were seated in a special restraining chair. During testing, surface electromyography at the oblique and long fibers of the vastus medialis, and at the vastus lateralis were recorded along with the knee moment of force. The integrated electromyographic signals associated with the peak torque for all of the vastus muscles, along with the vastus medialis obliquus:vastus lateralis and vastus medialis longus:vastus lateralis activity ratios showed no significant differences between groups nor between the three angles, suggesting that all vasti measured were consistently active throughout the studied range of motion. This suggests that the neural drive was not affected in the patellofemoral pain syndrome patients. However, when the five patients showing the largest Q angles were isolated, they revealed a significantly smaller vastus medialis obliquus:vastus lateralis ratio when compared to the other group. The same ratio was also significantly smaller at 15 degrees compared to 90 degrees.(ABSTRACT TRUNCATED AT 250 WORDS)

Relationships between electromyography and kinematics in human stepping strategies.

Kinematic and electromyographic (EMG) parameters were analyzed on phase plane diagrams of human stepping motions performed over an obstacle of varying height. A videomotion analysis system was used to record the displacement of 5 reflective markers placed on the subject's lower limb joints. Surface bipolar electrodes were placed on the rectus and biceps femoris muscles. Results revealed systematic differences in the EMG patterns of the rectus femoris between initial and subsequent trials in the 3 conditions of obstacle height. In the initial trials there was an episode of EMG activity right before or during the transition from knee flexion to extension. In the subsequent trials this episode was shifted toward maximum knee extension velocity, leaving a period of silence between the initial burst and this episode. On the other hand, the biceps femoris was rarely silent and particularly active during the flexion phase, reflecting its possible role in active knee flexion and postural control during the dynamic phase of stepping. The increase in phase plane size with obstacle height appears related to the amplitude of the initial EMG burst of the rectus femoris. Phase plane diagrams combined with EMG appear to be an appropriate approach to explore the dynamics of human movement strategies. The results suggest that control mechanisms for the flexion and extension phases are different and may be involved in fine tuning the stepping strategy for subsequent trials.