Reduced Muscle Activity Research Articles

Biomechanical Comparison of Two Lower Extremity Exercise Machines Used for Knee Injury Rehabilitation

2004 The goals of rehabilitation after an ACL reconstruction include improving cardiovascular and muscular fitness. Although stationary bicycles are commonly used to accomplish this task, recumbent stepping ergometers have been shown to achieve the same goals. Few data exist regarding the use of recumbent steppers by individuals who have ACL surgery. PURPOSE: To evaluate the knee joint reaction forces and lower extremity muscle activity during stepping and cycling. METHODS: Fourteen healthy individuals with no history of lower extremity injury (age: 22.9 ± 1.9 y, 173.6 ± 11.3 cm, 76.7 ± 13.2kg) and 10 individuals who had undergone ACL reconstruction in their right knees (age: 22.2 ± 4.1 y, 175.3 ± 7.6 cm, 73.0 ± 12.9 kg, 43 ± 32 months post-op) participated. Subjects were videotaped with two cameras as they performed moderate (RPE = 3) exercise on a stepper and a cycle. Four seat positions were utilized resulting in minimum knee flexion angles of 0° (full extension), 15°, 30°, and 45°. The right pedals of the stepper and bike were fitted with a load cell. Video (60Hz) and force data (900Hz) were synchronized and used to perform an inverse dynamics analysis. To assess the myoelectric activity of selected lower extremity muscles, six pairs of surface electrodes were attached to the right side of the body over the following muscles: vastus lateralis (VL), vastus medialis (VM), rectus femoris, biceps femoris, medial gastrocnemius, and soleus. RESULTS: Statistical analyses (ANOVA) revealed that proximal/distal forces were significantly decreased for the stepper compared to the cycle (p = .029) while medial/lateral forces increased (p<.001). When compared to the cycle, significantly reduced muscle activity was detected for the VL (p = .005) and VM (p = .001) for the stepper. Significant main effects (increased muscle activity with increasing knee angle) were found in the VM (p = .012) and VL (p = .058). CONCLUSION: Our results indicate that lower extremity biomechanics on the two machines tested are not affected by knee surgery status. The reduced compressive knee force, equal anterior shear force, and the similar muscle activity for 4 of the 6 lower extremity muscles tested, indicate that a recumbent stepper is a viable rehabilitation modality after ACL surgery. Supported by NuStep, Inc. (UPN#01061905)

Biomechanical Comparison of Two Lower Extremity Exercise Machines Used for Knee Injury Rehabilitation

2004 The goals of rehabilitation after an ACL reconstruction include improving cardiovascular and muscular fitness. Although stationary bicycles are commonly used to accomplish this task, recumbent stepping ergometers have been shown to achieve the same goals. Few data exist regarding the use of recumbent steppers by individuals who have ACL surgery. PURPOSE: To evaluate the knee joint reaction forces and lower extremity muscle activity during stepping and cycling. METHODS: Fourteen healthy individuals with no history of lower extremity injury (age: 22.9 ± 1.9 y, 173.6 ± 11.3 cm, 76.7 ± 13.2kg) and 10 individuals who had undergone ACL reconstruction in their right knees (age: 22.2 ± 4.1 y, 175.3 ± 7.6 cm, 73.0 ± 12.9 kg, 43 ± 32 months post-op) participated. Subjects were videotaped with two cameras as they performed moderate (RPE = 3) exercise on a stepper and a cycle. Four seat positions were utilized resulting in minimum knee flexion angles of 0° (full extension), 15°, 30°, and 45°. The right pedals of the stepper and bike were fitted with a load cell. Video (60Hz) and force data (900Hz) were synchronized and used to perform an inverse dynamics analysis. To assess the myoelectric activity of selected lower extremity muscles, six pairs of surface electrodes were attached to the right side of the body over the following muscles: vastus lateralis (VL), vastus medialis (VM), rectus femoris, biceps femoris, medial gastrocnemius, and soleus. RESULTS: Statistical analyses (ANOVA) revealed that proximal/distal forces were significantly decreased for the stepper compared to the cycle (p = .029) while medial/lateral forces increased (p<.001). When compared to the cycle, significantly reduced muscle activity was detected for the VL (p = .005) and VM (p = .001) for the stepper. Significant main effects (increased muscle activity with increasing knee angle) were found in the VM (p = .012) and VL (p = .058). CONCLUSION: Our results indicate that lower extremity biomechanics on the two machines tested are not affected by knee surgery status. The reduced compressive knee force, equal anterior shear force, and the similar muscle activity for 4 of the 6 lower extremity muscles tested, indicate that a recumbent stepper is a viable rehabilitation modality after ACL surgery. Supported by NuStep, Inc. (UPN#01061905)

The effects of lumbosacral orthoses on spine stability: What changes in EMG can be expected?

Antagonistic trunk muscle activity is normally required to stabilize the spine. A lumbosacral orthosis (LSO) might reduce the need for this antagonistic activity by providing passive stiffness to the trunk and increasing spine stability. The maximum reduction in trunk muscle EMG and in the resultant spine compression force due to the LSO was estimated using a biomechanical model. The lumbar spine stability was first quantified for the average trunk muscle EMG recorded from 11 male subjects performing various isometric trunk exertion tasks. Subsequently, the spine-stiffening effects of the LSO were implemented in the model and trunk muscle forces were reduced iteratively until the original level of spine stability without the LSO was achieved. The upper bound estimates of the reduction in trunk muscle EMG due to LSO ranged from 0.6% to 14.1% of the maximum voluntary activation depending on the task and the muscle. The resultant spine compression force averaged across all tasks decreased by only 355 N. A much larger variance of the experimental data precluded the detection of these effects at statistically significant levels. However, the small effects size does not necessarily exclude the possibility of functional benefits of slightly reducing muscle activity in patients with low back pain.

Fish exploiting vortices decrease muscle activity.

Fishes moving through turbulent flows or in formation are regularly exposed to vortices. Although animals living in fluid environments commonly capture energy from vortices, experimental data on the hydrodynamics and neural control of interactions between fish and vortices are lacking. We used quantitative flow visualization and electromyography to show that trout will adopt a novel mode of locomotion to slalom in between experimentally generated vortices by activating only their anterior axial muscles. Reduced muscle activity during vortex exploitation compared with the activity of fishes engaged in undulatory swimming suggests a decrease in the cost of locomotion and provides a mechanism to understand the patterns of fish distributions in schools and riverine environments.

Complex mechanisms of sensory tricks in cervical dystonia.

Muscle activities in 26 patients with predominantly rotational torticollis were quantified using surface electromyography. In the subgroup of 19 patients with an effective sensory trick, different modes and bilateral locations of trick application led to significant (P < 0.002) reduction of electromyographic (EMG) activity. A strong correlation was found between the efficacy of trick manoeuvres and the starting head position. Trick application in a neutral or even contralateral position was most effective while no reduction of muscle activity during trick application at the maximum dystonic head position was found (P < 0.001). We propose a two-phase model: First, normalisation of head posture is obtained by counterpressure or volitional antagonistic muscle activity. In a second step, this position can be stabilised using sensory tricks challenging central adaption of distorted sensorimotor integration.

Patient management and treatment technique

In contrast to the direct effect of augmentation materials, the smoothing effect on folds and wrinkles after a botulinum toxin treatment (BTX) is a reactive, secondary effect after achieving reduction of muscle activity first. Besides the relaxing effect on mimic muscles and overlying skin, the treatment leads to psychological effects as well. The interruption of long-established, reflexlike muscle hyperactivity or even pathologic muscle activity (tics) changes not only the patient's personal perception of his environment and the impression he leaves on his environment but also his entire aspect.

Adaptations in metabolic capacity of rat soleus after paralysis.

To determine whether long-term reductions in neuromuscular activity result in alterations in metabolic capacity, the activities of oxidative, i.e., succinate dehydrogenase (SDH) and citrate synthase (CS), and glycolytic, i.e., alpha-glycerophosphate dehydrogenase (GPD), enzyme markers were quantified in rat soleus muscles 1, 3, and 6 mo after a complete spinal cord transection (ST). In addition, the proportional content of lactate dehydrogenase (LDH) isozymes was used as a marker for oxidative and glycolytic capacities. The myosin heavy chain (MHC) isoform content of a fiber served as a marker of phenotype. In general, MHC isoforms shifted from MHC1 toward MHC2, particularly MHC2x, after ST. Mean SDH and CS activities were higher in ST than control at all time points. The elevated SDH and CS activities were indicative of an enhanced oxidative capacity. GPD activities were higher in ST than control rats at all time points. The increase in activity of SDH was larger than GPD. Thus the GPD-to-SDH (glycolytic-to-oxidative) ratio was decreased after ST. Compared with controls, total LDH activity increased transiently, and the LDH isozyme profile shifted from LDH-1 toward LDH-5, indicative of an enhanced glycolytic capacity. Combined, these results indicate that 1) the metabolic capacities of soleus fibers were not compromised, but the interrelationships among oxidative and glycolytic capacity and MHC content were apparently dissociated after ST; 2) enhancements in oxidative and glycolytic enzyme activities are not mutually exclusive; and 3) chronic reductions in skeletal muscle activity do not necessarily result in a reduced oxidative capacity.

Nondominant arm advantages in load compensation during rapid elbow joint movements.

This study was designed to examine interlimb asymmetries in responding to unpredictable changes in inertial loads, which have implications for our understanding of the neural mechanisms underlying handedness. Subjects made repetitive single joint speed constrained 20 degrees elbow flexion movements, while the arm was supported on a horizontal, frictionless, air-jet system. On random trials, a 2-kg mass was attached to the arm splint prior to the "go" signal. Subjects were not given explicit information about the mass prior to movement nor were they able to view their limb or the mass. Accordingly, muscle activity, recorded prior to peak tangential finger acceleration, was the same for loaded and baseline trials. After this point, substantial changes in muscle activity occurred. In both limbs, the load compensation response was associated with a reduction in extensor muscle activity, resulting in a prolonged flexion phase of motion. For the nondominant arm, this resulted in effective load compensation, such that no differences in final position accuracy occurred between loaded and baseline trials. However, the dominant arm response also included a considerable increase in flexor muscle activity. This substantially prolonged the flexor acceleration phase of motion, relative to that of the nondominant arm. As a result, the dominant arm overcompensated the effects of the load, producing a large and systematic overshoot of final position. These results indicate more effective load compensation responses for the nondominant arm; supporting a specialized role of the nondominant arm/hemisphere system in sensory feedback mediated error correction mechanisms. The results also suggest that specialization of the dominant arm system for controlling limb and task dynamics is specifically related to feedforward control mechanisms.

EMG-FEEDBACK FROM TRAPEZIUS AND FOREARM MUSCLES REDUCE TRAPEZIUS BUT NOT FOREARM ACTIVITY DURING COMPUTER WORK

High levels of electromyography (EMG) in muscle have been accompanied with development of work-related musculoskeletal disorders. EMG biofeedback is then hypothesised to be a useful means to reduce EMG activity in working muscles while typing. PURPOSE To test the effect of EMG biofeedback from m. trapezius (TRA) or m. extensor digitorum communis (EDC) on EMG activity of TRA and EDC during standardized computer work. METHODS Eleven healthy women (27–56 yrs) performed standardized computer work while receiving EMG biofeedback from either right TRA or EDC either visually or auditorially during either fixed or free work pace. The following variables were measured: EMG from TRA, EDC, subjective evaluation of the feedback usefulness, and duration of feedback variable above threshold. RESULTS Independent of feedback variable 82% (60–92%) of the subjects reported the EMG feedback to be useful. During feedback the muscle activity appeared above threshold for the given feedback muscle 34% (22–48%) of working time. EMG results analyzed for 4 persons showed the level of muscle activity in the control condition to be (mean±SD): 4.4±3.2% EMGref for TRA and 7.8±3.6% EMGmax for EDC. When receiving EMG biofeedback, subjects were able to significantly (p < 0.05) reduce the EMG activity in TRA to 2.5±2.7% EMGref compared to control independent of the feedback variables. The reduction in TRA was also significant (p < 0.05) when EMG feedback was given specifically from EDC (declined to 2.4±1.6% EMGref) compared to control. However, only a tendency toward reduction of EDC muscle activity was seen when receiving EMG biofeedback compared to control. EDC muscle activity declined to 6.7±3.5% EMGmax. CONCLUSION Biofeedback, in terms of EMG from the right TRA and EDC muscles, reduces muscle activity significantly in TRA but only marginally in EDC.

Serotonin agonists and antagonists in obstructive sleep apnea: therapeutic potential.

Obstructive sleep apnea hypopnea syndrome (OSAHS) is a prevalent disorder associated with substantial cardiovascular and neurobehavioral morbidity. Yet this is a disorder for which there are no widely effective pharmacotherapies. The pathophysiology of obstructive sleep apnea namely, normal respiration in waking with disordered breathing only in sleep, suggests that this disorder should be readily amenable to drug therapy. Over the past 10 years, we have gained tremendous insight into the neurochemical mechanisms involved in state-dependent control of respiration. It is apparent from this work that there are many potential avenues for pharmacotherapies, including several seemingly conflicting directions for serotonergic therapies. Serotonin delivery is reduced to upper airway dilator motor neurons in sleep, and this contributes, at least in part, to sleep-related reductions in dilator muscle activity and upper airway obstruction. The dilator motor neuron post-synaptic serotonin receptors are 5-HT(2A) and 5-HT(2C) subtypes, and in adults the presynaptic 5-HT receptor in motor nuclei is 5-HT(1B), an inhibitory receptor. Serotonin receptors are also found within central respiratory neuronal groups, and these receptor subtypes include 5-HT(1A) (inhibitory) and 5-HT(2) receptors. Peripherally, stimulation of 5-HT(2A), 5-HT(2C) and 5-HT(3) receptor subtypes have an inhibitory effect on respiration via action at the nodose ganglion. Many of these receptor subtypes and their signal transduction pathways may be affected by oxidative stress in obstructive sleep apnea. These alterations will make finding drug therapies for sleep apnea more challenging, but not insurmountable. Future directions are suggested for elucidating safe, well-tolerated serotonergic drugs for this disorder. Tryptophan was one of the first serotonergic drugs tested for OSAHS. This drug was withdrawn from the market as a result of reports linking tryptophan use with eosinophilic myalgia syndrome and life-threatening pulmonary hypertension. Newer drugs with serotonergic activity tested in persons with sleep-disordered breathing include buspirone, fluoxetine and paroxetine. Trials are presently being conducted to evaluate the effects of 5-HT(2A) and 5-HT(3) antagonists on OSAHS. Many of the drugs tested have not shown significant improvement in sleep apnea. However, with continued effort to elucidate the pharmacology of neurochemical control of state-dependent changes in respiratory control, the availability of pharmacological therapy for this disorder is not too far away.

Practice effects on coordination and control, metabolic energy expenditure, and muscle activation

One defining characteristic of skilled motor performance is the ability to complete the task with minimum energy expenditure. This experiment was designed to examine practice effects on coordination and control, metabolic energy expenditure, and muscle activation. Participants rowed an ergometer at 100 W for ten 16-min sessions. Oxygen consumption and perceived exertion (central and peripheral) declined significantly with practice and movement economy improved (reliably) by 9%. There was an associated but non-significant reduction in heart rate. Stroke rate decreased significantly. Peak forces applied to the ergometer handle were significantly less variable following practice and increased stability of the post-practice movement pattern was also revealed in more tightly clustered plots of hip velocity against horizontal displacement. Over practice trials muscle activation decreased, as revealed in integrated EMG data from the vastus lateralis and biceps brachii, and coherence analysis revealed the muscle activation patterns became more tightly coordinated. The results showed that practice reduced the metabolic energy cost of performance and practice-related refinements to coordination and control were also associated with significant reductions in muscle activation.

Modulation of upper airway collapsibility during sleep: influence of respiratory phase and flow regimen.

We hypothesized that upper airway collapsibility is modulated dynamically throughout the respiratory cycle in sleeping humans by alterations in respiratory phase and/or airflow regimen. To test this hypothesis, critical pressures were derived from upper airway pressure-flow relationships in six tracheostomized patients with obstructive sleep apnea. Pressure-flow relationships were generated by varying the pressure at the trachea and nose during tracheostomy (inspiration and expiration) (comparison A) and nasal (inspiration only) breathing (comparison B), respectively. When a constant airflow regimen was maintained throughout the respiratory cycle (tracheostomy breathing), a small yet significant decrease in critical pressure was found at the inspiratory vs. end- and peak-expiratory time point [7.1 +/- 1.6 (SE) to 6.6 +/- 1.9 to 6.1 +/- 1.9 cmH(2)O, respectively; P < 0.05], indicating that phasic factors exerted only a modest influence on upper airway collapsibility. In contrast, we found that the inspiratory critical pressure fell markedly during nasal vs. tracheostomy breathing [1.1 +/- 1.5 (SE) vs. 6.1 +/- 1.9 cmH(2)O; P < 0.01], indicating that upper airway collapsibility is markedly influenced by differences in airflow regimen. Tracheostomy breathing was also associated with a reduction in both phasic and tonic genioglossal muscle activity during sleep. Our findings indicate that both phasic factors and airflow regimen modulate upper airway collapsibility dynamically and suggest that neuromuscular responses to alterations in airflow regimen can markedly lower upper airway collapsibility during inspiration.

Afferent input-associated reduction of muscle activity in microgravity environment

Responses of electromyogram (EMG) of soleus, lateral portion of gastrocnemius (LG) and tibialis anterior (TA), and both afferent and efferent neurograms at the L 5 segmental level of the spinal cord, to altered gravity levels created by the parabolic flight of a jet airplane were investigated in adult rats. The EMG activity in antigravity soleus muscle gradually increased when the gravity was elevated from 1-G to 1.5-G (+23%) and 2-G (+67%) during the ascending phase of parabolic flight. The activity decreased ∼72% from the 1-G level immediately when the rat was exposed to microgravity. The EMG level was maintained low during the 20-s microgravity, but it was restored immediately once the gravity level was increased to 1.5-G and then 1-G during the descending and recovery phase. The EMG level of LG also increased gradually when the gravity level was elevated and the level then decreased when the rat was exposed to microgravity ( P>0.05). However, the activity level during the 20-s microgravity was identical to that obtained at 1-G. The EMG level of TA even increased insignificantly in response to the exposure to microgravity. The responses of afferent neurogram were similar to those of soleus EMG, even though the magnitude of the reduction of integrated neurogram level in response to microgravity exposure was small (∼26% vs. 1-G level) relative to that of soleus EMG. The level of efferent neurogram was also decreased, but only ∼9% vs. 1-G level, during the 20-s microgravity. The data in the current study suggest that the afferent input is closely associated with the gravity-dependent muscular activity.

Immediate and long-term effects of ankle-foot orthosis on muscle activity during walking: A randomized study of patients with unilateral foot drop

Geboers JF, Drost MR, Spaans F, Kuipers H, Seelen HA. Immediate and long-term effects of ankle-orthosis on muscle activity during walking: a randomized study of patients with unilateral foot drop. Arch Phys Med Rehabil 2002;83:240-5. Objectives: To determine (1) whether use of an ankle-foot orthosis (AFO) by patients with ankle dorsiflexor paresis leads to decreased muscle activity, immediately or 6 weeks after AFO use, and (2) whether this decrease (if present) differs between healthy and paretic subjects. Design: Cross-sectional and longitudinal randomized case-control study. Setting: Rehabilitation research center in the Netherlands. Participants: Fourteen healthy persons and 29 patients with foot drop. Interventions: Muscle activity was measured by surface electromyography. Electromyographic reproducibility was tested in 14 healthy volunteers walking with and without AFO. Acute changes in muscle activity from AFO use were compared between the 14 healthy persons and the 29 patients with foot drop. Adaptation effects of AFO use after 6 weeks were studied in 29 patients, randomly chosen 16 of whom had started using an AFO at the first measurement. Main Outcome Measures: Amount of change in mean rectified electromyographic activity (δ value) between walking with and without AFO. Follow-up measurements were conducted after 3 and 6 weeks. Results: Correlation coefficients, reflecting within-subject reproducibility, varied between.68 and.96 (mean,.86). In patients and healthy subjects, tibialis anterior muscle activity decreased by 7% and 20% (P =.01, P =.04), respectively, when using an AFO. In patients, this decrease was measured in the overall activity during the gait cycle; in healthy subjects, it was measured in the first 15% of the gait cycle. Overall electromyographic activity did not change during 6 weeks; δ values per muscle did not change during follow-up in the AFO group. Conclusion: AFO use immediately reduced muscle activity of the ankle dorsiflexors. However, using an AFO for 6 weeks did not lead to a generally lower electromyographic activity level nor did the amount of activity reduction accumulate in comparison with patients who did not use an AFO. It is, therefore, safe to use an AFO, even with recently paretic patients. © 2002 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation

Gait anlysis in patients with osteoarthritis of the knee before and after total knee replacement

The reduction in quadriceps femoris muscle activation is considered to be one of the major mechanisms involved in pathological motion patterns in patients with osteoarthritis (OA) of the knee. These activation deficits are, at least in part, reversible after total knee arthroplasty (TKA). The purpose of this study was to investigate the effects of TKA on motion patterns in patients with OA of the knee. 50 patients with knee OA were investigated prior and at 33 +/- 8 months after unilateral TKA. The control group comprised 17 healthy volunteers matched to the study group with respect to age. The gait analysis was performed with an optoelectronic motion analysis system (Elite(R), Italian). Step length, step duration and velocity increased after surgery (p < 0.05), but remained lower than the values of the controls (p < 0.05). Extension angles in stand and swing phase remained unchanged and significantly lower when compared to control values (p = 0.01, p = 0.04). Flexion angles in swing (p = 0.02) and stand phase (p = 0.01) increased on the operated side and were significantly higher than the flexion angle in the control group (p = 0.02, p = 0.03). Reextension in stand phase, which was investigated for quantification of functional range of motion, increased bilaterally after surgery (p < 0.01), but remained lower than the reextension of the controls (p </= 0.01). Although a significant improvement of pathological motion patterns in patients with OA of the knee after unilateral TKA occurred, a deficit persisted 33 +/- 8 months after surgery. The improvement of functional range of motion in stand phase was only caused by increased flexion. No influence was found with gait analysis on extension after TKA.

Sensomotor function while wearing lumbar support ortheses

Flexible lumbar corsets should contribute to an improved active and passive stabilisation of the lumbar spine. In the present study, the effects of flexible corsets on sensory-motor abilities have been investigated. 24 healthy subjects (m = 10, f = 14, 26.7 years) performed angle- and force-reproduction tasks with and without wearing a flexible corset. The subjects tried to reproduce trunk flexion angles of 20 degrees und 40 degrees as well as forces of 33% and 66% of the maximum strength of the back extensor muscles. Additionally, muscular reactions were measured during destabilizing stimuli. No significant differences were found for the reproduction tasks. When wearing the corset, the left erector spinae muscles showed a delayed onset (+15 ms, p < 0.05), and the obliquus abdominis reached maximal activity more rapidly (-18 ms, p < 0.05). In the preinnervation phase the obliquus abdominis (-33%, p < 0.01), in the eccentric phase the erector spinae (-27%, p < 0.05) and the right obliquus abdominis (-46%, p < 0.05) and in the concentric phase the right obliquus abdominis (-35%, p < 0.01) and the rectus abdominis (-32%, p < 0.05) showed lower activities. The reduced muscle activities with destabilizing stimuli can be interpreted as the result of an improved passive stabilisation of the lumbar region by wearing a flexible corset.

An investigation of ergonomic interventions in dental hygiene work

Alternative methods for viewing teeth while performing simulated dental procedures were investigated. The methods allowed participants to assume postures requiring less neck flexion than the standard direct view. One alternative used a video camera and monitor to view the mouth, the other incorporated 90° prism glasses. The study was conducted in two parts: (1) novice participants performing a targeting task; (2) dental hygienists performing a scaling task on a mouth model. Posture and subjective perceptions were assessed in Parts 1 and 2. Muscle activity and performance were also assessed in Part 1. The alternative methods significantly reduced muscle activity, neck flexion, and discomfort, compared to the direct view. Preferences were a function of criteria (general, comfort, productivity, or accuracy). Previously, recommendations for reducing ergonomic risk factor exposure of dental professionals emphasized reducing time spent performing dental procedures. This study shows ergonomic interventions offer alternative means of risk exposure reduction.

Effects of levodopa on laryngeal muscle activity for voice onset and offset in Parkinson disease.

The laryngeal pathophysiology underlying the speech disorder in idiopathic Parkinson disease (IPD) was addressed in this electromyographic study of laryngeal muscle activity. This muscle activity was examined during voice onset and offset gestures in 6 persons in the early stages of IPD who were not receiving medication. The purpose was to determine (a) if impaired voice onset and offset control for speech and vocal fold bowing were related to abnormalities in laryngeal muscle activity in the nonmedicated state and (b) if these attributes change with levodopa. Blinded listeners rated the IPD participants' voice onset and offset control before and after levodopa was administered. In the nonmedicated state, the IPD participants' vocal fold bowing was examined on nasoendoscopy, and laryngeal muscle activity levels were compared with normal research volunteers. The IPD participants were then administered a therapeutic dose of levodopa, and changes in laryngeal muscle activity for voice onset and offset gestures were measured during the same session. Significant differences were found between IPD participants in the nonmedicated state: those with higher levels of muscle activation had vocal fold bowing and greater impairment in voice onset and offset control for speech. Similarly, following levodopa administration, those with thyroarytenoid muscle activity reductions had greater improvements in voice onset and offset control for speech. In this study, voice onset and offset control difficulties and vocal fold bowing were associated with increased levels of laryngeal muscle activity in the absence of medication.

Electromyographic comparison of two lumbar support mechanisms intended for automotive seating applications

The purpose of this study was to compare and contrast the effect of two lumbar support mechanisms on low back muscle activity. These mechanisms were designed to provide either microadjustment or massage. In the control condition, the lumbar support mechanism was set to the full-off position (0 mm of mechanism prominence). In this position, the foam contour supplied only a minimal amount of support. This condition, which is representative of traditional lumbar support design, was used to obtain an indication of the typical amount of low back muscle activity expected from automotive sitting. The data were collected using surface electrodes located at the L3 and L5 levels. Both lumbar support mechanisms provided a small reduction in low back muscle activity during a 1 h session. This can be contrasted with the control condition, which resulted in a small increase. In this respect, the mechanisms were considered an improvement on traditional lumbar support designs.

The role of impact forces and foot pronation: a new paradigm.

This article discusses the possible association between impact forces and foot pronation and the development of running-related injuries, and proposes a new paradigm for impact forces and foot pronation. The article is based on a critical analysis of the literature on heel-toe running addressing kinematics, kinetics, resultant joint movements and forces, muscle activity, subject and material characteristics, epidemiology, and biologic reactions. However, this paper is not a review of the literature but rather an attempt to replace the established concepts of impact forces and movement control with a new paradigm that would allow explaining some of the current contradictions in this topic of research. The analysis included all papers published on this topic over the last 25 years. For the last few years, it concentrated on papers expressing critical concerns on the established concepts of impact and movement control. An attempt was made to find indications in the various publications to support or reject the current concept of impact forces and movement control. Furthermore, the results of the available studies were searched for indications expanding the current understanding of impact forces and movement control in running. Data were synthesized revealing contradictions in the experimental results and the established concepts. Based on the contradictions in the existing research publications, a new paradigm was proposed. Theoretical, experimental, and epidemiological evidence on impact forces showed that one cannot conclude that impact forces are important factors in the development of chronic and/or acute running-related injuries. A new paradigm for impact forces during running proposes that impact forces are input signals that produce muscle tuning shortly before the next contact with the ground to minimize soft tissue vibration and/or reduce joint and tendon loading. Muscle tuning might affect fatigue, comfort, work, and performance. Experimental evidence suggests that the concept of "aligning the skeleton" with shoes, inserts, and orthotics should be reconsidered. They produce only small, not systematic. and subject-specific changes of foot and leg movement. A new paradigm for movement control for the lower extremities proposes that forces acting on the foot during the stance phase act as an input signal producing a muscle reaction. The cost function used in this adaptation process is to maintain a preferred joint movement path for a given movement task. If an intervention counteracts the preferred movement path, muscle activity must be increased. An optimal shoe, insert, or orthotic reduces muscle activity. Thus, shoes, inserts, and orthotics affect general muscle activity and, therefore, fatigue, comfort, work, and performance. The two proposed paradigms suggest that the locomotor system use a similar strategy for "impact" and "movement control." In both cases the locomotor system keeps the general kinematic and kinetic situations similar for a given task. The proposed muscle tuning reaction to impact loading affects the muscle activation before ground contact. The proposed muscle adaptation to provide a constant joint movement pattern affects the muscle activation during ground contact. However, further experimental and theoretical studies are needed to support or reject the proposed paradigms.