Agonist-antagonist Co-contraction Research Articles

Acute effects of blood flow restriction on EMG activity of arm muscles during karate tsuki strike in young men

BackgroundThe positive effect of Blood Flow Restriction (BFR) on muscular performance have been documented in previous researches. However, its effect on agonist-antagonist cocontraction during fist movement has not been considered from a biomechanical perspective. The purpose of this research was to investigate the acute effect of BFR on EMG activity of arm muscles during karate tsuki strike in young men. Methods20 professional young, male karatekas voluntarily participated in this research. Biceps and triceps muscular activity and their cocontraction in the right arm (with BFR) and the left arm (control) were captured and compared using an EMG device during tsuki strike until exhaustion. ResultsThe results of independent t-test showed that biceps and triceps EMG activity in the BFR arm were significantly higher than the control arm (24.45% and 27.78% for biceps and triceps, respectively). Moreover, muscular cocontraction in BFR arm was significantly less than control arm (32.05%). ConclusionThe results indicate that acute arm BFR can increase arm muscle EMG activity during tsuki strike until exhaustion, which indirectly it is a sign for activation of type II motor units. However, BFR decreased agonist-antagonist cocontraction, which may increase the risk for injury during elbow extension in throw fist.

The impact of cerebellar transcranial direct current stimulation on isometric bench press performance in trained athletes

Athletic development centers on optimizing performance, including technical skills and fundamental motor abilities such as strength and speed. Parameters such as maximum contraction force and rate of force development, influence athletic success, although performance gains become harder to achieve as athletic abilities increase. Non-invasive transcranial direct current stimulation of the cerebellum (CB-tDCS) has been used successfully to increase force production in novices, although the potential effects in athletes remain unexplored. The present study examined the effects of CB-tDCS on maximum isometric voluntary contraction force (MVCiso) and isometric rate of force development (RFDiso) during a bench press task in well-trained athletes. 21 healthy, male, strength-trained athletes participated in a randomized, sham-controlled, double-blinded crossover design. Each participant completed the isometric bench press (iBP) task on two separate days, with at least 5 days between sessions, while receiving either CB-tDCS or sham stimulation. Electromyography (EMG) recordings of three muscles involved in iBP were acquired bilaterally to uncover differences in neuromuscular activation and agonist-antagonist co-contraction between conditions. Contrary to our hypothesis, no significant differences in MVCiso and RFDiso were observed between CB-tDCS and sham conditions. Furthermore, no tDCS-induced differences in neuromuscular activation or agonist-antagonist co-contraction were revealed. Here, we argue that the effects of CB-tDCS on force production appear to depend on the individual's training status. Future research should study individual differences in tDCS responses between athletes and novices, as well as the potential of high-definition tDCS for precise brain region targeting to potentially enhance motor performance in athletic populations.

Benchmarking the Effects on Human-Exoskeleton Interaction of Trajectory, Admittance and EMG-Triggered Exoskeleton Movement Control.

Nowadays, robotic technology for gait training is becoming a common tool in rehabilitation hospitals. However, its effectiveness is still controversial. Traditional control strategies do not adequately integrate human intention and interaction and little is known regarding the impact of exoskeleton control strategies on muscle coordination, physical effort, and user acceptance. In this article, we benchmarked three types of exoskeleton control strategies in a sample of seven healthy volunteers: trajectory assistance (TC), compliant assistance (AC), and compliant assistance with EMG-Onset stepping control (OC), which allows the user to decide when to take a step during the walking cycle. This exploratory study was conducted within the EUROBENCH project facility. Experimental procedures and data analysis were conducted following EUROBENCH's protocols. Specifically, exoskeleton kinematics, muscle activation, heart and breathing rates, skin conductance, as well as user-perceived effort were analyzed. Our results show that the OC controller showed robust performance in detecting stepping intention even using a corrupt EMG acquisition channel. The AC and OC controllers resulted in similar kinematic alterations compared to the TC controller. Muscle synergies remained similar to the synergies found in the literature, although some changes in muscle contribution were found, as well as an overall increase in agonist-antagonist co-contraction. The OC condition led to the decreased mean duration of activation of synergies. These differences were not reflected in the overall physiological impact of walking or subjective perception. We conclude that, although the AC and OC walking conditions allowed the users to modulate their walking pattern, the application of these two controllers did not translate into significant changes in the overall physiological cost of walking nor the perceived experience of use. Nonetheless, results suggest that both AC and OC controllers are potentially interesting approaches that can be explored as gait rehabilitation tools. Furthermore, the INTENTION project is, to our knowledge, the first study to benchmark the effects on human-exoskeleton interaction of three different exoskeleton controllers, including a new EMG-based controller designed by us and never tested in previous studies, which has made it possible to provide valuable third-party feedback on the use of the EUROBENCH facility and testbed, enriching the apprenticeship of the project consortium and contributing to the scientific community.

Increased co-contraction reaction during a surface perturbation is associated with unsuccessful postural control among older adults

BackgroundAs a strategy to maintain postural control, the stiffening strategy (agonist-antagonist co-contractions) is often considered dysfunctional and associated with poor physical capacity. The aim was to investigate whether increased stiffening is associated with unsuccessful postural control during an unpredictable surface perturbation, and which sensory and motor variables that explain postural stiffening.MethodsA sample of 34 older adults, 75.8 ± 3.8 years, was subjected to an unpredicted surface perturbation with the postural task to keep a feet-in-place strategy. The participants also completed a thorough sensory- and motor test protocol. During the surface perturbation, electromyography was measured from tibialis anterior and gastrocnemius to further calculate a co-contraction index during the feed-forward and feedback period. A binary logistic regression was done with the nominal variable, if the participant succeeded in the postural task or not, set as dependent variable and the co-contraction indexes set as independent variables. Further, the variables from the sensory and motor testing were set as independent variables in two separate Orthogonal Projections of Latent Structures (OPLS)-models, one with the feed-forward- and the other with the feedback co-contraction index as dependent variable.ResultsHigher levels of ankle joint stiffening during the feedback, but not the feed-forward period was associated with postural task failure. Feedback stiffening was explained by having slow non-postural reaction times, poor leg muscle strength and being female whereas feed-forward stiffening was not explained by sensory and motor variables.ConclusionsWhen subjected to an unpredicted surface perturbation, individuals with higher feedback stiffening had poorer postural control outcome, which was explained by poorer physical capacity. The level of feed-forward stiffening prior the perturbation was not associated with postural control outcome nor the investigated sensory and motor variables. The intricate causal relationships between physical capacity, stiffening and postural task success remains subject for future research.

Effects of gravitational versus iso-inertial resistance training on leg muscle force and metabolic cost of walking in healthy older adults.

The purpose was to compare the effects of 8-week resistance training programs (flywheel iso-inertial [FW] versus traditional gravity-dependent resistance training [GD]) performed twice a week at the same rate of perceived exertion (RPE), on muscle force and power capacities and physical performance in healthy older participants. Twenty-four participants were randomly assigned to either FW (male/female ratio: 7/5, age: 67.1±3.8 years) or GD (male/female ratio: 6/6, age 68.3±3.0 years) group. Knee extension maximal isometric voluntary contractions (MVC), lower limb maximal explosive power (MEP), Six-Minute Walking Test (6MWT), Timed Up-and-Go Test (TUG), metabolic cost of walking (CW) and agonist-antagonist co-contraction time (CCT) during walking were evaluated before and after training. absolute MEP and MEP normalized for body mass increased only in FW than GD group (+10.8% vs. +0.31%, P=0.056, respectively; +14.8% vs. +13.9%, P<0.001, respectively). Both training modalities improved MVC to a similar extent (+11.1% in FW vs. +13.4% in GD, P<0.001). Analogously, 6MWT distance increased in FW and GD (+5.2 and +5.5%, P<0.041, respectively). No effects of time and training modality were observed on the other parameters. The results of this study suggest that when FW and GD are administered at the same RPE with FW performed at higher movement speed in the concentric phase, both the trainings generate similar improvements in muscle strength but only the former can promote greater muscle power enhancements than GD in healthy older adults.

Characteristics of Lower Leg Muscle Activity in Patients with Cerebral Palsy during Cycling on an Ergometer.

Purpose Cycling on ergometer is often part of rehabilitation programs for patients with cerebral palsy (CP). The present study analyzed activity patterns of individual lower leg muscle during active cycling on ergometer in patients with CP and compared them to similar recordings in healthy participants. Methods Electromyographic (EMG) recordings of lower leg muscle activity were collected from 14 adult patients and 10 adult healthy participants. Activity of the following muscles was recorded: Musculus tibialis anterior, Musculus gastrocnemius, Musculus rectus femoris, and Musculus biceps femoris. Besides qualitative analysis also quantitative analysis of individual muscle activity was performed by computing the coefficient of variation of EMG signal amplitude. Results More irregular EMG patterns were observed in patients in comparison to healthy participants: agonist-antagonist cocontractions were more frequent, muscle activity measured at specific points of the cycle path was more variable, and dynamic range of muscle activity along the cycle path was narrower in patients. Hypertonicity was also more frequent in patients. Conclusion Muscle activity patterns during cycling differed substantially across patients. It showed irregular nature and occasional sharp high peaks. Dynamic range was also narrower than in controls. Observations underline the need for individualized cycling training to optimize rehabilitation effects.

Effects of hand configuration on muscle force coordination, co-contraction and concomitant intermuscular coupling during maximal isometric flexion of the fingers.

The mechanisms governing the control of musculoskeletal redundancy remain to be fully understood. The hand is highly redundant, and shows different functional role of extensors according to its configuration for a same functional task of finger flexion. Through intermuscular coherence analysis combined with hand musculoskeletal modelling during maximal isometric hand contractions, our aim was to better understand the neural mechanisms underlying the control of muscle force coordination and agonist-antagonist co-contraction. Thirteen participants performed maximal isometric flexions of the fingers in two configurations: power grip (Power) and finger-pressing on a surface (Press). Hand kinematics and force/moment measurements were used as inputs in a musculoskeletal model of the hand to determine muscular tensions and co-contraction. EMG-EMG coherence analysis was performed between wrist and finger flexors and extensor muscle pairs in alpha, beta and gamma frequency bands. Concomitantly with tailored muscle force coordination and increased co-contraction between Press and Power (mean difference: 48.08%; p < 0.05), our results showed muscle-pair-specific modulation of intermuscular coupling, characterized by pair-specific modulation of EMG-EMG coherence between Power and Press (p < 0.05), and a negative linear association between co-contraction and intermuscular coupling for the ECR/FCR agonist-antagonist muscle pair (r = - 0.65; p < 0.05). This study brings new evidence that pair-specific modulation of EMG-EMG coherence is related to modulation of muscle force coordination during hand contractions. Our results highlight the functional importance of intermuscular coupling as a mechanism contributing to the control of muscle force synergies and agonist-antagonist co-contraction.

Altered lower leg muscle activation patterns in patients with cerebral palsy during cycling on an ergometer.

ObjectiveCycling on a recumbent ergometer constitutes one of the most popular rehabilitation exercises in cerebral palsy (CP). However, no control is performed on how muscles are being used during training. Given that patients with CP present altered muscular activity patterns during cycling or walking, it is possible that an incorrect pattern of muscle activation is being promoted during rehabilitation cycling. This study investigated patterns of muscular activation during cycling on a recumbent ergometer in patients with CP and whether those patterns are determined by the degree of spasticity and of mobility.MethodsElectromyographic (EMG) recordings of lower leg muscle activation during cycling on a recumbent ergometer were performed in 14 adult patients diagnosed with CP and five adult healthy participants. EMG recordings were done with an eight-channel EMG system built in the laboratory. The activity of the following muscles was recorded: Musculus rectus femoris, Musculus biceps femoris, Musculus tibialis anterior, and Musculus gastrocnemius. The degree of muscle spasticity and mobility was assessed using the Modified Ashworth Scale and the Gross Motor Function Classification System, respectively. Muscle activation patterns were described in terms of onset and duration of activation as well as duration of cocontractions.ResultsMuscle activation in CP was characterized by earlier onsets, longer periods of activation, a higher occurrence of agonist–antagonist cocontractions, and a more variable cycling tempo in comparison to healthy participants. The degree of altered muscle activation pattern correlated significantly with the degree of spasticity.ConclusionThis study confirmed the occurrence of altered lower leg muscle activation patterns in patients with CP during cycling on a recumbent ergometer. There is a need to develop feedback systems that can inform patients and therapists of an incorrect muscle activation during cycling and support the training of a more physiological activation pattern.

Review of upper limb kinematics after cervical spinal cord injury: Implications for rehabilitation

The aim of this literature review is to provide a clear understanding of motor control and kinematic changes during open-chain upper limb (UL) movements after tetraplegia. Using data from MEDLINE between 1966 and August 2014, we investigated kinematic UL studies after tetraplegia. We included fourteen control case and three series case studies with a total of 161 SCI participants and 126 healthy control participants. SCI participants efficiently perform a broad range of tasks with their UL This is achieved by effective scapulothoracic and glenohumeral compensation which provide a dynamic mechanical coupling between the shoulder and elbow joints thus palliating elbow extension despite triceps brachii paralysis. The mechanism is incomplete, however, since C5-C6 SCI individuals are forced to reduce overhead workspace to keep the elbow extended and to maintain the mechanical dynamic interaction between the shoulder and elbow. Furthermore, motion slowing is a clear kinematic characteristic, caused by: – decreased strength; – triceps brachii paralysis disrupting normal agonist-antagonist co-contraction; – accuracy requirements at movement endpoint; – grasping. Grasping requires a prolonged deceleration phase during transport to ensure hand placement with respect to the to-be-grasped object then wrist extension during grasping to elicit either whole hand or lateral grip. Contrary to the normal pattern, where grasping is prepared during the transport phase, SCI individuals transport the wrist in flexion leading to passive finger opening that did not attest a grip preparation particularly if object size is greater than maximal grip aperture. The pattern (wrist flexed then extended) indicates that reaching and grasping are performed consecutively suggesting that these two phases are independent. Elbow extension restoration causes increased elbow stiffness resulting in increased movement velocity, reduced need for glenohumeral compensation, and overall improved motor control. Rehabilitation and surgical restoration should take these kinematic characteristics into account to reinforce proximal and distal compensations allowing elbow extension and grasp using tenodesis and consequently favoring greater autonomy of individuals after SCI.

Kinematics and knee muscle activation during sit-to-stand movement in women with knee osteoarthritis

BackroundThe purpose of this study was to compare joint kinematics, knee and trunk muscle activation and co-activation patterns during a sit-to-stand movement in women with knee osteoarthritis and age-matched controls. MethodsEleven women with knee osteoarthritis (mean and standard deviation, age: 66.90, 4.51years, height: 1.63, 0.02m, mass: 77.63, 5.4kg) and eleven healthy women (mean and standard deviation, age: 61.90, 3.12years, height: 1.63m, 0.03, mass: 78.30, 4.91kg) performed a Sit to Stand movement at a self-selected slow, normal and fast speed. Three-dimensional joint kinematics of the lower limb, vertical ground reaction forces and electromyographic activity of the biceps femoris vastus lateralis and erectus spinae were recorded bilaterally. FindingsA two-way ANOVA showed that the osteoarhtitis group performed the sit to stand task using a smaller knee and hip range of motion compared with the control group while no differences in temporal kinematics and ground reaction force-related parameters were observed. In addition, women with osteoarhtritis displayed significantly lower vastus lateralis coupled with a higher biceps feomoris electromyographic activity and higher agonist–antagonist co-contraction and co-activation than asymptomatic women. The activation of erectus spinae was not different between groups. InterpretationResults indicate that patients with moderate knee osteoarthritis rise from the chair using greater knee muscle co-contraction, earlier and greater activation of the hamstrings which results in reduced hip and knee range of motion. This may be a way to overcome the pain and potential muscle atrophy of knee extensor muscles without compromising overall task duration.

Tendon palpation during agonist contraction and antagonist co-contraction to assess wrist flexor and extensor muscle function

The aim of this study was to validate direct tendon palpation during agonist contraction and antagonist co-contraction as a method to assess wrist flexor and extensor muscle function in cases of upper limb paralysis. On one occasion, five doctors examined 17 patients with partial paralysis of the upper limb resulting from brachial plexus or cervical spinal cord injury. We asked examiners to determine if the extensor carpi radialis longus (ECRL), extensor carpi radialis brevis (ECRB), extensor carpi ulnaris (ECU), flexor carpi radialis (FCR), flexor carpi ulnaris (FCU) and palmaris longus (PL) were paralyzed, weak or strong in each patient. Examiners tested flexion – extension and radial – ulnar deviation against resistance and palpated wrist motor tendons. While palpating tendons, co-contractions were encouraged by soliciting finger extension to evaluate the FCU, thumb extension to evaluate the ECU, and finger flexion to evaluate the ECRB. Kappa values were 0.8 for the ECRL, 0.7 for the ECRB, 0.5 for the ECU, 0.8 for the FCR, 0.6 for the PL, and 0.8 for the FCU, indicating moderate to almost perfect agreement between examiners. Tendon palpation during muscle examination was adequate to identify complete paralysis, as well as weak and strong muscle contractions. This assessment helps to identify muscles that could be used during nerve or tendon transfer for reconstruction of extensive upper limb paralysis.

Upper limb kinematics after cervical spinal cord injury: a review.

Although a number of upper limb kinematic studies have been conducted, no review actually addresses the key-features of open-chain upper limb movements after cervical spinal cord injury (SCI). The aim of this literature review is to provide a clear understanding of motor control and kinematic changes during open-chain upper limb reaching, reach-to-grasp, overhead movements, and fast elbow flexion movements after tetraplegia. Using data from MEDLINE between 1966 and December 2014, we examined temporal and spatial kinematic measures and when available electromyographic recordings. We included fifteen control case and three series case studies with a total of 164 SCI participants and 131 healthy control participants. SCI participants efficiently performed a broad range of tasks with their upper limb and movements were planned and executed with strong kinematic invariants like movement endpoint accuracy and minimal cost. Our review revealed that elbow extension without triceps brachii relies on increased scapulothoracic and glenohumeral movements providing a dynamic coupling between shoulder and elbow. Furthermore, contrary to normal grasping patterns where grasping is prepared during the transport phase, reaching and grasping are performed successively after SCI. The prolonged transport phase ensures correct hand placement while the grasping relies on wrist extension eliciting either whole hand or lateral grip. One of the main kinematic characteristics observed after tetraplegia is motor slowing attested by increased movement time. This could be caused by (i) decreased strength, (ii) triceps brachii paralysis which disrupts normal agonist–antagonist co-contractions, (iii) accuracy preservation at movement endpoint, and/or (iv) grasping relying on tenodesis. Another feature is a reduction of maximal superior reaching during overhead movements which could be caused by i) strength deficit in agonist muscles like pectoralis major, ii) strength deficit in proximal synergic muscles responsible for scapulothoracic and glenohumeral joint stability, iii) strength deficit in distal synergic muscles preventing the maintenance of elbow extension by shoulder elbow dynamic coupling, iv) shoulder joint ankyloses, and/or v) shoulder pain. Further studies on open chain movements are needed to identify the contribution of each of these factors in order to tailor upper limb rehabilitation programs for SCI individuals.

Utility of multi‐channel surface electromyography in assessment of focal hand dystonia

Surface electromyography (SEMG) allows objective assessment and guides selection of appropriate treatment in focal hand dystonia (FHD). Sixteen-channel SEMG obtained during different phases of a writing task was used to study timing, activation patterns, and spread of muscle contractions in FHD compared with normal controls. Customized software was developed to acquire and analyze EMG signals. SEMG of FHD subjects (20) showed "early onset" during motor imagery, rapid proximal muscle recruitment, agonist-antagonist co-contraction involving proximal muscle groups, "delayed offset" after stopping writing, higher rectified mean amplitudes, and mirror activity in contralateral limb compared with controls (16). Muscle activation latencies were heterogenous in FHD. Anticipation, delayed relaxation, and mirror EMG activation were noted in FHD. A clear pattern of muscle activation cannot be ascertained. Multi-channel SEMG can aid in objective assessment of temporal-spatial distribution of activity and can refine targeted therapies like chemodenervation and biofeedback.

Commentaries on Viewpoint: Can muscle size fully account for strength differences between children and adults?

# {#article-title-2} to the editor: As comprehensively explained in their Viewpoint article ([1][1]), the answer to the question posed in the title has received conflicting answers. Since the contractile machinery in children is the same as that in adults, there is no reason to believe that

Quantifying established clinical assessment measures using 3D-movement analysis in individuals with Down syndrome

Purpose. A movement protocol for quantifying functional limitation in people with Down syndrome (DS) during the execution of a series of range of motion (ROM) tasks has been developed as a new assessment approach, combining quantitative measures of movement analysis and functional mobility with clinically established qualitative motor skill assessments.Methods. Fifteen subjects with DS and 11 subjects with typical development were evaluated using this movement protocol.Results. The results revealed longer durations in execution across all tasks in the DS group and were most likely due to low muscular tone and poor coordination. A significant difference in ankle ROM was found in the DS group during leg-lifting, with a wide plantar-flexion demonstrated during the entire movement. This result may be associated with the typical strategy for foot contact that generally favours the toe. Significant differences were also found in the trunk ROM and in the knee ROM and may likely reflect an increase of agonist–antagonist co-contraction, a strategy that may modify stability and dynamic equilibrium.Conclusion. The combined quantitative/qualitative protocol is an important advancement in evaluating individuals with DS and should be integrated into a more comprehensive evaluation of dynamic gait and lower limb analysis.

Effect Of Tendon Stiffness And Leg Stiffness On Running Economy In Well-trained Middle Distance Runners

PURPOSE: Previous research suggests that a stiff 'leg spring' during running can provide significant energy savings from the elastic properties in the leg‘s soft tissues. Knowing that the absorption and recovery of elastic energy in muscle-tendon units is important in determining the energy cost of running, researchers have used the leg stiffness measured during the ground contact phase in running as a surrogate for muscle-tendon stiffness. However, no research has examined both the leg spring stiffness and the stiffness of the major tendons simultaneously. The aim of the present study was to examine how leg stiffness and the mechanical properties of the primary ankle and knee tendons influence running economy. METHODS: Eleven well-trained male middle distance runners volunteered for the study. Medial gastrocnemius (MG) and vastus lateralis (VL) tendon force-elongation properties were measured during an isometric maximal voluntary contraction. Tendon forces were determined from joint moments measured by isokinetic dynamometry, corrected for agonist-antagonist co-contraction and joint rotation. Tendon elongation was measured using B-mode 2-D ultrasonography. Eccentric leg stiffness was measured during over-ground running using a previously published kinetic model. Total leg stiffness of the entire ground contact phase was derived from kinetic and kinematic measures. Running economy was measured using a standardised incremental treadmill protocol. RESULTS: Analysis revealed no correlation with MG or VL tendon stiffness and either eccentric (r = -0.03 and r = -0.07, respectively) or total leg stiffness (r = -0.06 and r = 0.16, respectively). Leg stiffness was not correlated with running economy; however, MG tendon stiffness and peak ankle joint moment were significantly correlated with economy (r = -0.66 and r = -0.68, respectively). CONCLUSIONS: Leg stiffness does not appear to be a surrogate for tendon stiffness and is not predictive of running economy in well-trained runners. Leg stiffness might be influenced by changes in leg mechanics or muscular co-contraction. Interestingly, Achilles tendon stiffness tends to be greater in more economical runners. Faster tendon recoil, indicative of stiffer tendons, could benefit power generation at the ankle joint during high speed running.

The relationships between muscle, external, internal and joint mechanical work during normal walking

Muscle mechanical work is an important biomechanical quantity in human movement analyses and has been estimated using different quantities including external, internal and joint work. The goal of this study was to investigate the relationships between these traditionally used estimates of mechanical work in human walking and to assess whether they can be used as accurate estimates of musculotendon and/or muscle fiber work. A muscle-actuated forward dynamics walking simulation was generated to quantify each of the mechanical work measures. Total joint work (i.e. the time integral of absolute joint power over a full gait cycle) was found to underestimate total musculotendon work due to agonist-antagonist co-contractions, despite the effect of biarticular muscle work and passive joint work, which acted to decrease the underestimation. We did find that when the net passive joint work over the gait cycle is negligible, net joint work (i.e. the time integral of net joint power) was comparable to the net musculotendon work (and net muscle fiber work because net tendon work is zero over a complete gait cycle). Thus, during walking conditions when passive joint work is negligible, net joint work may be used as an estimate of net muscle work. Neither total external nor total internal work (nor their sum) provided a reasonable estimate of total musculotendon work. We conclude that joint work is limited in its ability to estimate musculotendon work, and that external and internal work should not be used as an estimation of musculotendon work.

Application of an Entropy-Assisted Optimization Model in Prediction of Agonist and Antagonist Muscle Forces

Many existing optimization based biomechanical models fail to predict antagonist muscle activity. Some optimization models predict such a cocontraction, but either lack a compelling physiological basis or are computationally formidable. The current study takes advantage of the flexible definition of entropy as a scientific measure, and utilizes it in the objective function of an optimization formulation to construct a new optimization model for predicting agonist and antagonist muscle forces. In this model, the objective function of a nonlinear program consists of a weighted sum of two components: a linear or nonlinear term favoring agonist muscle exertions (reciprocal inhibition), and the entropy term enforcing cocontraction. The concept of the current optimization model is based on recent findings in neurophysiology that there exist two separate central nervous systems for generation of two motor patterns: agonist contraction and agonist-antagonist cocontraction.

LOCOMOTOR TRAINING IN INDIVIDUALS WITH INCOMPLETE SCI: FUNCTIONAL AND SPINAL REFLEX RELATED CHANGES ASSOCIATED WITH FOUR DIFFERENT BWS TRAINING PROTOCOLS.

PURPOSE/HYPOTHESIS: Body weight supported (BWS) locomotor training improves walking ability and other functions in individuals with incomplete spinal cord injury (SCI) but there is no consensus regarding optimal training parameters. Additionally, the relationship between changes in motor function and in spinal reflex activity remains to be investigated. Spinal reflexes become disorganized following SCI and this may be associated with movement disorders (spasticity, co-contraction, clonus, flexor spasms, etc). The purpose of this ongoing study is to assess functional changes associated with participation in different locomotor training protocols, and to assess the general effect (across groups) of locomotor training on spinal reflex activity. NUMBER OF SUBJECTS: Subjects were 18 individuals with chronic motor-incomplete SCI. MATERIALS/ METHODS: Subjects were assigned to 1 of 4 different 3-month BWS assisted-stepping protocols, including: 1) treadmill training with manual assistance (TM), 2) treadmill training with peroneal nerve stimulation (TS), 3) treadmill training with robotic assistance (Lokomat robotic orthosis; LR), or 4) overground training with peroneal nerve stimulation (using a Walkaidell stimulator; OG). We assessed walking speed, balance, lower extremity strength, EMG patterns and selected spinal reflexes prior to and following participation. Data were also acquired on spinal reflex measures including: a) agonist-antagonist reciprocal inhibition (RI), b) presynaptic inhibition (PI), c) Low frequency depression (LFD), d) flexor reflex threshold and amplitude, and d) H/M ratios. RESULTS: Overground walking speed improved in all groups: TM: +31.4%, TS: +30.8%, OG: +29.3%, LR: +42.9%. Mean initial strength scores (max= 50) were statistically different among training groups: TM= 36.2, TS= 27.8, OG= 39.8, LR= 20.5. There was a significant inverse correlation between change in walking speed and initial strength scores (r= -0.83). Mean balance scores (Berg Scale; max= 48) improved in 2/4 groups: TM= 2, TS= 0, OG= 5, LR= 0. EMG patterns were more robust following training in all groups. Reciprocal inhibition was increased following training (16%; pooled data). Flexor reflex threshold was not changed but the amplitude of response to a moderate stimulus (25mA) was decreased. There was a trend toward increased presynaptic (D1) inhibition and low frequency depression. CONCLUSIONS: Responses to locomotor training are similar across protocols and appear to be associated with changes in spinal reflex activity. These results suggest that repetitive performance of functional activity may be associated with plasticity of some spinal reflexes. CLINICAL RELEVANCE: In the case of RI, increased effectiveness of inhibition may be functionally important for reducing agonist-antagonist cocontraction. Decreases in flexor reflex amplitude may indicate a decrease in the hypereflexia associated with SCI. It is hypothesized that spinal reflex normalization may form the basis of more normal motor output following training.

The effects of release height on center of pressure and trunk muscle response following sudden release of stoop lifting tasks

Background and objectives: Sudden release of load during lifting threatens postural stability and is countered by trunk muscle response, which can generate high loads on the spine, and may be a cause of tissue injury. The postural threat following sudden release and the consequent muscular response are likely to depend on the posture at the time of release. This study investigates the effects of sudden release of load at two release heights of one- and three-quarters of the knee to shoulder distance during stoop lifting. Methods: Ten normal southern Chinese male volunteers were subject to sudden release of 20, 40, 60 and 80 N loads during stoop lifting trials. The release was randomly selected to be on the third, fourth or fifth cycle of a trial and was triggered at heights of one- and three-quarters of the total knee to shoulder lifting distance. The subjects stood on a force platform to allow the postural disturbance to be recorded by monitoring the center of pressure (COP), and electromyographic (EMG) data were collected from the rectus abdominus, internal oblique, external oblique, erector spinae and latissimus dorsi muscle groups. Results: The COP excursion moved closer to the posterior limit of stability with increasing release load, and this effect was significantly more marked for release from the lower of the two heights. The minimum posterior COP separation from the posterior limit of stability was significantly less for the lower release height at all loads ( p<0.001 in all cases). EMG data showed that the agonist–antagonist co-contraction durations were higher for the lower release height, and unlike sudden release from the higher level, showed a significant increase with increasing load. Conclusions: Sudden release at lower release height during stoop lifting results in significantly greater postural disturbance and spinal loading. The mean load predicted to result in fall or stumble at the lower release height (133 N) is significantly less than that predicted at the higher of the two release heights (245 N). A more marked effect of release load is also seen in the postural disturbance and trunk muscle co-contraction time for the lower release height, and particular care should therefore be taken when handling potentially unstable loads under these conditions. If the security of the load cannot be guaranteed, storage at a higher level may reduce the risk of injury due to sudden release of the load.