Reduced Muscle Activity Research Articles

Self-powered robots to reduce motor slacking during upper-extremity rehabilitation: a proof of concept study.

Robotic rehabilitation is a highly promising approach to recover lost functions after stroke or other neurological disorders. Unfortunately, robotic rehabilitation currently suffers from "motor slacking", a phenomenon in which the human motor system reduces muscle activation levels and movement excursions, ostensibly to minimize metabolic- and movement-related costs. Consequently, the patient remains passive and is not fully engaged during therapy. To overcome this limitation, we envision a new class of body-powered robots and hypothesize that motor slacking could be reduced if individuals must provide the power to move their impaired limbs via their own body (i.e., through the motion of a healthy limb). To test whether a body-powered exoskeleton (i.e. robot) could reduce motor slacking during robotic training. We developed a body-powered robot that mechanically coupled the motions of the user's elbow joints. We tested this passive robot in two groups of subjects (stroke and able-bodied) during four exercise conditions in which we controlled whether the robotic device was powered by the subject or by the experimenter, and whether the subject's driven arm was engaged or at rest. Motor slacking was quantified by computing the muscle activation changes of the elbow flexor and extensor muscles using surface electromyography. Subjects had higher levels of muscle activation in their driven arm during self-powered conditions compared to externally-powered conditions. Most notably, subjects unintentionally activated their driven arm even when explicitly told to relax when the device was self-powered. This behavior was persistent throughout the trial and did not wane after the initiation of the trial. Our findings provide novel evidence indicating that motor slacking can be reduced by self-powered robots; thus demonstrating promise for rehabilitation of impaired subjects using this new class of wearable system. The results also serve as a foundation to develop more sophisticated body-powered robots (e.g., with controllable transmissions) for rehabilitation purposes.

Effects of a passive exoskeleton on the mechanical loading of the low back in static holding tasks

With mechanical loading as the main risk factor for LBP in mind, exoskeletons are designed to reduce the load on the back by taking over a part of the required moment. The present study assessed the effect of a passive exoskeleton on back and abdominal muscle activation, hip and lumbar flexion and on the contribution of both the human and the exoskeleton to the L5/S1 net moment, during static bending at five different hand heights. Two configurations of the exoskeleton (LOW & HIGH) differing in angle-torque characteristics were tested. L5/S1 moments generated by the subjects were significantly reduced (15–20% for the most effective type) at all hand heights. LOW generated 4–11 Nm more support than HIGH at 50%, 25% and 0% upright stance hand height and HIGH generated 4–5 Nm more support than LOW at 100% and 75%. Significant reductions (11–57%) in back muscle activity were found compared to WITHOUT for both exoskeletons for some conditions. However, EMG reductions compared to WITHOUT were highly variable across subjects and not always significant. The device allowed for substantial lumbar bending (up to 70°) so that a number of participants showed the flexion-relaxation phenomenon, which prevented further reduction of back EMG by the device and even an increase from 2% to 6% MVC in abdominal activity at 25% hand height. These results indicate that flexion relaxation and its interindividual variation should be considered in future exoskeleton developments.

Soft Actuating Sit-to-Stand Trainer Seat

In this paper, we propose a novel type of soft robot for sit to stand (STS) training, which is made with soft bellow actuators. Analysis with five healthy human subjects revealed that there is a statistically significant decrease in the peak and mean muscle activation signal from three out of the four groups of lower limb muscle for STS transition, namely, tibialis anterior, hamstrings, and quadriceps. The peak muscle activation decreased most drastically on the quadriceps muscle group (0.726 ± 0.467 to 0.269 ± 0.334). As reduced muscle activation signal correlates to less muscular effort required by the users, the results show the effectiveness of the device in partially supporting the STS transition of the subject, which subsequently serves as an STS trainer device.

Enhanced availability of serotonin increases activation of unfatigued muscle but exacerbates central fatigue during prolonged sustained contractions.

Animal preparations have revealed that moderate synaptic release of serotonin (5-HT) onto motoneurones enhances motor activity via activation of 5-HT2 receptors, whereas intense release of 5-HT causes spillover of 5-HT to extrasynaptic 5-HT1A receptors on the axon initial segment to reduce motoneurone activity. We explored if increasing extracellular concentrations of endogenously released 5-HT (via the selective serotonin reuptake inhibitor paroxetine) influences the ability to perform unfatigued and fatigued maximal voluntary contractions in humans. Following the ingestion of paroxetine, voluntary muscle activation and torque generation increased during brief unfatigued maximal contractions. In contrast, the ability to generate maximal torque with increased 5-HT availability was compromised under fatigued conditions, which was consistent with paroxetine-induced reductions in motoneurone excitability and voluntary muscle activation. This is the first in vivo human study to provide evidence that 5-HT released onto the motoneurones could play a role in central fatigue. Brief stimulation of the raphe-spinal pathway in the turtle spinal cord releases serotonin (5-HT) onto motoneurones to enhance excitability. However, intense release of 5-HT via prolonged stimulation results in 5-HT spillover to the motoneurone axon initial segment to activate inhibitory 5-HT1A receptors, thus providing a potential spinal mechanism for exercise-induced central fatigue. We examined how increased extracellular concentrations of 5-HT affect the ability to perform brief, as well as sustained, maximal voluntary contractions (MVCs) in humans. Paroxetine was used to enhance 5-HT concentrations by reuptake inhibition, and three studies were performed. Study 1 (n= 14) revealed that 5-HT reuptake inhibition caused an ∼4% increase in elbow flexion MVC. However, when maximal contractions were sustained, time-to-task failure was reduced and self-perceived fatigue was higher with enhanced availability of 5-HT. Study 2 (n= 11) used twitch interpolation to reveal that 5-HT-based changes in motor performance had a neural basis. Enhanced 5-HT availability increased voluntary activation for the unfatigued biceps brachii and decreased voluntary activation of the biceps brachii by 2-5% following repeated maximal elbow flexions. The final study (n= 8) investigated whether altered motoneurone excitability may contribute to 5-HT changes in voluntary activation. F-waves of the abductor digiti minimi (ADM) were unaffected by paroxetine for unfatigued muscle and marginally affected following a brief 2-s MVC. However, F-wave area and persistence were significantly decreased following a prolonged 60-s MVC of the ADM. Overall, high serotonergic drive provides a spinal mechanism by which higher concentrations of 5-HT may contribute to central fatigue.

Feedforward and Feedback Control Share an Internal Model of the Arm's Dynamics.

Recent work has shown that, when countering external forces, the nervous system adjusts not only predictive (i.e., feedforward) control of reaching but also reflex (i.e., feedback) responses to mechanical perturbations. Here we show that altering the physical properties of the arm (i.e., intersegmental dynamics) causes the nervous system to adjust feedforward control and that this learning transfers to feedback responses even though the latter were never directly trained. Forty-five human participants (30 females) performed a single-joint elbow reaching task and countered mechanical perturbations that created pure elbow motion. In our first experiment, we altered intersegmental dynamics by asking participants to generate pure elbow movements when the shoulder joint was either free to rotate or locked by the robotic manipulandum. With the shoulder unlocked, we found robust activation of shoulder flexor muscles for pure elbow flexion trials, as required to counter the interaction torques that arise at the shoulder because of forearm rotation. After locking the shoulder joint, which cancels these interaction torques, we found a substantial reduction in shoulder muscle activity over many trials. In our second experiment, we tested whether such learning transfers to feedback control. Mechanical perturbations applied to the arm with the shoulder unlocked revealed that feedback responses also account for intersegmental dynamics. After locking the shoulder joint, we found a substantial reduction in shoulder feedback responses, as appropriate for the altered intersegmental dynamics. Our work suggests that feedforward and feedback control share an internal model of the arm's dynamics.SIGNIFICANCE STATEMENT Here we show that altering the physical properties of the arm causes people to learn new motor commands and that this learning transfers to their reflex responses to unexpected mechanical perturbations, even though the reflex responses were never directly trained. Our results suggest that feedforward motor commands and reflex responses share an internal model of the arm's dynamics.

Active Passive Nature of Assistive Wearable Gait Augment Suit for Enhanced Mobility

In this paper we discuss the active and passive nature of the assistive wearable gait augment suit (AWGAS). AWGAS is a soft, wearable, lightweight, and assists walking gait by reducing muscle activation during walking. It augments walking by reducing the muscle activation of the posterior and anterior muscles of the lower limb. The suit uses pneumatic gel muscles (PGM), foot sensors for gait detection, and pneumatic valves to control the air pressure. The assistive force is provided using the motion in loop feedforward control loop using foot sensors in shoes. PGMs are actuated with the help of pneumatic valves and portable air tanks. The elastic nature of the PGM allows AWGAS to assist walking in the absence of the air supply which makes AWGAS both active and passive walking assist suit. To evaluate the active and passive nature of the AWGAS, we experimented to measure surface EMG (sEMG) of the lower limb muscles. sEMG was recorded for unassisted walking, i.e., without the suit, passive assisted walking, i.e., wearing the suit with no air supply and active assisted walking, i.e., wearing the suit with air supply set at 60 kPa. The results shows reduction in the muscle activity for both passive and active assisted walking as compared to unassisted walking. The pilot trials of the AWGAS were conducted in collaboration with local farmers in the Hiroshima prefecture in Japan where feedback received is complementing the results obtained during the experiments.

Phasic reduction in respiratory muscle activity during rapid eye movement sleep in children

Phasic reduction in respiratory muscle activity during rapid eye movement sleep in children

Patterns of upper limb muscle activation in children with unilateral spastic cerebral palsy: Variability and detection of deviations

BackgroundThe aim of this study was two-fold: (1) to quantify the variability of upper limb electromyographic patterns during elbow movements in typically developing children and children with unilateral spastic cerebral palsy, and to compare different amplitude normalization methods; (2) to develop a method using this variability to detect (a) deviations in the patterns of a child with unilateral spastic cerebral palsy from the average patterns of typically developing children, and (b) changes after treatment to reduce muscle activation. MethodsTwelve typically developing children ([6.7–15.9yo]; mean 11.0 SD 3.0yo) and six children with unilateral spastic cerebral palsy ([7.9–17.4yo]; mean 12.4 SD 4.0yo) attended two sessions during which they performed elbow extension-flexion and pronation-supination movements. Surface electromyography of the biceps, triceps, brachioradialis, pronator teres, pronator quadratus, and brachialis muscles was recorded. The Likelihood method was used to estimate the inter-trial, inter-session, and inter-subject variability of the electromyography patterns for each time point in the movement cycle. Deviations in muscle patterns from the patterns of typically developing children and changes following treatment were evaluated in a case study of a child with cerebral palsy. FindingsNormalization of electromyographic amplitude by the mean peak yielded the lowest variability. The variability data were then used in the case study. This method detected higher levels of activation in specific muscles compared with typically developing children, and a reduction in muscle activation after botulinum toxin A injections. InterpretationUpper limb surface electromyography pattern analysis can be used for clinical applications in children with cerebral palsy.

Sleep bruxism: an overview for clinicians

Bruxism is characterised by clenching or grinding of the teeth due to contraction of the masseter, temporalis and other jaw muscles. Bruxism may lead to masticatory muscle hypertrophy, tooth surface loss, fracture of restorations or teeth, hypersensitive or painful teeth and loss of periodontal support. Sleep bruxism has previously been viewed as a dysfunctional movement or pathological condition, whereas it is now accepted as a centrally controlled condition with various systemic risk factors. It has been postulated that sleep bruxism may have a protective role during sleep, for example in relation to airway maintenance or in stimulating saliva flow. A diagnosis of sleep bruxism may be made via patient report and clinical interview, clinical examination, intraoral appliances or recording of muscle activity. Bruxism in itself does not require treatment: management is only indicated where problems arise as a result of bruxism. Oral appliances primarily aim to protect the dentition from damage caused by clenching/grinding, although they may reduce muscle activity. Irreversible occlusal adjustments have no basis in evidence in the management of bruxism. Behavioural strategies include biofeedback, relaxation and improvement of sleep hygiene. Administration of botulinum toxin (Botox) to the masticatory muscles appears to reduce the frequency of bruxism, but concerns have been raised regarding possible adverse effects. Dentists should be aware of the potential aetiology, pathophysiology and management strategies of sleep bruxism.

Biomechanical Testing of an Upper-Extremity Occupational Exoskeleton - Preliminary Report on Methodologies and Pilot Data

Current and rapidly emerging occupational exoskeleton and exosuit technologies target increases in worker performance and safety through reductions of potentially harmful biomechanical exposures. While long-term effectiveness of these technologies cannot be readily evaluated, short-term effectiveness could be; however, the development of standardized assessment methods for exoskeleton and exosuit performance lags behind. In this study, three generalized methods to assess the performance characteristics of active and passive exoskeleton modes, including kinematic compliance, are introduced. The methods consisted of an isometric pull test, an isometric hold test, and an anatomical functional range of motion (FRM) test. Pilot data was captured from one subject wearing a low-cost actively and passively controlled occupational exoskeleton prototype called Robotic Exoskeleton for Upper Extremity Strength Augmentation (REUESA). Comparison of data between assessment measurements taken with and without the use of the REUESA system showed reductions in surface electromyographic (sEMG) muscle activity levels and reductions in FRMs. The experimental assessment methodology introduced in this paper may provide a platform for the ongoing development and improvement of standardized assessment methods for exoskeleton and exosuits, especially within the efforts of ASTM International Standards Committee F48.

P118-The clinical use of an Asymmetry Index (AI) calculated from Centre of Mass data to distinguish between normal and pathological gait

We have recently demonstrated that the triceps surae muscles energy cost (ECTS) represents a substantial portion of the total metabolic cost of running (Erun). Therefore, it seems relevant to evaluate the factors which dictate ECTS, namely the amount and velocity of shortening, since it is likely these factors will dictate Erun. Erun and triceps surae morphological and AT mechanical properties were obtained in 46 trained and elite male and female distance runners using ultrasonography and dynamometry. ECTS (J·stride−1) at the speed of lactate threshold (sLT) was estimated from AT force and crossbridge mechanics and energetics. To estimate the relative impact of these factors on ECTS, mean values for running speed, body mass, resting fascicle length (Lf), Achilles tendon stiffness and moment arm and maximum isometric plantarflexion torque were obtained. ECTS was calculated across a range (mean ± 1 sd) of values for each independent factor. Average sLT was 233 m·min−1. At this speed, ECTS was 255 J·stride−1. Estimated fascicle shortening velocity was 0.08 Vmax and the level of muscle activation was 84.7% of maximum isometric torque. Compared to the ECTS calculated from the lowest range of values obtained for each independent factor, higher AT stiffness was associated with a 39% reduction in ECTS, 81% reduction in fascicle shortening velocity and a 31% reduction in muscle activation. Longer AT moment arms and elevated body masses were associated with an increase in ECTS of 18% and 23%, respectively. These results demonstrate that a low ECTS is achieved primarily from a high AT stiffness and low body mass, which is exemplified in elite distance runners.

Evaluation of a Novel Active Exoskeleton for Tasks at or Above Head Level

Tasks at or above head level have been known to cause discomfort and musculoskeletal disorders. This letter presents a novel wearable active pneumatic exoskeletal support system with a power tool based control interface to make work at head level or above more comfortable. Subjective task effort (Borg CR10), movement velocities, and muscle activity (surface electromyography) have been evaluated and different passive and active operation modes are being compared. We show that the exoskeleton is capable of performing a substantial reduction in muscle activity, increasing upper arm elevation velocities, and reducing task effort. The comparison of an active support mode with a trigger button at the tool grip with a passive support mode showed that users reported less resistance for lowering the arm and increased velocities when they were able to adjust the support with a simple button. We show that for tasks with a varying weight of loads, an adaptive active exoskeleton is greatly beneficial over a passive one as it eases usability and acceptance.

ForceHand Glove: A Wearable Force-Feedback Glove With Pneumatic Artificial Muscles (PAMs)

The upper limb of the human body has a higher likeliness of getting affected by muscle fatigue and injury, due to repetitive motion. This letter presents an interactive glove for the wrist of the human body. The assistive model is enabled with pneumatic actuators and stretch sensors, which support the user in performing wrist flexion, extension, pronation, and supination. The purpose of this glove is to provide the subject with a force-feedback feeling, and while a repetitive practice of such training is done, rehabilitation can be achieved. For evaluation, surface electromyographical signals were first used to study the effect of force feedback on associated muscle activation. Average % maximum voluntary isometric contraction of all subjects showed statistically reduced muscle activity in at least one associated muscle while performing all motions, enabled with actuators. Second, the force feedback generated by the glove at the user's wrist has been modeled using experiments with a force transducer. An application was then developed where the user was given the ability to interact with virtual objects and receive feedback through the glove. The evaluation of the feasibility of this application was done by measuring and comparing force profiles while generating an equal force in an ideal and a predicted environment.

Design of novel running leggings with thermoplastic polyurethane membrane compression zones

Compression garments are widely used by athletes to improve athletic performance and to avoid potential injuries. Some compression garments are developed to exert pressure on muscle groups via thermoplastic polyurethane (TPU) membrane layers laminated on the textile surface. This study investigates the effect of novel TPU membrane patterns on muscle performance of the lower extremities and on the comfort parameters of air and water vapor permeability. Three novel running leggings with TPU membrane compression zones were designed to exert pressure on the major muscle groups used during running. Electromyography (EMG) measurements of the female participants wearing the designed leggings with TPU membranes, conventional leggings and shorts were recorded during a standardized squat protocol via a wireless surface EMG system. A repeated measures analysis of variance with a Greenhouse–Geisser correction determined that the mean root of mean square values for the EMG signals retrieved from the rectus femoris, vastus lateralis, gastrocnemius and hamstring muscles while wearing a specific legging design revealed statistically significant reductions in muscle activation. On the other hand, comfort tests exhibited low water vapor permeability and air permeability results when the textile surface was laminated with the TPU membrane. TPU membranes laminated on athletic wear to create compression zones could be effective in reducing muscle activation. Comfort performance is another essential design parameter that should be integrated into the design decisions. Large surfaces of solid TPU membranes should be minimized and surface textures should be employed for increased breathability.

A Soft-Inflatable Exosuit for Knee Rehabilitation: Assisting Swing Phase During Walking.

In this paper, we present a soft-inflatable exosuit to assist knee extension during gait training for stroke rehabilitation. The soft exosuit is designed to provide 25% of the knee moment required during the swing phase of the gait cycle and is integrated with inertial measurement units (IMUs) and smart shoe insole sensors to improve gait phase detection and controller design. The stiffness of the knee joint during level walking is computed using inverse dynamics. The soft-inflatable actuators, with an I cross-section, are mechanically characterized at varying angles to enable generation of the required stiffness outputs. A linear relation between the inflatable actuator stiffness and internal pressure as a function of the knee angle is obtained, and a two-layer stiffness controller is implemented to assist the knee joint by providing appropriate stiffness during the swing phase. Finally, to evaluate the ability of the exosuit to assist in swing motion, surface-electromyography (sEMG) sensors are placed on the three muscle groups of the quadriceps and two groups of the hamstrings, on three healthy participants. A reduction in muscle activity of the rectus femoris, vastus lateralis, and vastus medialis is observed, which demonstrates feasibility of operation and potential future usage of the soft inflatable exosuit by impaired users.

Re-thinking floor mat design from an ergonomics perspective: Can a two-part mat system reduce biomechanical loads during normal mat handling tasks?

Floor mats are commonplace in commercial buildings, particularly in entry ways. These mats are routinely handled by delivery personnel as the mats are picked up for cleaning and clean mats are deployed. A new two-part mat design, which eliminates the need to move the rubber base during mat change operations, was hypothesized to reduce the physical demands on delivery personnel. Electromyographic data from back and shoulder muscles and spinal kinematics were obtained as 12 volunteers simulated mat selection, mat deployment, and mat pick-up tasks. Other factors considered in this study included mat size, pick-up method, and mat textile orientation during deployment. Results indicated that the two-part design reduced muscle activation levels across all tasks. Biomechanical benefits were also found when the mats were picked-up using a kick-fold as opposed to hand-fold method and when mats were deployed with the textile component rolled towards the inside of the roll.

The Influence of Directional Compression Tights onMuscle Activity and Performance in Recreational Alpine Skiers

INTRODUCTION Recent studies reported reduced muscle activity in competitive alpine skiers using directional compression (DC). It is not known whether the effects of DC are limited to competitive skiers, or if similar changes would be observed in recreational skiers. The purpose of this study was to examine changes in hip and leg muscle EMG patterns in recreational alpine skiers when skiing with and without a lower body DC garment. METHODS 11 intermediate and expert skiers volunteered for this study. Subjects skied 2 days, 2 weeks apart, with DC and non-compressive (TNC) base layer in a randomized order. EMG of the gluteus medius (GMED), rectus femoris (RF), and adductor longus (ADL) were recorded using surface EMG during measurement runs. Two measurement runs with standardized turns were taken on each visit. Subjects free skied for 1.75 hrs between measurement runs. This sequence of ski runs was replicated on the second testing day. Standardized turns were normalized to 100% turn duration and averaged together for each trial in each condition. A 2x2 ANOVA with repeated measures was used to compare turn time, edge angle, RMS, and MF within trials. Paired t-tests were used to compare percent change (%Δ) RMS, MF and self-paced skiing between trials. RESULTS Subjects skied more runs (8.0 ± 1.5 vs. 5.6 ± 1.8; p<0.05) and vertical (1969 ± 489 m vs. 1382 ± 304 m; p<0.05) during free skiing with DC than TNC. No differences were observed between trials for turn duration or edge angle. Although no statistical differences in %Δ RMS or MF were found, there was a trend towards smaller magnitude %Δ MF with DC (Table 1). CONCLUSIONS Although there were no differences in muscle activity between trials in either condition, subjects improved their self-paced skiing performance in the DC condition. There was also a trend towards smaller %Δ MF the DC condition. Further research should investigate the biomechanical influence of DC on skiing performance.Table: No title available.

Relationship Between Muscle Activation and Force Recovery Following Sustained Maximal Voluntary Isometric Contractions

Previously we observed dissociation of skeletal muscle activation and force recovery following sustained maximal voluntary isometric contractions (MVIC). Fatigue was specifically related to reduced muscle activation within bouts; yet, incomplete recovery of force between bouts occurred with maximal muscle activation, suggesting interference in excitation-contraction coupling. PURPOSE: to investigate time-dependency of nervous system recovery following MVIC. METHODS: Men (n=29) performed two bouts (B1, B2) of sustained handgrip MVIC preceded/followed by 10-min recovery periods. Force (dynamometer) and surface EMG (sEMG) from the brachioradialis (BR), flexor carpi radialis (FCR), flexor carpi ulnaris (FCU), and flexor digitorum profundus (FDP) were collected continuously (1000 Hz) during bouts. sEMG signals were band pass filtered, rectified, and integrated (iEMG), then normalized (nEMG) to initial B1 iEMG. Force and sEMG were analyzed at 0.5 second intervals every 15 seconds. RESULTS: Recovery (B2 initial force >90% of B1) response fell into two groups; recovered (R, n=13) and not recovered (NR; n=16). Force decreased similarly in B1 (R: 77%; NR: 78%) and B2 (R: 79%; NR: 80%), yet initial B2 force was less (22%) than B1 in NR. Fatigue progressed in two-phases; B1 fast-phase decreased more rapidly with slow phase inflection occurring earlier (60 sec; 35% initial force) in R compared to NR (90 sec; 26% initial force). In R, B2 had a similar rate of decline in force and inflection (60 sec; 32% initial force). NR B2 showed a similar rate of decline in force, but inflection occurred sooner (75 sec) at the same level of initial B1 force (22%). Muscle activation (iEMG) during B1 was reduced similarly in R and NR (R: BR: 68%; FCR: 63%; FCU: 73%; FDP: 75%; NR: BR: 61%; FCR: 67%; FCU: 43%; FDP: 57%). During B2, R and NR showed similar decreases as B1, but all muscles except FCR showed lower initial B2 muscle activation compared to B1. CONCLUSIONS: Fatigue manifested in two phases; fast and slow. Recovery of force following sustained MVIC appears to be related to initial rate of decline of force (fast phase) with earlier onset of phase two. Earlier onset of phase shift may be related to changes in external compressive force and blood flow through the muscle, reducing metabolic perturbation and reflex inhibition of activation.

Theoretical considerations for muscle-energy savings during distance running

We have recently demonstrated that the triceps surae muscles energy cost (ECTS) represents a substantial portion of the total metabolic cost of running (Erun). Therefore, it seems relevant to evaluate the factors which dictate ECTS, namely the amount and velocity of shortening, since it is likely these factors will dictate Erun. Erun and triceps surae morphological and AT mechanical properties were obtained in 46 trained and elite male and female distance runners using ultrasonography and dynamometry. ECTS (J·stride−1) at the speed of lactate threshold (sLT) was estimated from AT force and crossbridge mechanics and energetics. To estimate the relative impact of these factors on ECTS, mean values for running speed, body mass, resting fascicle length (Lf), Achilles tendon stiffness and moment arm and maximum isometric plantarflexion torque were obtained. ECTS was calculated across a range (mean ± 1 sd) of values for each independent factor. Average sLT was 233 m·min−1. At this speed, ECTS was 255 J·stride−1. Estimated fascicle shortening velocity was 0.08 Vmax and the level of muscle activation was 84.7% of maximum isometric torque. Compared to the ECTS calculated from the lowest range of values obtained for each independent factor, higher AT stiffness was associated with a 39% reduction in ECTS, 81% reduction in fascicle shortening velocity and a 31% reduction in muscle activation. Longer AT moment arms and elevated body masses were associated with an increase in ECTS of 18% and 23%, respectively. These results demonstrate that a low ECTS is achieved primarily from a high AT stiffness and low body mass, which is exemplified in elite distance runners.

Evaluation of a passive exoskeleton for static upper limb activities

The aim of this study was to evaluate the effect of a passive upper body exoskeleton on muscle activity, perceived musculoskeletal effort, local perceived pressure and subjective usability for a static overhead task. Eight participants (4 male, 4 female) held a load (0 kg and 2 kg) three times overhead for a duration of 30 s each, both with and without the exoskeleton. Muscle activity was significantly reduced for the Biceps Brachii (49%) and Medial Deltoid (62%) by the device for the 2 kg load. Perceived effort of the arms was significantly lower with the device for the 2 kg load (41%). The device did not have a significant effect on trunk or leg muscle activity (for the 2 kg load) or perceived effort. Local perceived pressure was rated below 2 (low pressure levels) for all contact areas assessed. Half of the participants rated the device usability as acceptable. The exoskeleton reduced muscle activity and perceived effort by the arms, and had no significant negative effect on the trunk and lower body with regards to muscle activity, perceived effort and localised discomfort.