EMG Intensity Research Articles

The effect of prolonged walking on leg muscle activity patterns and vulnerability to perturbations

Understanding the consequences and ecological relevance of muscle fatigue is important to guide the development of strategies to preserve independence. However, few studies have examined walking-related fatigue and the effects on walking instability. Our purpose was to investigate the effects of prolonged walking on leg muscle activity and vulnerability to balance perturbations. Eighteen healthy young adults completed a 30-min walking trial at their preferred walking speed while leg muscle activities were recorded. Before and after the 30-min walk, participants responded to five 5% body weight lateral force perturbations. Time-frequency analysis with wavelet transformation and principal component analyses assessed neuromuscular adaptations of muscles to prolonged walking. Following prolonged walking, we observed a time-dependent increase in EMG intensities at slower frequencies for the soleus and tibialis anterior and a decrease in mean amplitudes for the soleus, lateral gastrocnemius, and semitendinosus. Mean mediolateral CoM displacement following perturbations averaged 21% larger after the 30-min walk. Our results suggest that walking for 30 min at a comfortable speed elicits complex neuromuscular adaptations indicative of local muscle fatigue and an increased vulnerability to walking balance perturbations. These findings could inform fatigue monitoring systems or walking assistive devices aimed at reducing walking-related fatigue and maintaining independent mobility.

Heartbeat detection from high-density EMG electrodes on the upper arm at different EMG intensity levels using Zephlet

Background and objectivesA significant number of global deaths caused by cardiac arrhythmias can be prevented with accurate and immediate identification. Wearable devices can play a critical role in such identification by continuously monitoring cardiac activity using electrocardiogram (ECG). The existing body of research has focused on extracting cardiac information from the body surface by investigating various electrode locations and algorithm development for ECG interpretation. The present study was designed for heartbeat detection using the signals recorded from the upper arm. MethodsFirstly, optimal electrode locations on the upper arm were identified for Rest and elbow flexion (EF) conditions. Next, a synthesized ECG was generated using the selected electrodes with generalized weights over subjects and trials, and then zero-phase wavelet (Zephlet) was applied for feature extraction. Heartbeat detection was finally performed using the extracted detail coefficients incorporated with a multiagent detection scheme (MDS). ResultsThe F1-score for heartbeat detection was 0.94 ± 0.16, 0.86 ± 0.22, 0.79 ± 0.26, and 0.67 ± 0.31 for Rest and EF with three different levels of muscle contraction (C1 to C3), respectively. Changing the acceptable distance between the detected and actual heartbeats from 50 ms to 20 ms, the F1-score changed to 0.81 ± 0.20, 0.66 ± 0.26, 0.57 ± 0.26, and 0.44 ± 0.26 for Rest and C1 to C3, respectively. ConclusionThese findings make several contributions to the current literature, summarized as precise and consistent electrode localization for various muscle contraction levels and accurate heartbeat detection method development for each of these conditions.

Central Commands to the Elbow and Shoulder Muscles During Circular Planar Movements of Hand With Simultaneous Generation of Tangential Forces.

This study examines some of the non-linear effects of signal transduction in the human motor system, with particular emphasis on muscle hysteresis. The movement tests were analyzed in a group of eight subjects, which were asked to develop tangential force using visual biofeedback while performing slow, externally imposed, circular movements of right hand holding a moving handle operated by a computerized mechatronic system. The positional changes in the averaged EMGs of the elbow and shoulder muscles were compared for all combinations of direction of movement and generated force. Additionally, for one of the subjects, there was carried out MRI identification and 3D printing of the bones of the forelimb, shoulder, scapula and collarbone, which made it possible to reconstruct for him the length and force traces of all the muscles under study. The averaged EMG traces in muscles of both joints show their close correspondence to the related force traces, however, the co-activation patterns of activity in agonists and antagonists were also often encountered. The EMG waves related to the respective force waves were strongly dependent on the predominant direction of the muscle length changes within the correspondent force wave locations: the EMG intensities were higher for the shortening muscle movements (concentric contractions) and lower during muscle lengthening (eccentric contractions). The data obtained allows to suggest that for two-joint movements of the forelimbs, it is sufficient to consider the force and activation synergies (patterns of simultaneous activity in different muscles), ignoring at the first stage the effects associated with kinematic synergy. On the other hand, the data obtained indicate that the movement kinematics has a strong modulating effect on the activation synergy, dividing it into concentric and eccentric subtypes, in accordance with the known non-linear features of the muscle dynamics. It has been shown that the concentric and eccentric differences in the responses of the shoulder muscles are more clearly distinguishable than those in the elbow muscles. The shoulder muscles also have a more pronounced symmetry of the averaged EMG responses with respect to the ascending and descending phases of force waves, while demonstrating a lower degree of antagonist cocontraction. The data obtained suggest that the central commands in two-joint movements are determined mainly by the interdependence of force and activation synergies including both intra- and inter-joint components, while kinematic synergy can be interpreted as a potent modulator of activation synergy.

EMG Signals Can Reveal Information Sharing between Consecutive Pedal Cycles.

Producing a steady cadence and power while cycling results in fairly consistent average pedal forces for every revolution, although small fluctuations about an average force do occur. This force can be generated by several combinations of muscles, each with slight fluctuations in excitation for every pedal cycle. Fluctuations such as these are commonly thought of as random variation about average values. However, research into fluctuations of stride length and stride time during walking shows information can be contained in the order of fluctuations. This order, or structure, is thought to reveal underlying motor control strategies. Previously, we found persistent structure in the fluctuations of EMG signals during cycling using entropic half-life analysis. These EMG signals contained fluctuations across multiple timescales, such as those within a burst of excitation, between the burst and quiescent period of a cycle, and across multiple cycles. It was not clear which sources of variation contributed to the persistent structure in the EMG. In this study, we manipulated variation at different timescales in EMG intensity signals to identify the sources of structure observed during cycling. Nine participants cycled at a constant power and cadence for 30 min while EMG was collected from six muscles of the leg. We found persistent structure across multiple pedal cycles of average EMG intensities, as well as average pedal forces and durations. In addition, we found the entropic half-life did not quantify fluctuations within a burst of EMG intensity; instead, it detected unstructured variation between the burst and quiescent period within a cycle. The persistent structure in average EMG intensities suggests that fluctuations in muscle excitation are regulated from cycle to cycle.

The Effects of Inter-Set Recovery Time on Explosive Power, Electromyography Activity, and Tissue Oxygenation during Plyometric Training.

Performing continuous sets to failure is fatiguing during the plyometric training. Cluster sets have been used to redistribute total rest time to create short frequent sets so that muscle fatigue can be avoided. The purpose of the study was to investigate the effects of inter-set recovery time on lower extremity explosive power, neuromuscular activity, and tissue oxygenation during plyometric exercise and recovery. An integrated assessment of explosive power, muscle electrical activity, and tissue oxygenation was adopted in the present study to help understand local muscle metabolism and fatigue during plyometric exercise and recovery. Ten university male basketball players participated in this study. Subjects performed 4 groups of exercise, each group comprised of 3 sets of jumps: 1, 2, 3, or 5 min. Surface electromyography (sEMG) signals were collected from 9 lower extremity muscles; near-infrared spectroscopy (NIRS) was recorded on vastus lateralis; mechanical data during plyometric exercise were collected from a force plate. No significant differences among sets and among groups were found regarding explosive power, jump height, EMG intensity, mean power frequency, the rate of tissue saturation index, and HbO2 changes between baseline and recovery. The current study has shown no muscular fatigue induced during the 4 groups of exercise. The results of this study may help inform recommendations concerning the recovery time during plyometric exercises at low loads (30% 1 RM).

Quantifying muscle coactivation in individuals with incomplete spinal cord injury using wavelets

BackgroundIndividuals with incomplete spinal cord injury often have decreased gait function and coactivation of antagonistic muscle pairs. Common ways of quantifying coactivation using electromyographic signals do not consider frequency information in the signal. As electromyographic signals from different motor unit types have different frequency components and muscle fiber type can change in individuals with spinal cord injury, it may be beneficial to consider frequency components. The aims were to demonstrate the utility of using a method which considers temporal and frequency components of the electromyographical signal to quantify coactivation in lower extremity muscles in individuals with incomplete spinal cord injury through 1) comparison with able-bodied individuals and 2) comparison before and after body weight supported treadmill training. MethodsFrequency decomposition techniques were applied to electromyographical signals to consider the temporal and frequency components of the electromyographical signals to quantify coactivation over a range of frequencies. ResultsOur main findings show that correlation coefficients between total EMG intensities of rectus femoris-biceps femoris and medial gastrocnemius-tibialis anterior were significantly different between able-bodied individuals and those with incomplete spinal cord injury (p = 0006, p = 0.01). The correlation spectra of medial gastrocnemius-tibialis anterior of the spinal cord injury group were substantially different than those the able-bodied group, while the EMG normalcy score was significantly different (p = 0.002). We also found that there was a change in coactivation of ankle muscles after body weight supported treadmill training. InterpretationOur findings indicate that there may be frequency specific differences in muscle coactivation between able-bodied individuals and those with incomplete spinal cord injury. Changes in coactivation were also observed before and after body weight supported treadmill training. These differences may reflect the changes in recruitment patterns of different motor unit types.

During Cycling What Limits Maximum Mechanical Power Output at Cadences above 120 rpm?

A key determinant of muscle coordination and maximum power output during cycling is pedaling cadence. During cycling, the neuromuscular system may select from numerous solutions that solve the task demands while producing the same result. For more challenging tasks, fewer solutions will be available. Changes in the variability of individual muscle excitations (EMG) and multimuscle coordination, quantified by entropic half-life (EnHL), can reflect the number of solutions available at each system level. We, therefore, ask whether reduced variability in muscle coordination patterns occur at critical cadences and if they coincide with reduced variability in excitations of individual muscles. Eleven trained cyclists completed an array of cadence-power output conditions. The EnHL of EMG intensity recorded from 10 leg muscles and EnHL of principal components describing muscle coordination were calculated. Multivariate adaptive regressive splines were used to determine the relationships between each EnHL and cycling condition or excitation characteristics (duration, duty cycle). Muscle coordination became more persistent at cadences up to 120 rpm, indicated by increasing EnHL values. Changes in EnHL at the level of the individual muscles differed from the changes in muscle coordination EnHL, with longer EnHL occurring at the slowest (<80 rpm) and fastest (>120 rpm) cadences. The EnHL of the main power producing muscles, however, reached a minimum by 80 rpm and did not change across the faster cadences studied. Muscle coordination patterns, rather than the contribution of individual muscles, are key to power production at faster cadences in trained cyclists. Reductions in maximum power output at cadences above 120 rpm could be a function of the time available to coordinate orientation and transfer of forces to the pedals.

EFFECTS OF A LIGHTER DISCUS ON SHOULDER MUSCLE ACTIVITY IN ELITE THROWERS, IMPLICATIONS FOR INJURY PREVENTION

Performance in the discus throw requires high forces and torques generated from the shoulder of the throwing arm, making shoulder muscles at risk of overuse injury. Little is known on muscle activation patterns in elite discus throw. The purpose of this study was to compare the body kinematics and muscle activation patterns of arm and shoulder muscles involved in discus throwing when using discs of different mass (1.7 kg vs 2.0 kg). It was hypothesized that the use of a lighter discus would modify the activation of the shoulder musculature compared to a standard discus. Case-control laboratory study. Seven male elite discus throwers performed five throws using a standard discus (STD, 2.0 kg) and five throws using a lighter weight discus (LGT, 1.7 kg). Surface EMG was recorded for the biceps brachii (BB), deltoideus anterior (DA), deltoideus medialis (DM), clavicular head of the pectoralis major (PM), latissimus dorsi (LD), and trapezius medialis (TM). Three-dimensional high-speed video analysis was utilized to record discus speed and identify the different temporal phases of each throw from the preparation phase (P1) to the delivery phase (P5). The EMG activation of LD lasted longer (p < 0.01) in P1 and was initiated later in P5 with the LGT discus compared to STD. In P5, the EMG intensity of BB decreased (p = 0.02) with LGT (%EMGmax = 50.4 ± 49.6%) compared to STD (64.8 ± 77.9%) and the activation of PM increased (p < 0.01) with LGT (86.2 ± 40.3%) compared to STD (66.2 ± 26.9%). The discus speed at release was increased (p = 0.04) by using the LGT discus (20.62 ± 0.75m.s-1) compared to STD (19.61 ± 0.57m.s-1). The throwing distance was also increased (P < 0.01) with the LGT (43.1 ± 4.3m) discus compared to STD (39.4 ± 3.4m). /Clinical relevance: A lighter discus could be used by elite athletes in training to add variability in muscle solicitation and thus limit the overload on certain muscles of the shoulder region. These results may have implications regarding lowering the risk of injury in discus throw. Level 3.

Altered multi-muscle coordination patterns in habitual forefoot runners during a prolonged, exhaustive run

Introduction: In response to fatigue during an exhaustive treadmill run, forefoot runner’s muscles must adapt to maintain their pace. From a neuromuscular control perspective, certain muscles may not be able to sustain the force to meet the run’s demands; thus, there may be alternative muscle coordination in the lower extremity that allows for continued running for an extended period of time. The aim of this study was to quantify the change in muscle coordination during a prolonged run in forefoot runners.Methods: Thirteen forefoot runners performed exhaustive treadmill runs (mean duration: 15.4 ± 2.2 min). The muscle coordination of seven lower extremity muscles was quantified using a high-resolution time–frequency analysis together with a pattern recognition algorithm.Results: The mean EMG intensity for the lateral and medial gastrocnemius muscles decreased with the run (p = 0.02; 0.06). The weight factors of the second principal pattern decrease by 128.01% by the end of run (p = 0.05, Cohen’s d = 0.42) representing a relatively greater biceps femoris activation in midstance but smaller midstance rectus femoris, vastus medialis, triceps surae, and tibialis anterior activation.Discussion: These results suggest that forefoot runners cannot sustain plantar flexor activation throughout an exhaustive run and change their muscle coordination strategy as a compensation. Understanding the underlying compensation mechanisms humans use to cope with fatigue will help to inform training modalities to enhance these late stage muscle activation strategies for athletes with the goal of improving performance and reducing injury.

How Do the Mechanical Demands of Cycling Affect the Information Content of the EMG?

The persistence of phase-related information in EMG signals can be quantified by its entropic half-life (EnHL). It has been proposed that the EnHL would increase with the demands of a movement task, and thus increase as the pedaling power increased during cycling. However, simulation work on the properties of EMG signals suggests that the EnHL depends on burst duration and duty cycle in the EMG that may not be related to task demands. This study aimed to distinguish between these alternate hypotheses. The EnHL was characterized for 10 muscles from nine cyclists cycling at a range of powers (35 to 260 W) and cadences (60-140 rpm) for the raw EMG, phase-randomized surrogate EMG, EMG intensity, and the principal components describing the muscle coordination patterns. There was phase-related information in the raw EMG signals and EMG intensities that was related to the EMG burst duration, duty cycle pedaling cadence, and power. The EnHL for the EMG intensities of the individual muscles (excluding quadriceps) and for the coordination patterns decreased as cycling power and cadence increased. The EnHL provide information on the structure of the motor control signals and their constituent motor unit action potentials, both within and between muscles, rather than on the mechanical demands of the cycling task per se.

The Movement- and Load-Dependent Differences in the EMG Patterns of the Human Arm Muscles during Two-Joint Movements (A Preliminary Study).

Slow circular movements of the hand with a fixed wrist joint that were produced in a horizontal plane under visual guidance during conditions of action of the elastic load directed tangentially to the movement trajectory were studied. The positional dependencies of the averaged surface EMGs in the muscles of the elbow and shoulder joints were compared for four possible combinations in the directions of load and movements. The EMG intensities were largely correlated with the waves of the force moment computed for a corresponding joint in the framework of a simple geometrical model of the system: arm - experimental setup. At the same time, in some cases the averaged EMGs exit from the segments of the trajectory restricted by the force moment singular points (FMSPs), in which the moments exhibited altered signs. The EMG activities display clear differences for the eccentric and concentric zones of contraction that are separated by the joint angle singular points (JASPs), which present extreme at the joint angle traces. We assumed that the modeled patterns of FMSPs and JASPs may be applied for an analysis of the synergic interaction between the motor commands arriving at different muscles in arbitrary two-joint movements.

Application of Cortexin for correcting the consequences of hypoxic-ischemic damage in the brain of infant rats

To verify if the peptide preparation Cortexin can be used to correct pathological processes in the CNS during perinatal ontogenesis, a set of physiological parameters (EMG, ECG, respiration, vagosympathetic balance) were recorded and analyzed in a rat perinatal hypoxic-ischemic brain damage (PHIBD) model and control infant rats. In experimental 7-day-old animals, PHIBD was modeled under inhalation ether anesthesia by ligation of the left common carotid artery and subsequent exposure to a gas mixture of 8% oxygen and 92% nitrogen. 1 h after exposure, animals were first injected intraperitoneally with cortexin (1 mg/kg); the preparation was then injected at a daily basis for 10 days. Both control animals and those after surgery but non-treated were injected with physiological solution. After 10 and 30 days after surgery, animals exhibited a delayed body weight gain versus control and significant differences in EMG intensity and spectral structure. Cortexin treatment induced on day 10 a transient improvement in EMG spectral structure but not amplitude; by day 30, the positive effect of cortexin was no longer observed. The respiration rate both in treated and non-treated animals was higher than in control. No significant changes in the heart rate were revealed in animals with PHIBD, but non-treated animals exhibited on day 30 a tendency towards its decrease. The heart rate variability (HRV) analysis showed that 10 days after trauma both non-treated and cortexin-treated animals exhibited a statistically significant shift in the vagosympathetic balance towards a parasympathetic prevalence. On day 30, cortexin treatment yields positive effects, while in non-treated animals the vagosympathetic balance shifts towards a humoral-metabolic and sympathetic prevalence. Cortexin injection to intact rats leads to significant disturbances in the vagosympathetic balance, cardiac and, to a lesser extent, respiratory rhythms, and can cause stable disturbances in activity of the somatic and vegetative nervous systems.

Altered corticomuscular coherence elicited by paced isotonic contractions in individuals with cerebral palsy: A case-control study

The purpose of the study was to analyze corticomuscular coherence during planning and execution of simple hand movements in individuals with cerebral palsy (CP) and healthy controls (HC).Fourteen individuals with CP and 15 HC performed voluntary paced movements (opening and closing the fist) in response to a warning signal. Simultaneous scalp EEG and surface EMG of extensor carpi radialis brevis were recorded during 15 isotonic contractions. Time–frequency corticomuscular coherence (EMG-C3/C4) before and during muscular contraction, as well as EMG intensity, onset latency and duration were analyzed.Although EMG intensity was similar in both groups, individuals with CP exhibited longer onset latency and increased duration of the muscular contraction than HC. CP also showed higher corticomuscular coherence in beta EEG band during both planning and execution of muscular contraction, as well as lower corticomuscular coherence in gamma EEG band at the beginning of the contraction as compared with HC.In conclusion, our results suggest that individuals with CP are characterized by an altered functional coupling between primary motor cortex and effector muscles during planning and execution of isotonic contractions. In addition, the usefulness of corticomuscular coherence as a research tool for exploring deficits in motor central processing in persons with early brain damage is discussed.

Variations in Muscle Activity of Vastus Lateralis and Gastrocnemius Lateralis with Increasing Draft Load

IntroductionMinimal research has examined the effect of draft loading on equine muscle activity. It is hypothesised that increases in draft load of 10 and 20% body mass (BM) will increase EMG intensity and duration of vastus lateralis (VL) and gastrocnemius lateralis (GL) during walk.MethodsData were collected from six Irish Cob geldings (11 ± 4 years; height: 1.47 ± 0.05 m; BM: 490 ± 65 kg) all trained to draft‐load. A modified lorry tyre was used as a loading platform. Surface EMG electrodes were positioned over VL and GL. An accelerometer placed on the left hind hoof collected kinematic data. EMG and accelerometer data at a draft load equivalent to 0 (zero load), 10 and 20% BM were recorded as the horses were led at walk in a 20 m straight line. A stopwatch determined mean speed over 20 m. Mean kinematic and EMG values were determined for 10 consecutive strides. EMG intensities were normalised to each horse. Kolmogorov‐Smirnov test explored data normality. Friedman test (P&lt;0.05) compared between conditions. A pair‐wise Wilcoxon signed ranks test identified where the differences lay.ResultsData for one horse was excluded because of poor accelerometer data. The remaining horses (n = 5) maintained a constant velocity (1.10 ± 0.02 m/s {SD}) between conditions (P&gt;0.7). VL and GL increased their muscular activity in terms of EMG intensity and duration of contraction with increasing draft load from 0 to 10% and 10 to 20% BM (P = 0.04; n = 5).ConclusionsDraft loading can be utilised for strength training following injury or to improve athletic performance.Ethical Animal ResearchThe study was approved by the Royal Veterinary College Ethics Committee. Sources of funding: Royal Veterinary College. Competing interests: none.

Accuracy of gastrocnemius muscles forces in walking and running goats predicted by one-element and two-element Hill-type models

Hill-type models are commonly used to estimate muscle forces during human and animal movement—yet the accuracy of the forces estimated during walking, running, and other tasks remains largely unknown. Further, most Hill-type models assume a single contractile element, despite evidence that faster and slower motor units, which have different activation–deactivation dynamics, may be independently or collectively excited. This study evaluated a novel, two-element Hill-type model with “differential” activation of fast and slow contractile elements. Model performance was assessed using a comprehensive data set (including measures of EMG intensity, fascicle length, and tendon force) collected from the gastrocnemius muscles of goats during locomotor experiments. Muscle forces predicted by the new two-element model were compared to the forces estimated using traditional one-element models and to the forces measured in vivo using tendon buckle transducers. Overall, the two-element model resulted in the best predictions of in vivo gastrocnemius force. The coefficient of determination, r2, was up to 26.9% higher and the root mean square error, RMSE, was up to 37.4% lower for the two-element model than for the one-element models tested. All models captured salient features of the measured muscle force during walking, trotting, and galloping (r2=0.26–0.51), and all exhibited some errors (RMSE=9.63–32.2% of the maximum in vivo force). These comparisons provide important insight into the accuracy of Hill-type models. The results also show that incorporation of fast and slow contractile elements within muscle models can improve estimates of time-varying, whole muscle force during locomotor tasks.

Comparison of four specific dynamic office chairs with a conventional office chair: Impact upon muscle activation, physical activity and posture

Prolonged and static sitting postures provoke physical inactivity at VDU workplaces and are therefore discussed as risk factors for the musculoskeletal system. Manufacturers have designed specific dynamic office chairs featuring structural elements which promote dynamic sitting and therefore physical activity. The aim of the present study was to evaluate the effects of four specific dynamic chairs on erector spinae and trapezius EMG, postures/joint angles and physical activity intensity (PAI) compared to those of a conventional standard office chair. All chairs were fitted with sensors for measurement of the chair parameters (backrest inclination, forward and sideward seat pan inclination), and tested in the laboratory by 10 subjects performing 7 standardized office tasks and by another 12 subjects in the field during their normal office work. Muscle activation revealed no significant differences between the specific dynamic chairs and the reference chair. Analysis of postures/joint angles and PAI revealed only a few differences between the chairs, whereas the tasks performed strongly affected the measured muscle activation, postures and kinematics. The characteristic dynamic elements of each specific chair yielded significant differences in the measured chair parameters, but these characteristics did not appear to affect the sitting dynamics of the subjects performing their office tasks.

Piper rhythm of the electromyograms of the abductor pollicis brevis muscle during isometric contractions

A temporal pattern coding, synchronization and rhythmicity form an integral part of central nervous system information controlling the muscle activation. Rhythmic oscillations of muscles at frequencies of 35–60 Hz were already noted in the electromyograms by Piper (1907). The purpose of this study was to resolve the Piper rhythm in the EMG of the APB muscle and report the pacing frequencies of the Piper rhythm. The Piper rhythm was identified using the power of the EMG signals extracted by a wavelet transform at higher frequencies (170–271 Hz). The results showed distinct power of the intensity extracted by the wavelets in a frequency band ranging from about 30–60 Hz. The band was reflected in the power spectra of the EMG intensity and in the first eigenvector of a principal component analysis of the power spectra. The fact that the Piper rhythm shown in this study for the APB muscle yielded a large contribution to the total power means that one can use the frequency and amplitude of the Piper rhythm in future analysis of EMG signals to monitor the influence and changes of the central command.

Quadriceps Muscle Function during Recreational Alpine Skiing

Since the introduction of carving skis, muscle activity has been investigated primarily on expert-level skiers with respect to EMG intensities. The three-part aim of this recreational skiing study was to analyze functional differences within the quadriceps muscle, to analyze the topographical influence, and to apply a time-frequency analysis of the EMG intensities using wavelets. Seven female subjects performed two runs through a standardized corridor on a slope with different inclinations (13 degrees , 29 degrees , and 21 degrees ). Knee angle and EMG of vastus lateralis (VL) and rectus femoris (RF) of the right leg were measured during the runs. The recorded EMG signal was resolved with a set of 10 wavelets (11-432 Hz) into a time-frequency space. Subsequently, the EMG intensity and mean frequency (MF) were calculated for different time windows (inside leg; outside leg). For RF, a significantly higher MF (+15.5%, P = 0.009) but similar EMG intensities were detected in the inside leg compared with the outside leg. For VL, the MF (-9.6%, P = 0.053) and EMG intensities (-54.3%, P = 0.010) were lower in the inside leg compared with the outside leg. Both muscles responded with higher EMG intensities on increasing slope inclination (VL = 90.8%, P = 0.022; RF = 115%, P = 0.01). MF is not directly related to inclination. Contrary to previously suggested coloading of the inside leg while carving, our results do not support this hypothesis for VL. However, the functional demand for RF in the inside leg is very high when skiing recreationally. The ability of a situation-dependent loading (RF as knee extensor) and unloading (RF as hip flexor) of the inside leg seems to be a crucial point with respect to specific fatigue during a skiing day.

Muscle coordination is key to the power output and mechanical efficiency of limb movements

The purpose of this study was to determine which features of muscle mechanics and muscle coordination affect the power output from a limb during locomotion. Eight subjects were tested while cycling at maximum exertion for 25 min on a stationary dynamometer. Cadence and load were varied to span a range of power outputs and myoelectric activity was measured from 10 muscles in the leg. Cycle-by-cycle variations in muscle coordination, cadence and power output were observed and the EMG intensity across all muscles was used as an estimate of the metabolic cost for each cycle. Data for the cycles at greatest power output were separated into three groups: maximum power, 80% power but lower EMG intensity and 80% power and higher EMG intensity. Torque-angular velocity relations were determined for the ankle and knee joints. During cycling at maximum power output the ankle joint was not extending at the velocity necessary for maximum power output; thus, maximum limb power occurs when some of the individual muscles cannot be generating maximum power output. Increases in EMG intensity occurred with no increase in power output from the limb: these corresponded to decreases in the efficiency and changes in coordination. Increases in power were achieved that were not matched by equivalent increases in EMG intensity, but did occur with changes in coordination. It is proposed that the power output from the limb is limited by the coordination pattern of the muscles rather than the maximum power output from any one muscle itself.

Classification of multi muscle activation patterns of osteoarthritis patients during level walking

The study compares the timing and frequency changes of surface EMGs recorded from osteoarthritis patients with previous traumatic ankle injury and normal subjects during level walking. EMG intensity (power) was obtained by a wavelet analysis. There were intensity values for each frequency characterized by the wavelets for every time point. The intensities were compounded into Multi Muscle Patterns (MMP) simultaneously showing the time and spectral aspects of the lower leg muscle activity. The aim of the study was to test the hypothesis that the differences between the group of the MMPs from the affected leg (AFL) and the not affected leg (NAL) allow detecting whether a newly measured MMP results from an AFL or NAL. This hypothesis was tested by a spherical classification procedure yielding the correctly classified MMPs thus indicating the significance of the differences between the MMPs of the AFL and NAL. The hypothesis was supported (not falsified) by the results. Thus there were common features of muscle activity in the AFL of most osteoarthritis patients that allowed detecting whether the MMP of a new patient was of the kind seen in most other osteoarthritis patients. The spectral, timing and intensity factors in the MMP that allowed this classification were visualized in the mean MMPs of the patients and the control group. The comparison revealed where on average the relative timing and spectral differences of the muscle activation of osteoarthritis patients and control subjects occurred.