A. Bertrand Arsenault

Plantarflexor weakness as a limiting factor of gait speed in stroke subjects and the compensating role of hip flexors

OBJECTIVE To determine, using the Muscular Utilization Ratio (MUR) method, whether plantarflexor weakness is among the factors preventing stroke subjects from walking at faster speeds. Potential compensations by the hip flexors were also examined. DESIGN A convenience sample of 17 chronic stroke subjects in a context of a descriptive study. BACKGROUND Gait speed is correlated with the residual strength of the muscles involved in gait in stroke subjects. However, it has not been established if this residual strength limits gait speed. METHODS Kinetic and kinematic data for comfortable and maximal gait speeds were collected on the paretic side, and were used to determine the moments in plantarflexion (mechanical demand: MUR numerator) during the push-off phase. The maximal potential moment (MUR denominator) of the plantarflexors during gait was predicted using an equation derived from dynamometric data collected with a Biodex system. The MURs of the plantarflexors were then calculated at every 1% interval of the push-off phase. The pull-off phase of gait and the hip flexor strength were also examined. RESULTS Ten subjects of the sample had a MUR value between 80 and 150% at maximal gait speed. These subjects produced the lowest peak torques in plantarflexion. Each of the four fastest subjects of this group had a large hip flexion moment during the pull-off phase of gait and produced high hip flexion torque values on the dynamometer. Each of the seven remaining subjects had a MUR value under 70% when they walked at maximal speed. CONCLUSIONS Weakness of the plantarflexors should be considered as one factor limiting gait speed in 10 hemiparetic subjects. Some subjects with weak plantarflexors could walk rapidly because they compensated with the hip flexors. For the remaining stroke subjects, factors other than weakness of the plantarflexors have to be considered in order to explain the reduction in their gait speed.

The influence of an increase in the level of force on the EMG power spectrum of elbow extensors

SummaryIt has been proposed that the mean power frequency (MPF) of the electromyogram (EMG) power spectrum increases gradually with force of contraction and that this increase is a function of the fiber-type content of the muscle investigated and the inter-electrode distance (IED) used when recording the EMG signals. In order to test these hypotheses, the values of the MPF of two elbow extensor muscles, triceps brachii (TB, 65% fast twitch fibers) and anconeus (AN, 65% slow twitch fibers), were compared at different levels of contraction. Subjects (n =13) produced ten static ramp elbow extensions [0–100% maximum voluntary contraction (MVC)]. EMG signals of each muscle were recorded with two pairs of surface miniature electrodes having IEDs of 6 mm and 30 mm respectively. MPFs were obtained at each of the following levels: 10, 20, 40, 60, 80 and 100% MVC. Statistical analyses indicated that the MPF of AN increased significantly (P<0.05) up to 60% MVC. In contrast, the MPF values for TB showed no significant change across different levels of contraction (P>0.05). Since skinfold was on average 3.2 times thicker over TB than over AN it is suggested that the low-pass filtering effect of the skin could have prevented the observation of an increase of the MPF for TB. It thus appears that changes of the MPF with the level of force, as disclosed by surface electrode recordings, is specific to each muscle. Consequently one has to account for factors such as thickness of the skinfold when it comes to the determination of the fiber-type content of different muscles within a subject.

Normality and stationarity of EMG signals of elbow flexor muscles during ramp and step isometric contractions

The purpose of this study was to test the stationarity and normality of electromyographic (EMG) signals obtained while exerting isometric contractions: (a) where a steady force level is maintained (step contractions); and (b) where the force level is increased linearly over time (ramp contractions). Ramp elbow flexions were performed from 0 to 100% of the maximum voluntary contraction (MVC) in a 5-s period. For the step contractions, four force levels (20, 40, 60 and 80% MVC) were maintained for a period of 3 s each. EMG signals of the biceps brachii (BB) and brachioradialis (BR) muscles of 16 subjects were recorded with surface electrodes and digitized at a sampling frequency of 2000 Hz. Tests of normality (Shapiro-Wilk test) and stationarity (reverse arrangement test) were performed locally on short finite time records (512-ms windows). Results show that, in general, EMG signals present a non-Gaussian amplitude distribution and are stationary. Furthermore, the amplitude distribution characteristics and the stationarity of the signal were not dependent on the muscle investigated, nor on the type of contraction or force level tested. The finding of local stationarity for both tasks is important, because it suggests that performing standard spectral analysis is applicable for both step and ramp contractions. It also allows a direct comparison between results obtained under both conditions.

Surface electromyography assessment of back muscle intrinsic properties

The purpose of this study was to assess (1) the reliability and (2) the sensitivity to low back pain status and gender of different EMG indices developed for the assessment of back muscle weakness, muscle fiber composition and fatigability. Healthy subjects (men and women) and chronic low back pain patients (men only) performed, in a static dynamometer, maximal and submaximal static trunk extension tasks (short and long duration) to assess weakness, fiber composition and fatigue. Surface EMG signals were recorded from four (bilateral) pairs of back muscles and three pairs of abdominal muscles. To assess reliability of the different EMG parameters, 40 male volunteers (20 controls and 20 chronic low back pain patients) were assessed on three occasions. Reliable EMG indices were achieved for both healthy and chronic low back pain subjects when specific measurement strategies were applied. The EMG parameters used to quantify weakness and fiber composition were insensitive to low back status and gender. The EMG fatigue parameters did not detect differences between genders but unexpectedly, healthy men showed higher fatigability than back pain patients. This result was attributed to the smaller absolute load that was attributed to the patients, a load that was defined relative to their maximal strength, a problematic measure with this population. An attempt was made to predict maximal back strength from anthropometric measurements but this prediction was prone to errors. The main difficulties and some potential solutions related to the assessment of back muscle intrinsic properties were discussed.

Gait study of patients with patellofemoral pain syndrome

Patellofemoral pain syndrome is a frequent knee impairment in young adults. This study investigated the kinematic and kinetic gait patterns of individuals suffering from patellofemoral pain syndrome (PFPS). It was hypothesized that PFPS subjects modify their gait pattern in order to reduce loading on the painful patellofemoral joint. To verify this, the gait pattern of five subjects with right chronic PFPS was compared with that of five healthy subjects. Spatiotemporal, kinematic and kinetic data were collected from five gait cycles. The joint moments at the hip, knee and ankle joints were calculated using an inverse dynamic approach and the values were normalized to body weight (N·m/kg). Individual joint moments were expressed as a percentage of the support moment in order to quantify possible compensatory strategies. The kinematic analysis revealed a significant reduction of the knee flexion angle (ANOVAs, P 0.05) as far as the individual joint moments and their contribution to the support moment were concerned. However, modifications were observed in the knee and hip moments between 10% and 20% of the gait cycle. These modifications may suggest that PFPS subjects alter their gait pattern in order to reduce loading of the patellofemoral joint to avoid pain.

EMG power spectra of elbow extensors during ramp and step isometric contractions

SummaryThe goal of the present study was to compare electromyogram (EMG) power spectra obtained from step (constant force level) and ramp (progressive increase in the force level) isometric contractions. Data windows of different durations were also analysed for the step contractions, in order to evaluate the stability of EMG power spectrum statistics. Fourteen normal subjects performed (1) five ramp elbow extensions ranging from 0 to 100% of the maximum voluntary contraction (MVC) and (2) three stepwise elbow extensions maintained at five different levels of MVC. Spectral analysis of surface EMG signals obtained from triceps brachii and anconeus was performed. The mean power frequency (MPF) and the median frequency (MF) of each power spectrum were obtained from 256-ms windows taken at 10, 20, 40, 60 and 80% MVC for each type of contraction and in addition on 512-, 1024-and 2048-ms windows for the step contractions. No significant differences (P>0.05) were found in the values of both spectral statistics between the different window lengths. Even though no significant differences (P>0.05) were found between the ramp and the step contractions, significant interactions (P<0.05) between these two types of contraction and the force level were found for both the MPF and the MF data. These interactions point out the existence of different behaviours for both the MPF and the MF across force levels between the two types of contraction.

Changes in the electromyographic spectrum power distribution caused by a progressive increase in the force level

The purpose of the present study was to determine the specific changes occurring in the power spectrum with an increasing force level during isometric contractions. Surface electromyographic signals of the triceps brachii (TB) and the anconeus (AN) of 29 normal subjects were recorded during isometric ramp contractions performed from 0 to 100% of the maximum voluntary contraction (MVC) in a 5-s period. Power spectra were obtained at 10, 20, 30, 40, 50, 60, 70, 80 and 90% MVC. Changes in the shape of these spectra were evaluated visually and with the calculation of several statistical parameters related to the distribution of power along the frequency axis, such as median frequency and mean power frequency, standard deviation, skewness, first and third quartiles and half-power range. For the AN, the behaviour of the spectrum was relatively similar across subjects, presenting a shift toward higher frequencies without any major change in the shape of the spectrum. For the TB, subjects with a thin skinfold thickness presented similar behaviours. In subjects with a thicker skinfold, however, a loss of power in the high frequency region paralleled the increase in the force level. Significant correlations were obtained between the extent of the change in the value of higher order statistical parameters across force and the thickness of the skin. This points out the importance of the skinfold layer when recording with surface electrodes. Furthermore, the use of a combination of several parameters appears to provide a better appreciation of the changes occurring in the spectrum than any single parameter taken alone.

Assessment of the paraspinal muscles of subjects presenting an idiopathic scoliosis: an EMG pilot study.

BackgroundIt is known that the back muscles of scoliotic subjects present abnormalities in their fiber type composition. Some researchers have hypothesized that abnormal fiber composition can lead to paraspinal muscle dysfunction such as poor neuromuscular efficiency and muscle fatigue. EMG parameters were used to evaluate these impairments. The purpose of the present study was to examine the clinical potential of different EMG parameters such as amplitude (RMS) and median frequency (MF) of the power spectrum in order to assess the back muscles of patients presenting idiopathic scoliosis in terms of their neuromuscular efficiency and their muscular fatigue.MethodsL5/S1 moments during isometric efforts in extension were measured in six subjects with idiopathic scoliosis and ten healthy controls. The subjects performed three 7 s ramp contractions ranging from 0 to 100% maximum voluntary contraction (MVC) and one 30 s sustained contraction at 75% MVC. Surface EMG activity was recorded bilaterally from the paraspinal muscles at L5, L3, L1 and T10. The slope of the EMG RMS/force (neuromuscular efficiency) and MF/force (muscle composition) relationships were computed during the ramp contractions while the slope of the EMG RMS/time and MF/time relationships (muscle fatigue) were computed during the sustained contraction. Comparisons were performed between the two groups and between the left and right sides for the EMG parameters.ResultsNo significant group or side differences between the slopes of the different measures used were found at the level of the apex (around T10) of the major curve of the spine. However, a significant side difference was seen at a lower level (L3, p = 0.01) for the MF/time parameter.ConclusionThe EMG parameters used in this study could not discriminate between the back muscles of scoliotic subjects and those of control subject regarding fiber type composition, neuromuscular efficiency and muscle fatigue at the level of the apex. The results of this pilot study indicate that compensatory strategies are potentially seen at lower level of the spine with these EMG parameters.

A triaxial dynamometer to monitor lateral bending and axial rotation moments during static trunk extension efforts

OBJECTIVE The purpose of the study was to describe a new static triaxial dynamometer designed to monitor the lateral bending and axial rotation moments during trunk extension efforts. BACKGROUND Most studies on back muscle function using electromyographic spectral analysis have not controlled moments produced about the three orthopaedic axes during trunk extension efforts. Criteria to control lateral bending and axial rotation moments during extension efforts have not been proposed in the literature. METHODS Fourteen healthy subjects performed three trunk extension ramp contractions (0-100% of the maximal voluntary contraction). Triaxial L5/S1 moments at 20%, 40%, 60% and 80% of the maximal voluntary contraction in extension were extracted. RESULTS During the extension efforts, the lateral bending and axial rotation moments at L5/S1 increased significantly across the force levels and reached 6.2 Nm (SD: 6.6) and 6.1 Nm (SD: 4.5), respectively, at 80% of the maximal voluntary contraction. Tolerance limits were proposed to control these associated efforts in the context of the electromyographic analysis of back muscles. Relevance. Simultaneous measurement of lateral bending and axial rotation moments at L5/S1 during extension efforts might help to explain and control load sharing between back muscles during extension efforts.

Influence of gender on the EMG power spectrum during an increasing force level.

The aim of the present study was to contrast, between men and women, the behavior of EMG power spectrum statistics (median frequency (MF) and mean power frequency (MPF) obtained across increasing force levels. Thirteen men and sixteen women produced ramp (single contractions with the force increasing linearly) elbow flexions and extensions from 0 to 100% of the maximum voluntary contraction (MVC). Each ramp was produced in a 5-s period. Surface EMG signals from triceps brachii (TB), anconeus (AN), and biceps brachii (BB) were recorded with miniature surface electrodes placed 6 mm apart. These signals were sampled at 2,000 Hz. The MPF and MF of power spectra, obtained from single 256-ms windows, were computed for each muscle at 10, 20, 40, 60, and 80% MVC. Significant differences (two-way analyses of variance (ANOVAs); p < 0.05) in the behaviors of the MPF and MF across force levels were found between men and women. In general, the MPF and MF showed more pronounced increases across increasing force levels for men than for women. It is proposed that this gender effect could be explained by differences in skinfold thickness and fiber type characteristics between the two groups of subjects.