Voluntary Contraction Level Research Articles

Olfactory Effects on Attention Allocation and Perception of Exertion

The aim of the current study was to test the effectiveness of pleasant odors on perception of exertion and attention allocation. A secondary purpose was to employ a placebo-control design and measure perceived smell intensity during task performance; methods that have been overlooked in previous olfaction studies in the sport and exercise domain. Seventy-six college students (35 females, 41 males) were recruited to perform a handgrip task. They were randomly assigned to one of 4 groups: control, placebo, lavender odor, and peppermint odor. Adhesive strips were placed under the noses of those in the latter three groups. The placebo group had a strip with no odor. The lavender and peppermint odor groups had a drop of concentration on the strip. After establishing a maximal voluntary contraction (MVC) level, participants performed at 30% of their MVC level for as long as they could tolerate, during which they provided ratings of perceived exertion (or effort; RPE), attention, and smell intensity at 30s intervals, and affect every 60s. MANOVA procedures failed to reveal significant differences among the treatment and nontreatment groups on rate of perceived exertion, attention allocation, and total time duration on the task. However, statistical differences were found between both odor groups and the placebo group on perceived attention diversion. The lavender group reported that the odor diverted attention to a higher degree than both the peppermint and placebo groups. Although nonsignificant, findings revealed a trend suggesting that odors may have an effect on cognitive processes, and on performance. There is a need for additional research to better capture these effects. Directions for further research, with an emphasis on methodological issues are outlined.

Neural compensation mechanisms to regulate motor output during physical fatigue

Central fatigue refers to a progressive decline in the ability to activate muscles voluntarily. Although the neural mechanisms of central inhibition of the motor area during physical fatigue have been widely investigated, mechanisms supporting motor output during fatigue remain to be clarified. In this study, the compensation mechanisms to regulate physical fatigue were examined using a 160-channel whole-head-type magnetoencephalographic (MEG) system. The study group consisted of nine right-handed healthy participants. After enrollment, participants performed a fatigue-inducing physical task session in which they performed repetitive grips of the right hand at maximal voluntary contraction levels every second. Before and after the session, imagery of maximum grips of the right hand was performed for evaluation with MEG. Although beta-band event-related desynchronization (ERD) level of the motor movement-evoked magnetic field in the left sensorimotor area showed a trend toward reduction after the fatigue session, the ERD level of the motor movement-evoked magnetic field in the right sensorimotor area was increased after the session. The ERD level in the prefrontal area was increased after the fatigue-inducing session. The ERD level in the left sensorimotor area was positively associated with that in the right sensorimotor area after the fatigue-inducing task session. In addition, ERD levels in the left and right sensorimotor areas had trends toward positive correlations with that in the prefrontal area. These results suggest that the ipsilateral sensorimotor and prefrontal areas are brain regions associated with compensation mechanisms to support motor output under the condition of physical fatigue.

Neuromuscular fatigue induced by alternating isometric contractions of the ankle plantar and dorsiflexors

Ankle muscle activity is important in regulating postural control as well as more complex movement tasks. Fatigue of these muscles clearly influences postural stability; however, the mechanisms responsible for this change have not been well characterized. In this study the fatigue produced in the plantar (PF) and dorsiflexors (DF) during intermittent, isometric contractions was examined and the recovery process was monitored for ten minutes post-fatigue. Fifteen healthy participants alternated between isometric PF and DF contractions until the torque was reduced to >50% of the pre-fatigue maximal voluntary contraction level in both directions. Peripheral fatigue was identified by measuring the change in the twitch torque and M-wave amplitude pre and post-fatigue. Central fatigue was determined by comparing the level of voluntary activation in the PF and DF between pre and post-fatigue. The fatigue protocol decreased the torque production in PF and DF to similar levels; however, the characteristics and recovery of the fatigue were different for the two muscle groups. This study demonstrates that although the torque produced by two antagonist muscles can be reduced to the same level, the mechanisms responsible for this change may not be similar and therefore may not impact motor tasks in the same way.

Reliability of spike and turn variables of surface EMG during isometric voluntary contractions of the biceps brachii muscle

The present study was performed to investigate the reliability of initial value and the rate of change of eight spike and turn variables in biceps brachii muscle of eleven subjects during sustained isometric voluntary contractions. Three levels of contraction were studied: 20%, 50% and 80% of the maximal voluntary contraction (MVC) level. Reliability was investigated using the intraclass correlation coefficient (ICC) and the standard error of measurement (SEM) of the estimates for each subject. The reliability of the spike and turn variables was compared with four classical variables (ARV: average rectified value, RMS: root mean square MDF: median frequency and MNF: mean frequency) for the same recorded signals. Tukey’s HSD (honestly significant difference) test was applied to detect significant differences at the 95% confidence level ( p < 0.05) between the %MVC levels for each of the eight variables. The main findings of this study can be summarized as follows: (i) the initial value estimation with eight spike and turn variables showed good reliability but the estimation of rate of change showed poor reliability. (ii) The reliability of the classical variables for estimation of the initial value was slightly lower than that of the spike and turn variables, but the four classical variables showed better reliability than the spike and turn variables for estimation of the slope. (iii) The initial values of the amplitude variables (MTA: mean turn amplitude and MSA: mean spike amplitude) increased and the rate of changes of the frequency variables (MTF: mean turn frequency, MSF: mean spike frequency and ZC: zero-crossing) decreased significantly when the contraction level increased from 20% MVC to 80% MVC.

Neuromuscular function in healthy occlusion

This study aimed to measure neuromuscular function for the masticatory muscles under a range of occlusal conditions in healthy, dentate adults. Forty-one subjects conducted maximum voluntary clenches under nine different occlusal loading conditions encompassing bilateral posterior teeth contacts with the mandible in different positions, anterior teeth contacts and unilateral posterior teeth contacts. Surface electromyography was recorded bilaterally from the anterior temporalis, superficial masseter, sternocleidomastoid, anterior digastric and trapezius muscles. Clench condition had a significant effect on muscle function (P = 0.0000) with the maximum function obtained for occlusions with bilateral posterior contacts and the mandible in a stable centric position. The remaining contact points and moving the mandible to a protruded position, whilst keeping posterior contacts, resulted in significantly lower muscle activities. Clench condition also had a significant effect on the per cent overlap, anterior-posterior and torque coefficients (P = 0.0000-0.0024), which describe the degree of symmetry in these muscle activities. Bilateral posterior contact conditions had significantly greater symmetry in muscle activities than anterior contact conditions. Activity in the sternocleidomastoid, anterior digastric and trapezius was consistently low for all clench conditions, i.e. <20% of the maximum voluntary contraction level. In conclusion, during maximum voluntary clenches in a healthy population, maximum masticatory muscle activity requires bilateral posterior contacts and the mandible to be in a stable centric position, whilst with anterior teeth contacts, both the muscle activity and the degree of symmetry in muscle activity are significantly reduced.

In vivo estimation of the short-range stiffness of cross-bridges from joint rotation

Short-range stiffness (SRS) is a mechanical property of muscles that is characterized by a disproportionally high stiffness within a short length range during both lengthening and shortening movements. SRS is attributed to the cross-bridges and is beneficial for stabilizing a joint during, e.g., postural conditions. Thus far, SRS has been estimated mainly on isolated mammalian muscles. In this study we presented a method to estimate SRS in vivo in the human wrist joint. SRS was estimated at joint level in the angular domain (N m/rad) for both flexion and extension rotations of the human wrist in nine healthy subjects. Wrist rotations of 0.15 rad at 3 rad/s were imposed at eight levels of voluntary contraction ranging from 0 to 2.1 N m by means of a single axis manipulator. Flexion and extension SRS of the wrist joint was estimated consistently and accurately using a dynamic nonlinear model that was fitted onto the recorded wrist torque. SRS increased monotonically with torque in a way consistent with previous studies on isolated muscles. It is concluded that in vivo measurement of joint SRS represents the population of coupled cross-bridges in wrist flexor and extensor muscles. In its current form, the presented technique can be used for clinical applications in many neurological and muscular diseases where altered joint torque and (dissociated) joint stiffness are important clinical parameters.

Individual-specific muscle maximum force estimation using ultrasound for ankle joint torque prediction using an EMG-driven Hill-type model

EMG-driven models can be used to estimate muscle force in biomechanical systems. Collected and processed EMG readings are used as the input of a dynamic system, which is integrated numerically. This approach requires the definition of a reasonably large set of parameters. Some of these vary widely among subjects, and slight inaccuracies in such parameters can lead to large model output errors. One of these parameters is the maximum voluntary contraction force (Fom). This paper proposes an approach to find Fom by estimating muscle physiological cross-sectional area (PCSA) using ultrasound (US), which is multiplied by a realistic value of maximum muscle specific tension. Ultrasound is used to measure muscle thickness, which allows for the determination of muscle volume through regression equations. Soleus, gastrocnemius medialis and gastrocnemius lateralis PCSAs are estimated using published volume proportions among leg muscles, which also requires measurements of muscle fiber length and pennation angle by US. Fom obtained by this approach and from data widely cited in the literature was used to comparatively test a Hill-type EMG-driven model of the ankle joint. The model uses 3 EMGs (Soleus, gastrocnemius medialis and gastrocnemius lateralis) as inputs with joint torque as the output. The EMG signals were obtained in a series of experiments carried out with 8 adult male subjects, who performed an isometric contraction protocol consisting of 10s step contractions at 20% and 60% of the maximum voluntary contraction level. Isometric torque was simultaneously collected using a dynamometer. A statistically significant reduction in the root mean square error was observed when US-obtained Fom was used, as compared to Fom from the literature.

Cervical EMG profile differences between patients of neck pain and control

Purpose. The objective of this study was to investigate EMG signals of cervical muscles in five directional efforts from chronic neck pain patients and compare them with those of the healthy controls to discern differences between patients and controls with respect to strength and EMG characteristics.Method. Seventeen male and 17 female idiopathic and non-specific chronic neck pain patients without any diagnosed pathologies or prior surgery in the age group 18–65 years were recruited into the study. The controls consisted of 30 male and 33 female subjects with no history of neck pain in the past 12 months. Both patients and controls performed the experimental activities of flexion, left anterolateral flexion, left lateral flexion, left posterolateral extension and extension. The patients exerted to their 20% maximum voluntary contraction (MVC), pain threshold and pain tolerance levels in three separate contractions. Similarly, the control subjects exerted to their 20% MVC, 60% MVC and MVC in random order. The descriptive statistics for strength, normalised peak EMG, median frequency (MF), 10% frequency bands and their power were calculated. Eight levels of wavelet decomposition and their coefficients were calculated and subjected to principal component analysis. These variables were subjected to analysis of variance and regression analysis to distinguish between patients and controls. The full wave rectified linear envelope detected EMG of patients and controls were plotted against time to reveal pattern differences.Results. There was a lack of significant difference in the MF of the two samples indicating that the muscle conduction velocity was not disturbed by the pain. Significant differences were also found in 10 percentile frequency bands between patients and controls (p < 0.05). The wavelet decomposition with principal component analysis revealed that patients and controls could be identified as such 100% of the time at 20% MVC; and, patients and controls could be identified correctly 100% and 90% of the time respectively at pain threshold/60% MVC.Conclusion. Thus, a combination of EMG spectral frequency banding and wavelet decomposition with regression can be used to distinguish chronic pain patients from controls.

Uncovering chaotic structure in mechanomyography signals of fatigue biceps brachii muscle

The mechanomyography (MMG) signal reflects mechanical properties of limb muscles that undergo complex phenomena in different functional states. We undertook the study of the chaotic nature of MMG signals by referring to recent developments in the field of nonlinear dynamics. MMG signals were measured from the biceps brachii muscle of 5 subjects during fatigue of isometric contraction at 80% maximal voluntary contraction (MVC) level. Deterministic chaotic character was detected in all data by using the Volterra–Wiener–Korenberg model and noise titration approach. The noise limit, a power indicator of the chaos of fatigue MMG signals, was 22.20±8.73. Furthermore, we studied the nonlinear dynamic features of MMG signals by computing their correlation dimension D 2, which was 3.35±0.36 across subjects. These results indicate that MMG is a high-dimensional chaotic signal and support the use of the theory of nonlinear dynamics for analysis and modeling of fatigue MMG signals.

膝伸展疲労課題中の内側広筋斜頭および長頭の機能に関する筋電図学的検討

To clarify the function of the vastus medialis obliquus (VMO) and longus (VML), we investigated the electromyographic properties of superficial quadriceps femoris during fatiguing knee extension task.Ten healthy male and 10 female volunteers participated in this study. The subjects performed fatiguing knee extension task at maximal effort (10 times×10 set at 60 deg/sec). Surface electromyography (EMG) was recorded from the VMO, VML, vastus lateralis(VL) and rectus femoris(RF) during isometric knee extension (knee bent at 60 deg and 30% of the maximal voluntary contraction level) with a 90-sec interval between each set. Knee extension peak torque, integrated EMG (IEMG) and median power frequency (MDF) of four muscles during each set were compared using Dunnett's test.Knee extension peak torque decreased gradually and peak torque decreased significantly starting from set 9 in male subjects, while there was no significant decrease in female subjects. IEMG from VMO and VL increased linearly. IEMG increased significantly from set 7 in VMO and set 6 in VL for both male and female subjects. Then, IEMG from both VML and RF increased significantly starting from set 9 in VML and set 8 in RF in male subjects, while there was no significant change in female subjects.MDF of VMO, VL and RF also increased significantly at timepoints similar to those showing increases in IEMG in male subjects. However, MDF of VML increased significantly from set 4 in male subjects. On the other hand, MDF did not change significantly in female subjects.As a result of this study, it was suggested that VMO and VL were facilitated to maintain target knee extension torque during fatigue. Therefore, it was considered that VML and RF facilitated the maintenance of target torque under fatiguing conditions.

Assessment of voluntary muscle activation using magnetic stimulation

The present study investigated the applicability of magnetic stimulation (MS) for estimating activation capacity. Ten men performed isometric knee extensions at 95% of maximum voluntary contraction level on two testing sessions. MS and electrical stimulation (ES) were applied by placing the coil and stimulating electrodes, respectively, on the quadriceps muscle group. In session 1, MS and ES were applied to allow a comparison between the two stimulating devices. During session 2, MS was applied again to assess the reproducibility of MS measurements. Activation level was estimated with the interpolated twitch technique (ITT) and the central activation ratio (CAR). Activation level determined with MS was highly repeatable, but was only comparable to ES when using ITT (standard error < 1.45%, mean difference < 0.63%, for all). MS with CAR overestimated activation by 3% compared to ES (standard error = 1.13%, mean difference = 2.76%) because the power of the magnetic field was limited. These results indicate that MS is a promising tool for reliable and valid quantification of activation levels when using the ITT approach and potentially suitable for studies involving children or other subjects where the pain of conventional ES is a problem.

Muscular capabilities and workload of flight attendants for pushing and pulling trolleys aboard aircraft

Increasingly in the recent years, passengers’ services are extended into the ascent and descent flight phases on short distance flights. Trolleys containing the required meal and beverage items are used for these service operations and pushed or pulled along the aisles of the aircraft. Flight attendants reported about increased musculo-skeletal disorders and had been complaining about high physical workload from handling trolleys. In order to ensure acceptable load levels for pushing or pulling operations of trolleys, the physical capabilities of the collective “flight attendants” had been investigated by means of force measurements at maximum voluntary contraction (MVC) level and associated relevant anthropometrical and biometrical data. CEN and ISO standards as well as international and national German methods were used to derive recommended force limits for pushing/pulling operations with respect to the physical capabilities of the target group “flight attendants”. Comparing these recommended limits with the force requirements per shift under various conditions of trolley handlings (inclination of the floor, type and weight of trolley, mode of handling, frequency of operation) showed that especially for higher floor inclinations and trolley weights, flight attendants work (substantially) above recommended limits. It became also apparent that the handling of half-size trolleys is unexpectedly high demanding due to high vertical force components, caused by the unfavorable location of their center of gravity. Relevance to industry On short-distance flights, passengers’ services are performed by means of trolleys from the ascent to the descent flight phases. Flight attendants had been complaining about high physical workload. This study offers recommendations on maximum force limits for the handling of trolleys with respect to the muscular capacities and work situations of flight attendants on short-distance flights.

Effects of the physiological parameters on the signal-to-noise ratio of single myoelectric channel

BackgroundAn important measure of the performance of a myoelectric (ME) control system for powered artificial limbs is the signal-to-noise ratio (SNR) at the output of ME channel. However, few studies illustrated the neuron-muscular interactive effects on the SNR at ME control channel output. In order to obtain a comprehensive understanding on the relationship between the physiology of individual motor unit and the ME control performance, this study investigates the effects of physiological factors on the SNR of single ME channel by an analytical and simulation approach, where the SNR is defined as the ratio of the mean squared value estimation at the channel output and the variance of the estimation.MethodsMathematical models are formulated based on three fundamental elements: a motoneuron firing mechanism, motor unit action potential (MUAP) module, and signal processor. Myoelectric signals of a motor unit are synthesized with different physiological parameters, and the corresponding SNR of single ME channel is numerically calculated. Effects of physiological multi factors on the SNR are investigated, including properties of the motoneuron, MUAP waveform, recruitment order, and firing pattern, etc.ResultsThe results of the mathematical model, supported by simulation, indicate that the SNR of a single ME channel is associated with the voluntary contraction level. We showed that a model-based approach can provide insight into the key factors and bioprocess in ME control. The results of this modelling work can be potentially used in the improvement of ME control performance and for the training of amputees with powered prostheses.ConclusionThe SNR of single ME channel is a force, neuronal and muscular property dependent parameter. The theoretical model provides possible guidance to enhance the SNR of ME channel by controlling physiological variables or conscious contraction level.

Quantifying the effects of voluntary contraction and inter-stimulus interval on the human soleus H-reflex

The human soleus H-reflex is commonly tested as an indicator of the reflex excitability of the calf muscles with infrequent stimuli to a subject seated and at rest. However, the reflex varies widely with the level of voluntary contraction and with the time history of stimulation. We studied two aspects of this variation. Antagonist (tibialis anterior) activation decreases the response, while increasing agonist (soleus) activation increases the H-reflex to a peak after which it declines. In subjects with large H-reflexes at rest, the reflex peaked at low levels of contraction. In contrast, in subjects with small H-reflexes at rest, the reflex peaked at higher levels of contraction for reasons that were elucidated using a realistic computer model. A parabolic curve fitted the maximum amplitude of the H-reflex in the model and over the entire range of contractile levels studied. The second aspect studied was post-activation depression or homosynaptic depression (HD), which has been described previously as a reduction of a second H-reflex elicited shortly after an initial reflex. We confirmed the presence of HD in resting, seated subjects for intervals up to 4 s. However, by voluntarily activating the soleus muscle, HD was drastically reduced when seated and abolished when standing. This suggests that HD may be absent in normal, functional movements and perhaps in clinical conditions that alter H-reflexes. Meaningful, quantitative measurements of reflex excitability can only be made under voluntary activity that mimics the condition of interest.

Relationship between Strength Level and Pedal Rate

The purpose of this study was to examine the relationship between strength capacity and preferred and optimal cadence in well trained cyclists. Eighteen cyclists participated in this study. Each subject completed three sessions. The initial session was to evaluate the maximal isokinetic voluntary contraction level of lower limb. The second session was an incremental test to exhaustion. During the third session subjects performed a constant cycling exercise (20 min) conducted at five randomly cadences (50, 70, 90, 110 rpm) and at the preferred cadence (FCC) at the power reached at ventilatory threshold. Cardiorespiratory and EMG values were recorded. A metabolic optimum (EOC) was observed at 63.5 +/- 7.8 rpm different from preferred cadence (FCC, 90.6 +/- 9.1 rpm). No difference was found between FCC and the neuromuscular optimal cadence (NOC, 93.5 +/- 4). Significant relationships were found between EOC, NOC and strength capacities (r = - 0.75 and - 0. 63), whereas FCC was only related with VO2max (r = 0.59). The main finding of this study was that during submaximal cycling energetically optimal cadence or neuromuscular optimum in trained cyclists was significantly related with strength capacity and whereas preferred cadence seems to be related with endurance training status of cyclists.

Muscle force generation and force control of finger movements in children with spastic hemiplegia during isometric tasks

Force control ability was investigated in 10 males and 10 females, between 5 and 15 years old with spastic hemiplegia (mild and moderate hand dysfunction), and an aged-matched control group (eight males, 12 females). An isometric force production task at five different levels of maximum voluntary contraction (MVC) was performed. Results showed that MVC generated with the affected hand (AH) was only one-third of that generated by the non-affected hand (NAH; p <0.001), time to peak was almost twofold at the highest force level (p <0.001), and the coefficient of variation was twice as high (p <0.001). Results for the NAH did not differ from those of the control children. Correlations between clinical and experimental variables were significant for the relation between Ash worth score for elbow flexors, MVC and variability at the highest force level. In conclusion, the findings for the AH suggests that strength training should be considered for agonist spastic muscles.

The Effect of Poststroke Impairments on Brachialis Muscle Architecture as Measured by Ultrasound

Li L, Tong KY, Hu X. The effect of poststroke impairments on brachialis muscle architecture as measured by ultrasound. Objective To evaluate the joint angle dependence of brachialis muscle architecture at rest and changes in brachialis muscle architecture during isometric voluntary contractions in people after stroke. Design The pennation angle and fascicle length of the brachialis muscle were measured in the affected and unaffected sides of people after stroke at 9 different elbow angles ranging from 10° to 90° at the rest condition. Measurements were also carried out at a fixed joint angle of 90° while the subjects were performing isometric muscle contractions at 5 incremental levels of maximal voluntary contraction (MVC) from 20% to 100% of MVC. The data obtained from the affected and unaffected sides of the subjects were compared. Setting A research laboratory in a rehabilitation center. Participants Seven hemiplegic adults after stroke with passive range of motion in the elbow from 10° to 90° and Modified Ashworth Scale score larger than 1 were recruited. Interventions Not applicable. Main Outcome Measures Ultrasonography was used to measure brachialis muscle pennation angle and fascicle length at the rest condition and during isometric contractions. Results The measured pennation angles and fascicle lengths were found to be joint-angle-dependent in both the affected and unaffected groups at the rest condition. Further comparisons found that the pennation angles of the affected brachialis muscle were significantly larger ( P<.05) than the unaffected muscle in the most extended positions (<50°), whereas the affected fascicle lengths were significantly shorter ( P<.05) than the unaffected muscle in most flexed positions (>20°). As the level of isometric voluntary contraction was increased incrementally from 20% to 100% of MVC, the results showed that pennation angle increased significantly ( P<.05) while fascicle length decreased significantly ( P<.01) in the unaffected muscle. However, the contraction level has a significant effect only on pennation angle ( P<.05) but not on fascicle length in the affected side. In addition, the measured fascicle lengths in the unaffected group were significantly shorter than those in the affected group for isometric contractions above 40% MVC. Conclusions Our findings suggest that the architectural parameters of the brachialis muscle in people after stroke are elbow-joint-angle-dependent at the rest condition. In the affected side, pennation angle changed the most when the muscle was in an extension position and fascicle length changed the most in a flexed position when compared with the unaffected side. Immobilization and contracture might cause a shortening of the fascicle and an increase in pennation angle in the affected side. Smaller pennation angle and fascicle length changes in the affected side during isometric contraction might be due to weakness in the muscle after the onset of stroke.

Relationship between occlusal tooth contact patterns and tightness of proximal tooth contact

The aim of this study was to investigate the relationship between occlusal tooth contact patterns and the tightness of proximal tooth contact (TPTC) during clenching. Twenty young adult volunteers with healthy dentition participated in the study. TPTC between the left second premolar (P2) and the first molar (M1) was measured during clenching at the 50% maximum voluntary contraction level in the intercuspal position (ICP). A silicone impression material was used to make an interocclusal record at the ICP in each subject, and interocclusal records were analysed using an image-processing system. Subjects were classified according to the presence or absence of each type (A, B or C) of occlusal contact. Statistical analysis was performed using the Mann-Whitney U-test. The results of this study exhibited a relationship between B-type contact and the TPTC of maxillary teeth. The experimental group with a lack of B-type contact on maxillary P2 and/or M1 showed a statistically greater TPTC than the group with B-type contact on both of these teeth (P < 0.01). These results suggest that occlusal tooth contact patterns have an influence on TPTC during clenching.

The tonic stretch reflex and spastic hypertonia after spinal cord injury

The operational definition of spasticity is focused on increased resistance of joints to passive rotation and the possible origin of this increased resistance in the induced tonic stretch reflex (TSR). This term is applied in the context of both cerebral and spinal injury, implying that a similar reflex mechanism underlies the two disorders. From recent studies it is clear that increased passive joint resistance in resting limbs following stroke is highly correlated with the induced TSR, but this evidence is lacking in spinal injury. The contribution of the TSR to hypertonia in spinal cord injury (SCI) is unclear and it is possible that hypertonia has a different origin in SCI. The contribution of resting and activated TSR activity to joint stiffness was compared in SCI and normal subjects. The magnitude of the TSR in ankle dorsiflexors (DF) and plantarflexors (PF) and mechanical ankle resistive torque were measured at rest and over a range of contraction levels in normal subjects. Similar measures were made in 13 subjects with SCI to the limits of their range of voluntary contraction. Normals and SCI received a pseudo-sinusoidal stretch perturbation of maximum amplitude +/- 20 degrees and frequency band 0.1-3.5 Hz that was comparable to that used in manual clinical testing of muscle tone. Elastic resistance and resonant frequency of the ankle joint, after normalization for limb volume, were significantly lower in complete and incomplete SCI than normal subjects. No reflex response related to stretch velocity was observed. Resting DF and PF TSR gain, when averaged over the tested band of frequencies, were significantly lower in complete SCI than in resting normal subjects (<0.5 microV/deg). Linear regression analysis found no significant relationship between TSR gain and resting joint stiffness in SCI. Mean TSR gain of DFs and PFs at rest was not correlated with the subject variables: age, time since SCI, level of injury, Frankel score, number of spasms per day, Ashworth score or anti-spastic medication. DF and PF reflex gain were linearly related to voluntary contraction level and regression analysis produced similar slopes in incomplete SCI and normal subjects. Hence TSR loop gain was not significantly increased in SCI at any equivalent contraction level. Extrapolation of the regression lines to zero contraction level predicted that reflex threshold was not reduced in SCI. Low frequency passive stretches did not induce significant TSR activity in the resting limbs of any member of this SCI group. The TSR thus did not contribute to their clinical hypertonia. Other reflex mechanisms must contribute to hypertonia as assessed clinically. This result contrasts with our similar study of cerebral spasticity after stroke, where a comparable low frequency stretch perturbation produced clear evidence of increased TSR gain that was correlated with the hypertonia at rest. We conclude that a low frequency stretch perturbation clearly distinguished between spasticity after stroke and SCI. Spasticity in the two conditions is not equivalent and care should be taken in generalizing results between them.

Force enhancement at different levels of voluntary contraction in human adductor pollicis

There is substantial evidence that the steady-state isometric force following active muscle stretch is greater than the corresponding isometric force for sub-maximal and maximal electrically induced contractions, and for maximal voluntary contractions (MVC). This so-called force enhancement (FE) has not been investigated systematically for sub-MVC. Recently, we found that FE only occurred in about half of all tested subjects at 30% of MVC in the adductor pollicis muscle, suggesting that FE for voluntary contractions might depend on the level of activation. Therefore, the purpose of this study was to determine if FE during voluntary contractions is associated with activation. We hypothesized that FE increases with increasing levels of activation, and thus increasing levels of effort. Subjects (n=12) performed voluntary contractions of the adductor pollicis using a custom-built dynamometer at 10 and 60% of maximal effort. There was consistent FE in 4 of 12 subjects at 10% and in 10 of 12 subjects at 60% of maximal voluntary effort. Furthermore, the absolute FE increased from 10 to 60% of maximal voluntary effort. The results of this study, together with previous results on the same subject group at 30% and other subjects at 100% of maximal voluntary effort, suggest that FE systematically increases with increasing levels of voluntary contraction. The mechanisms responsible for this increased FE remain unclear.