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Muscle Activation (muscle + activation)
Terms modified by Muscle Activation Selected AbstractsOn functional motor adaptations: from the quantification of motor strategies to the prevention of musculoskeletal disorders in the neck,shoulder regionACTA PHYSIOLOGICA, Issue 2010P. Madeleine Abstract Background:, Occupations characterized by a static low load and by repetitive actions show a high prevalence of work-related musculoskeletal disorders (WMSD) in the neck,shoulder region. Moreover, muscle fatigue and discomfort are reported to play a relevant initiating role in WMSD. Aims: To investigate relationships between altered sensory information, i.e. localized muscle fatigue, discomfort and pain and their associations to changes in motor control patterns. Materials & Methods:, In total 101 subjects participated. Questionnaires, subjective assessments of perceived exertion and pain intensity as well as surface electromyography (SEMG), mechanomyography (MMG), force and kinematics recordings were performed. Results:, Multi-channel SEMG and MMG revealed that the degree of heterogeneity of the trapezius muscle activation increased with fatigue. Further, the spatial organization of trapezius muscle activity changed in a dynamic manner during sustained contraction with acute experimental pain. A graduation of the motor changes in relation to the pain stage (acute, subchronic and chronic) and work experience were also found. The duration of the work task was shorter in presence of acute and chronic pain. Acute pain resulted in decreased activity of the painful muscle while in subchronic and chronic pain, a more static muscle activation was found. Posture and movement changed in the presence of neck,shoulder pain. Larger and smaller sizes of arm and trunk movement variability were respectively found in acute pain and subchronic/chronic pain. The size and structure of kinematics variability decreased also in the region of discomfort. Motor variability was higher in workers with high experience. Moreover, the pattern of activation of the upper trapezius muscle changed when receiving SEMG/MMG biofeedback during computer work. Discussion:, SEMG and MMG changes underlie functional mechanisms for the maintenance of force during fatiguing contraction and acute pain that may lead to the widespread pain seen in WMSD. A lack of harmonious muscle recruitment/derecruitment may play a role in pain transition. Motor behavior changed in shoulder pain conditions underlining that motor variability may play a role in the WMSD development as corroborated by the changes in kinematics variability seen with discomfort. This prognostic hypothesis was further, supported by the increased motor variability among workers with high experience. Conclusion:, Quantitative assessments of the functional motor adaptations can be a way to benchmark the pain status and help to indentify signs indicating WMSD development. Motor variability is an important characteristic in ergonomic situations. Future studies will investigate the potential benefit of inducing motor variability in occupational settings. [source] Resistance training increases in vivo quadriceps femoris muscle specific tension in young menACTA PHYSIOLOGICA, Issue 1 2010R. M. Erskine Abstract Aim:, The present study investigated whether in vivo human quadriceps femoris (QF) muscle specific tension changed following strength training by systematically determining QF maximal force and physiological cross-sectional area (PCSA). Methods:, Seventeen untrained men (20 ± 2 years) performed high-intensity leg-extension training three times a week for 9 weeks. Maximum tendon force (Ft) was calculated from maximum voluntary contraction (MVC) torque, corrected for agonist and antagonist muscle activation, and moment arm length (dPT) before and after training. QF PCSA was calculated as the sum of the four component muscle volumes, each divided by its fascicle length. Dividing Ft by the sum of the component muscle PCSAs, each multiplied by the cosine of the respective fascicle pennation angle, provided QF specific tension. Results:, MVC torque and QF activation increased by 31% (P < 0.01) and 3% (P < 0.05), respectively, but there was no change in antagonist co-activation or dPT. Subsequently, Ft increased by 27% (P < 0.01). QF volume increased by 6% but fascicle length did not change in any of the component muscles, leading to a 6% increase in QF PCSA (P < 0.05). Fascicle pennation angle increased by 5% (P < 0.01) but only in the vastus lateralis muscle. Consequently, QF specific tension increased by 20% (P < 0.01). Conclusion:, An increase in human muscle specific tension appears to be a real consequence of resistance training rather than being an artefact of measuring errors but the underlying cause of this phenomenon remains to be determined. [source] Electrical activation of the orbicularis oculi muscle does not increase the effectiveness of botulinum toxin type A in patients with blepharospasmEUROPEAN JOURNAL OF NEUROLOGY, Issue 3 2010A. Conte Background:, Our primary aim in this study was to determine whether electrically induced activation of the injected muscle increases effectiveness of botulinum type A toxin (BonT-A) in patients with blepharospasm (BPS). The second aim was to assess the safety of BonT-A by investigating whether BonT-A injection alters the excitability of blink reflex circuits in the brainstem. Methods:, Twenty-three patients with BPS received BonT-A (Botox) injected bilaterally into the orbicularis oculi muscle at a standard dose. In 18 patients, electrically induced muscle activation of the orbicularis oculi muscle on one side was performed for 60 min (4 Hz frequency) in a single session, immediately after BonT-A injection and in five patients for 60 min once a day for five consecutive days. The severity of BPS was assessed clinically with the BPS score. Compound muscle action potential (cMAPs) from the orbicularis oculi muscles were measured bilaterally. The blink reflex recovery cycle was studied at interstimulus intervals of 250 and 500 ms. Participants underwent clinical and neurophysiological assessment before BonT-A injection (T0) and 2 weeks thereafter (T1). Results:, Compound muscle action potential amplitude significantly decreased at T1 but did not differ between stimulated and non-stimulated orbicularis oculi in the two groups. BonT-A injection left the blink reflex recovery cycle tested on the stimulated and non-stimulated sides unchanged. Conclusions:, In patients with BPS, the electrically induced muscle activation neither increases the effectiveness of BonT-A nor produces larger electrophysiological peripheral effects. The lack of BonT-A-induced changes in the blink reflex recovery cycle provides evidence that BonT-A therapy is safe in patients with BPS. [source] Fibre-type composition of rabbit jaw muscles is related to their daily activityEUROPEAN JOURNAL OF NEUROSCIENCE, Issue 11 2005T. van Wessel Abstract Skeletal muscles contain a mixture of fibres with different contractile properties, such as maximum force, contraction velocity and fatigability. Muscles adapt to altered functional demands, for example, by changing their fibre-type composition. This fibre-type composition can be changed by the frequency, duration and presumably the intensity of activation. The aim of this study was to analyse the relationship between the spontaneous daily muscle activation and fibre-type composition in rabbit jaw muscles. Using radio-telemetry combined with electromyography, the daily activity of five jaw muscles was characterized in terms of the total duration of muscle activity (duty time) and the number of activity bursts. Fibre-type composition of the muscles was classified by analysing the myosin heavy chain content of the fibres. The amount of slow-type fibres was positively correlated to the duty time and the number of bursts only for activations exceeding 20,30% of the maximum activity per day. Furthermore, cross-sectional areas of the slow-type fibres were positively correlated to the duty time for activations exceeding 30% of the maximum activity. The present data indicate that the amount of activation above a threshold (> 30% peak activity) is important for determining the fibre-type composition and cross-sectional area of slow-type fibres of a muscle. Activation above this threshold occurred only around 2% of the time in the jaw muscles, suggesting that contractile properties of muscle fibres are maintained by a relatively small number of powerful contractions per day. [source] Contractile Properties, Fatigue and Recovery are not Influenced by Short-Term Creatine Supplementation in Human MuscleEXPERIMENTAL PHYSIOLOGY, Issue 4 2000J. M. Jakobi There have been several studies on the effect of short-term creatine (Cr) supplementation on exercise performance, but none have investigated both voluntary and stimulated muscle contractions in the same experiment. Fourteen moderately active young men (19-28 years) were randomly assigned, in a double blind manner, to either a creatine (Cr) or placebo (P) group. The subjects supplemented their regular diet 4 times a day for 5 days with either 5 g Cr + 5 g maltodextrin (Cr group), or 5 g maltodextrin (P group). Isometric maximal voluntary contraction (MVC), muscle activation, as assessed using the modified twitch interpolation technique, electrically stimulated contractile properties, electromyography (EMG), endurance time and recovery from fatigue were measured in the elbow flexors. The fatigue protocol involved both voluntary and stimulated contractions. Following supplementation there was a significant weight gain in the Cr group (1.0 kg), whereas the P group did not change. For each group, pre-supplementation measures were not significantly different from post-supplementation for MVC, twitch and tetanic tensions at rest, time to peak tension, half-relaxation time and contraction duration. Prior to Cr supplementation time to fatigue was 10 ± 4 min (mean ± S.E.M.) for both groups, and following supplementation there was a non-significant increase of 1 min in each group. MVC force, muscle activation, EMG, stimulated tensions and durations were similar for the Cr and P groups over the course of the fatigue protocol and did not change after supplementation. Furthermore, recovery of MVC, stimulated tensions and contractile speeds did not differ as a result of Cr supplementation. These results indicate that short-term Cr supplementation does not influence isometric elbow flexion force, muscle activation, stimulated contractile properties, or delay time to fatigue or improve recovery. [source] Multichannel surface electromyography in ergonomics: Potentialities and limitsHUMAN FACTORS AND ERGONOMICS IN MANUFACTURING & SERVICE INDUSTRIES, Issue 4 2010Marco Gazzoni Abstract The prevention of work-related musculoskeletal disorders is one of the main goals in ergonomics. Among others, surface electromyography (sEMG) is an important tool for the evaluation of risks related to work activity. Three main issues have been approached in ergonomics via sEMG: 1) the analysis of muscle activation, 2) the analysis of exerted forces and torques, and 3) the analysis of muscle fatigue. Many studies have been carried out in static conditions. In ergonomics, however, it is more relevant to study muscle activity and fatigue during real tasks that are, in general, dynamic. From isometric to dynamic contractions, the complexity of the interpretation of sEMG signals increases considerably. Changes in sEMG signals are related to the continuous modifications in force output, muscle fiber length, and relative position of surface electrodes and sources. To increase the reliability of the information extracted from sEMG, multichannel detection systems have been applied, showing the possibility of overcoming some limits of the standard technique. Some illustrative laboratory and field studies are reported in this work to illustrate the potentialities and the open problems in the use of multichannel sEMG in ergonomics. Case 1 is a laboratory study investigating the myoelectric manifestations of fatigue in the biceps brachii (BB) during dynamic elbow flexion/extension. Case 2 is a laboratory study investigating the myoelectric manifestations of fatigue during a repetitive lifting task. Case 3 is a field study, carried out in an automotive plant, investigating muscle activation during the welding of a car door. Many factors play a leading role in the correct interpretation of information provided by sEMG. Even though multichannel sEMG provides information able to improve the estimation of force and/or fatigue during working tasks, many problems related to the signal acquisition and interpretation are still open. Further improvements are necessary to develop multichannel sEMG into an effective tool supporting other methodologies for the evaluation of work-related risks. © 2010 Wiley Periodicals, Inc. [source] Influence of neck rotation and neck lateroflexion on mandibular equilibriumJOURNAL OF ORAL REHABILITATION, Issue 5 2010H. J. SCHINDLER Summary, Neuromuscular interaction between neck and jaw muscles has been reported in several studies. However, the influence of experimentally modified posture of the neck on jaw muscle activity during isometric biting was not investigated so far. The aim of the present study was to test by the aid of simultaneous electromyographic and intraoral bite force measurements whether neck rotation and lateroflexion, in contrast to a straightforward neck position, change the isometric cocontraction patterns of masticatory muscles under identical submaximum bite forces of 50,200 N. Electric muscle activity of all masticatory muscles and changes of the reduction point (RP) of the resultant bite force vectors were examined. An anteroposterior displacement of the RPs could be observed for the rotated and lateroflexed neck position in comparison with the straightforward position. On the other hand, the results revealed no significant differences between bilateral muscle activation under the different test conditions. These findings suggest a force transmission between the neck and the masticatory system, but no essential activity changes in the masticatory muscles under short time posture modification of the neck. [source] Ageing and surface EMG activity patterns of masticatory musclesJOURNAL OF ORAL REHABILITATION, Issue 4 2010F. A. CECÍLIO Summary, The purpose of this study was to evaluate the influence of age on the electromyographic activity of masticatory muscles. All volunteers were Brazilian, fully dentate (except for Group I , mixed dentition), Caucasian, aged 7,80, and divided into five groups: I (7,12 years), II (13,20 years), III (21,40 years), IV (41,60 years) and V (61,80 years). Except for Group V, which comprised nine women and eight men, all groups were equally divided with respect to gender (20 M/20 F). Surface electromyographic records of masticatory muscles were obtained at rest and during maximal voluntary contraction, right and left laterality, maximal jaw protrusion and maximal clenching in the intercuspal position. Statistically significant differences (P < 0·05) were found in all clinical conditions among the different age groups. Considerably different patterns of muscle activation were found across ages, with greater electromyographic activity in children and youth, and decreasing from adults to aged people. [source] Motor control of jaw muscles in chewing and in isometric biting with graded narrowing of jaw gapeJOURNAL OF ORAL REHABILITATION, Issue 10 2008P. A. PRÖSCHEL Summary, When a certain bite force is applied during unilateral chewing, the combination of jaw elevator muscle activities is different than when a comparable force is applied in unilateral isometric biting, e.g. on a force transducer. Masticatory peak force is generated in a nearly isometric phase of the chewing cycle, with a jaw gape of about 1 mm. In contrast, peak force in isometric biting on force measuring equipment usually induces jaw gapes of 6 mm or even more. Therefore, we tested the hypothesis that the jaw gape influences relative activation of elevator muscles in unilateral isometric biting. We further examined whether such influence could explain the different activity combinations of chewing and isometric biting. In thirty asymptomatic males, masseter and temporalis activities were recorded during intermittent isometric biting with jaw gapes of 6, 5, 3, 2 and 1 mm and during unilateral chewing. Activity combinations were described by working/balancing ratios and by temporalis/masseter ratios. With decreasing jaw gape the working/balancing ratio of the posterior temporalis decreased (P < 0·002) while that of the masseter increased (P < 0·001). Likewise, the temporalis/masseter ratio on the balancing side increased (P < 0·001). With decreasing jaw gape, activity ratios of isometric biting approached ratios of chewing. We conclude that: (i) relative jaw muscle activation in isometric biting depends on the jaw gape, (ii) relative muscle activation in chewing resembles relative activation of isometric biting with a small ,chewing-like' gape. This suggests that characteristic activity combinations in chewing are mainly a result of the approximately isometric contraction during the slow closing phase of the chewing cycle. [source] Differential activity patterns in the masseter muscle under simulated clenching and grinding forcesJOURNAL OF ORAL REHABILITATION, Issue 8 2005H. J. SCHINDLER summary, The aim of this study was to investigate (i) whether the masseter muscle shows differential activation under experimental conditions which simulate force generation during clenching and grinding activities; and (ii) whether there are (a) preferentially active muscle regions or (b) force directions which show enhanced muscle activation. To answer these questions, the electromyographic (EMG) activity of the right masseter muscle was recorded with five intramuscular electrodes placed in two deep muscle areas and in three surface regions. Intraoral force transfer and force measurement were achieved by a central bearing pin device equipped with three strain gauges (SG). The activity distribution in the muscle was recorded in four different mandibular positions (central, left, right, anterior). In each position, maximum voluntary contraction (MVC) was exerted in vertical, posterior, anterior, medial and lateral directions. The investigated muscle regions showed different amount of EMG activity. The relative intensity of the activation, with respect to other regions, changed depending on the task. In other words, the muscle regions demonstrated heterogeneous changes of the EMG pattern for the various motor tasks. The resultant force vectors demonstrated similar amounts in all horizontal bite directions. Protrusive force directions revealed the highest relative activation of the masseter muscle. The posterior deep muscle region seemed to be the most active compartment during the different motor tasks. The results indicate a heterogeneous activation of the masseter muscle under test conditions simulating force generation during clenching and grinding. Protrusively directed bite forces were accompanied by the highest activation in the muscle, with the posterior deep region as the most active area. [source] Muscle stabilization strategies in people with medial knee osteoarthritis: The effect of instabilityJOURNAL OF ORTHOPAEDIC RESEARCH, Issue 9 2008Laura C. Schmitt Abstract The sensation of knee instability (shifting, buckling. and giving way) is common in people with medial knee osteoarthritis (OA). Its influence on knee stabilization strategies is unknown. This study investigated the influence of knee instability on muscle activation during walking when knee stability was challenged. Twenty people with medial knee OA participated and were grouped as OA Stable (OAS) (n,=,10) and OA Unstable (OAU) (n,=,10) based on self-reported knee instability during daily activities. Quadriceps strength, passive knee laxity, and varus alignment were assessed and related to knee instability and muscle cocontraction during walking when the support surface translated laterally. Few differences in knee joint kinematics between the groups were seen; however, there were pronounced differences in muscle activation. The OAU group used greater medial muscle cocontraction before, during, and following the lateral translation. Self-reported knee instability predicted medial muscle cocontraction, but medial laxity and limb alignment did not. The higher muscle cocontraction used by the OAU subjects appears to be an ineffective strategy to stabilize the knee. Instability and high cocontraction can be detrimental to joint integrity, and should be the focus of future research. © 2008 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 26:1180,1185, 2008 [source] A prospective analysis of incidence and severity of quadriceps inhibition in a consecutive sample of 100 patients with complete acute anterior cruciate ligament ruptureJOURNAL OF ORTHOPAEDIC RESEARCH, Issue 5 2004Terese L. Chmielewski Background: Weakness of the quadriceps femoris muscle after anterior cruciate ligament injury and reconstruction has been attributed to incomplete voluntary activation of the muscle. The literature is conflicting on the incidence of incomplete voluntary quadriceps activation after anterior cruciate ligament injury because of differences in testing methods and population biases. The purpose of this study was to systematically examine the incidence and severity of quadriceps voluntary activation failure in both lower extremities after acute anterior cruciate ligament injury. We hypothesized that the incidence of quadriceps inhibition would be higher in the anterior cruciate ligament injured limbs than the uninvolved limbs, that the incidence of inhibition in the anterior cruciate ligament deficient limbs would be larger than in our historical sample of healthy young individuals tested in the same manner and that there would be no difference in inhibition by gender. Study design: Prospective, descriptive. Methods: One hundred consecutive patients with acute anterior cruciate ligament rupture (39 women and 61 men) were tested when range of motion was restored and effusion resolved, an average of 6 weeks after injury. A burst superimposition technique was used to assess quadriceps muscle activation and strength in all patients. Dependent t -tests were used to compare side-to-side differences in quadriceps strength. Independent t -tests were used to compare incidence of activation failure by gender and make comparisons to historical data on young, active individuals. Results: The average involved side quadriceps activation was 0.92, and ranged from 0.60 to 1.00. The incidence of incomplete activation in the involved side quadriceps was 33 per cent and uninvolved side quadriceps was 31 per cent after acute anterior cruciate ligament rupture. The incidence of incomplete activation bilaterally was 21 per cent. There was no difference in incidence of quadriceps inhibition by gender. Conclusion: The incidence of voluntary quadriceps inhibition on the involved side was three times that of uninjured, active young subjects, but the magnitude was not large. The incidence of quadriceps inhibition on the uninjured side was similar to the injured side. Clinical relevance: Both the incidence and magnitude of quadriceps inhibition after ACL rupture are lower than have previously been reported. The conventional wisdom, therefore, that quadriceps inhibition is a significant problem in this population is challenged by the results of this study. Differences between this study and others include sufficient practice to ensure a maximal effort contraction and rigorous inclusion criteria. The findings have implications for strength testing as well as rehabilitation. The quadriceps index, an assessment of the injured side quadriceps strength deficit may be affected by the presence of voluntary activation failure in the uninvolved side. © 2004 Orthopaedic Research Society. Published by Elsevier Ltd. All rights reserved. [source] Hypertonia in childhood secondary dystonia due to cerebral palsy is associated with reflex muscle activation,MOVEMENT DISORDERS, Issue 7 2009Johan van Doornik PhD Abstract It is often assumed that co-contraction of antagonist muscles is responsible for increased resistance to passive movement in hypertonic dystonia. Although co-contraction may certainly contribute to hypertonia in some patients, the role of reflex activation has never been investigated. We measured joint torque and surface electromyographic activity during passive flexion and extension movements of the elbow in 8 children with hypertonic arm dystonia due to dyskinetic cerebral palsy. In all cases, we found significant phasic electromyographic activity in the lengthening muscle, consistent with reflex activity. By correlating activation with position or velocity of the limb, we determined that some children exhibit position-dependent activation, some exhibit velocity-dependent activation, and some exhibit a mixed pattern of activation. We conclude that involuntary or reflex muscle activation in response to stretch may be a significant contributor to increased tone in hypertonic dystonia, and we conjecture that this activation may be more important than co-contraction for determining the resistance to passive movement. © 2009 Movement Disorder Society [source] The effect of cutaneous input on intracortical inhibition in focal task-specific dystoniaMOVEMENT DISORDERS, Issue 9 2007Michelle N. McDonnell PhD Abstract In normal subjects short interval intracortical inhibition (SICI) is topographically modulated by cutaneous input, which may be important for focusing muscle activation during tasks. In patients with writer's cramp, a task-specific focal dystonia characterized by inappropriate and excessive muscle activation of the upper limb during certain motor tasks, intracortical inhibition is reduced at rest and lacks the normal topographically-specific modulation during motor tasks. In the present study we investigated whether cutaneous input modulated SICI in a group of patients with writer's cramp and a control group of subjects. Electromyographic recordings were made from the right first dorsal interosseous (FDI), abductor pollicis brevis (APB), and abductor digiti minimi (ADM) muscles. Brief electrical stimuli were applied to either digit II or digit V with ring electrodes. SICI was investigated using a paired transcranial magnetic stimulation paradigm employing interstimulus intervals of 1,15 ms. Cutaneous input from both digit II and digit V modulated motor evoked potentials and SICI in a topographically-specific manner in control subjects. In contrast, cutaneous input failed to modulate motor evoked potentials or SICI in the focal hand dystonia patients. These results provide further evidence of abnormal sensorimotor integration in focal hand dystonia. © 2007 Movement Disorder Society [source] Compliance of the bladder neck supporting structures: Importance of activity pattern of levator ani muscle and content of elastic fibers of endopelvic fasciaNEUROUROLOGY AND URODYNAMICS, Issue 4 2003Matija Barbi Abstract Aims Firm bladder neck support during cough, suggested to be needed for effective abdominal pressure transmission to the urethra, might depend on activity of the levator ani muscle and elasticity of endopelvic fascia. Methods The study group of 32 patients with stress urinary incontinence and hypermobile bladder neck, but without genitourinary prolapse, were compared with the control group of 28 continent women with stable bladder neck. The height of the bladder neck (HBN) and compliance of the bladder neck support (C) were assessed, the latter by the quotient of the bladder neck mobility during cough and the change in abdominal pressure. By using wire electrodes, the integrated full-wave rectified electromyographic (EMGave) signal of the levator ani muscle was recorded simultaneously with urethral and bladder pressures. The pressure transmission ratio (PTR), time interval between the onset of muscle activation and bladder pressure increment (,T), and area under the EMGave curve during cough (EMGcough) were calculated. From bioptic samples of endopelvic fascia connecting the vaginal wall and levator ani muscle, elastic fiber content was assessed by point counting method. Mann-Whitney test was used to compare all the variables. Correlations between the parameters were evaluated by using the Spearman correlation coefficient. Results In the study group, HBN was significantly lower (P,<,0.001), C was significantly greater (P,<,0.001), and PTR was significantly lower (P,<,0.001). In the study group, the muscular activation started later (median, ,Tl, ,0.147 second; ,Tr, ,0.150 second), and in the control group, it preceded (,Tl, 0.025 second; P,<,0.001; ,Tr, 0.050 second; P,<,0.001) the bladder pressure increment. EMGcough on the left side was significantly greater in the study group (P,<,0.046). Elastic fiber content showed no difference between the groups. The analysis of all patients revealed negative correlations between C and PTR (r,=,,0.546; P,<,0.001) and between C and ,Tl (r,=,,0.316; P,<,0.018). Conclusions Firm bladder neck support enables effective pressure transmission. Timely activation of the levator ani seems to be an important feature. Neurourol. Urodynam. 22:269,276, 2003. © 2003 Wiley-Liss, Inc. [source] Mechanical and neural stretch responses of the human soleus muscle at different walking speedsTHE JOURNAL OF PHYSIOLOGY, Issue 13 2009Neil J. Cronin During human walking, a sudden trip may elicit a Ia afferent fibre mediated short latency stretch reflex. The aim of this study was to investigate soleus (SOL) muscle mechanical behaviour in response to dorsiflexion perturbations, and to relate this behaviour to short latency stretch reflex responses. Twelve healthy subjects walked on a treadmill with the left leg attached to an actuator capable of rapidly dorsiflexing the ankle joint. Ultrasound was used to measure fascicle lengths in SOL during walking, and surface electromyography (EMG) was used to record muscle activation. Dorsiflexion perturbations of 6 deg were applied during mid-stance at walking speeds of 3, 4 and 5 km h,1. At each walking speed, perturbations were delivered at three different velocities (slow: ,170 deg s,1, mid: ,230 deg s,1, fast: ,280 deg s,1). At 5 km h,1, fascicle stretch amplitude was 34,40% smaller and fascicle stretch velocity 22,28% slower than at 3 km h,1 in response to a constant amplitude perturbation, whilst stretch reflex amplitudes were unchanged. Changes in fascicle stretch parameters can be attributed to an increase in muscle stiffness at faster walking speeds. As stretch velocity is a potent stimulus to muscle spindles, a decrease in the velocity of fascicle stretch at faster walking speeds would be expected to decrease spindle afferent feedback and thus stretch reflex amplitudes, which did not occur. It is therefore postulated that other mechanisms, such as altered fusimotor drive, reduced pre-synaptic inhibition and/or increased descending excitatory input, acted to maintain motoneurone output as walking speed increased, preventing a decrease in short latency reflex amplitudes. [source] Determinants of force rise time during isometric contraction of frog muscle fibresTHE JOURNAL OF PHYSIOLOGY, Issue 3 2007K. A. P. Edman Force,velocity (F,V) relationships were determined for single frog muscle fibres during the rise of tetanic contraction. F,V curves obtained using isotonic shortening early in a tetanic contraction were different from those obtained at equivalent times with isovelocity shortening, apparently because changing activation early in the contraction leads, in isovelocity experiments, to changing force and changing series elastic extension. F,V curves obtained with isotonic and with isovelocity shortening are similar if the shortening velocity in the isovelocity trials is corrected for series elastic extension. There is a progressive shift in the scaling of force,velocity curves along the force axis during the course of the tetanic rise, reflecting increasing fibre activation. The time taken for F,V curves to reach the steady-state position was quite variable, ranging from about 50 ms after the onset of contraction (1,3°C) to well over 100 ms in different fibres. The muscle force at a fixed, moderately high shortening velocity relative to the force at this velocity during the tetanic plateau was taken as a measure of muscle activation. The reference velocity used was 60% of the maximum shortening velocity (Vmax) at the tetanic plateau. The estimated value of the fractional activation at 40 ms after the onset of contraction was used as a measure of the rate of activation. The rate of rise of isometric tension in different fibres was correlated with the rate of fibre activation and with Vmax during the plateau of the tetanus. Together differences in rate of activation and in Vmax accounted for 60,80% of the fibre-to-fibre variability in the rate of rise of isometric tension, depending on the measure of the force rise time used. There was not a significant correlation between the rate of fibre activation and Vmax. The steady-state F,V characteristics and the rate at which these characteristics are achieved early in contraction are seemingly independent. A simulation study based on F,V properties and series compliance in frog muscle fibres indicates that if muscle activation were instantaneous, the time taken for force to rise to 50% of the plateau value would be about 60% shorter than that actually measured from living fibres. Thus about 60% of the force rise time is a consequence of the time course of activation processes and about 40% represents time taken to stretch series compliance by activated contractile material. [source] Visual and non-visual control of landing movements in humansTHE JOURNAL OF PHYSIOLOGY, Issue 1 2001Marco Santello 1The role of vision in controlling leg muscle activation in landing from a drop was investigated. Subjects (n= 8) performed 10 drops from four heights (0.2, 0.4, 0.6 and 0.8 m) with and without vision. Drop height was maintained constant throughout each block of trials to allow adaptation. The aim of the study was to assess the extent to which proprioceptive and vestibular information could substitute for the lack of vision in adapting landing movements to different heights. 2At the final stages of the movement, subjects experienced similar peak centre of body mass (CM) displacements and joint rotations, regardless of the availability of vision. This implies that subjects were able to adapt the control of landing to different heights. The amplitude and timing of electromyographic signals from the leg muscles scaled to drop height in a similar fashion with and without vision. 3However, variables measured throughout the execution of the movement indicated important differences. Without vision, landings were characterised by 10 % larger ground reaction forces, 10 % smaller knee joint rotations, different time lags between peak joint rotations, and more variable ground reaction forces and times to peak CM displacement. 4We conclude that non-visual sensory information (a) could not fully compensate for the lack of continuous visual feedback and (b) this non-visual information was used to reorganise the motor output. These results suggest that vision is important for the very accurate timing of muscle activity onset and the kinematics of landing. [source] | |||||||||||||