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Voluntary Activation (voluntary + activation)
Selected AbstractsDifferential age-related changes in motor unit properties between elbow flexors and extensorsACTA PHYSIOLOGICA, Issue 1 2010B. H. Dalton Abstract Aim:, Healthy adult ageing of the human neuromuscular system is comprised of changes that include atrophy, weakness and slowed movements with reduced spinal motor neurone output expressed by lower motor unit discharge rates (MUDRs). The latter observation has been obtained mostly from hand and lower limb muscles. The purpose was to determine the extent to which elbow flexor and extensor contractile properties, and MUDRs in six old (83 ± 4 years) and six young (24 ± 1 years) men were affected by age, and whether any adaptations were similar for both muscle groups. Methods:, Maximal isometric voluntary contraction (MVC), voluntary activation, twitch contractile properties, force,frequency relationship and MUDRs from sub-maximal to maximal intensities were assessed in the elbow flexors and extensors. Results:, Both flexor and extensor MVCs were significantly (P < 0.05) less (,42% and ,46% respectively) in the old than in the young. Contractile speeds and the force,frequency relationship did not show any age-related differences (P > 0.05). For the elbow flexors contraction duration was ,139 ms and for the extensors it was ,127 ms for both age groups (P > 0.05). The mean MUDRs from 25% MVC to maximum were lower (,10% to ,36%) in the old than in the young (P < 0.01). These age-related differences were larger for biceps (Cohen's d = 8.25) than triceps (Cohen's d = 4.79) brachii. Conclusion:, Thus, at least for proximal upper limb muscles, mean maximal MUDR reductions with healthy adult ageing are muscle specific and not strongly related to contractile speed. [source] Exercise Heat Stress does not Reduce Central Activation to non-exercised Human Skeletal MuscleEXPERIMENTAL PHYSIOLOGY, Issue 6 2003Julian Saboisky In this study we measured the central activation ratio (CAR) of the leg extensors and the elbow flexor muscles before and after exhaustive exercise in the heat to determine whether exercise-induced hyperthermia affects the CNS drive to exercised (leg extensors) and/or non-exercised (forearm flexors) muscle groups. Thirteen subjects exercised at fixed intensities representative of a percentage of peak power output (PPO) for 10 min periods (50%, 40%, 60%, 50%) and then at 75% PPO until exhaustion in ambient conditions of 39.3 ± 0.8 °C and 60.0 ± 0.8% relative humidity. Before and immediately following exercise subjects performed a series of maximal voluntary contractions (MVCs) with the leg extensors (exercised muscles) and forearm flexors (non-exercised muscles). The degree of voluntary activation during the sustained MVCs was assessed by superimposing electrical stimulation to the femoral nerve and the biceps brachii. Exercise to exhaustion increased the rectal temperature from 37.2 ± 0.2 to 38.8 ± 0.2 °C (P < 0.0001). The mean heart rate at the end of exercise to exhaustion was 192 ± 3 beats min,1. Leg extensor voluntary force was significantly reduced from 595 ± 143 to 509 ± 105 N following exercise-induced hyperthermia but forearm flexor force was similar before and after exercise. The CAR of the leg extensors decreased from 94.2 ± 1.3% before exercise to 91.7 ± 1.5% (P < 0.02) following exercise-induced hyperthermia. However, the CAR for the forearm flexors remained at similar levels before and after exercise. The data suggest that the central nervous system selectively reduces central activation to specific skeletal muscles as a consequence of exercise-induced hyperthermia. [source] Ankle eversion torque response to sudden ankle inversion Torque response in unbraced, braced, and pre-activated situationsJOURNAL OF ORTHOPAEDIC RESEARCH, Issue 2 2005Lars Konradsen Abstract In 13 young ankle stable subjects, ankle eversion torque and peroneal EMG were simultaneously recorded in response to sudden ankle inversion. The eversion torque response was bi-phasic. The initial development of torque, which was responsible for 30% of the maximal eversion torque response, was observed 135ms after the start of platform rotation and correlated well with the onset of the automatic postural peroneal EMG response. The remaining eversion torque response commenced after 305 ms, strongly correlating with the onset of the peroneal long latency voluntary EMG activity. With the ankle unbraced, 66% of the maximal torque level was reached in 326ms. While braced, the same torque magnitude was reached using 230ms (p < 0.02), and pre-activation of the peroneal muscles allowed the subjects to reach the same level of torque in 89ms (p < 0.0005). Prior to the study, a common reaction pattern to sudden inversion was expected in an ankle stable population, but review of the eversion torque and EMG data from the 13 subjects revealed three different voluntary reaction patterns: 10 subjects showed an efficient activation of evertor muscles; two subjects stiffened their ankles with activation of both in- and evertor muscles; and one subject showed a marginal voluntary activation of the ankle evertors. The results of the study indicate that the reaction to sudden ankle inversion is not solely automatic. The main part of the torque response is voluntarily mediated and inter-individual differences in strategy seem to exist in healthy subjects. © 2004 Orthopaedic Research Society. Published by Elsevier Ltd. All rights reserved. [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] Effect of transcranial magnetic stimulation on voluntary activation in patients with quadriceps weaknessMUSCLE AND NERVE, Issue 2 2005Dietmar Urbach MD Abstract Joint disease causes weakness and wasting of adjacent muscles, in part because of inability to fully activate these muscles voluntarily. Previous findings suggest that transcranial magnetic stimulation (TMS) paired with muscle contractions enhances maximal voluntary contraction force (MVC) in healthy subjects by improving voluntary activation (VA). The aim of the present study was to evaluate whether such an effect is also present in subjects suffering from diminished muscle force due to decreased VA. Three single TMS over resting motor threshold were applied in 10 patients with a mean age of 62 years after total-knee arthroplasty either during MVC or during muscle relaxation (control experiment) in a blinded randomized crossover study. MVC and VA were determined using a twitch-interpolation technique at 1, 15, 30, and 60 min after stimulation. There was a significant effect of TMS on MVC if applied in synchrony with muscle contraction, and this persisted for at least 60 min beyond stimulation. In patients suffering from joint disease, TMS might make physiotherapy more effective. Muscle Nerve, 2005 [source] Protocol for clinical neurophysiologic examination of the pelvic floorNEUROUROLOGY AND URODYNAMICS, Issue 6 2001Simon Podnar Abstract Clinical neurophysiologic examination of the pelvic floor is performed worldwide, but there is no consensus on the choice of tests, nor on technical details of individual methods. Standardized methods are, however, necessary to obtain their valid application in different laboratories for the purpose of collection of normative data, comparison of patient data and organization of multi-center studies. It is proposed that in patients with suspected "lower motor neuron" type lesions concentric needle electromyography (CNEMG) is the most informative test to detect pelvic floor denervation/reinnervation, and the external anal sphincter (EAS) muscle is the most appropriate muscle to be examined (either in isolation,when a selective lesion is suspected,or in addition to examination of other muscles). An algorithm consisting of standardized tests including a standardized approach to CNEMG examination of the EAS is presented. The proposed electrophysiologic assessment consists of a computer-assisted analysis of denervation and reinnervation features of the CNEMG signal, a qualitative assessment of reflex and voluntary activation of EAS motor units, and of electrical (or mechanical) elicitation of the bulbocavernosus reflex in those patients in whom manual anogenital stimulation failed to elicit a robust response in the EAS. The proposed protocol could serve as a basis for further studies on validity, sensitivity and specificity of electrophysiologic assessment in patients with different types of "lower motor neuron" involvement of pelvic floor muscles and sacral dysfunction. Neurourol. Urodynam. 20:669,682, 2001. © 2001 Wiley-Liss, Inc. [source] Neural control of shortening and lengthening contractions: influence of task constraintsTHE JOURNAL OF PHYSIOLOGY, Issue 24 2008Jacques Duchateau Although the performance capabilities of muscle differ during shortening and lengthening contractions, realization of these differences during functional tasks depends on the characteristics of the activation signal discharged from the spinal cord. Fundamentally, the control strategy must differ during the two anisometric contractions due to the lesser force that each motor unit exerts during a shortening contraction and the greater difficulty associated with decreasing force to match a prescribed trajectory during a lengthening contraction. The activation characteristics of motor units during submaximal contractions depend on the details of the task being performed. Indexes of the strategy encoded in the descending command, such as coactivation of antagonist muscles and motor unit synchronization, indicate differences in cortical output for the two types of anisometric contractions. Furthermore, the augmented feedback from peripheral sensory receptors during lengthening contractions appears to be suppressed by centrally and peripherally mediated presynaptic inhibition of Ia afferents, which may also explain the depression of voluntary activation that occurs during maximal lengthening contractions. Although modulation of the activation during shortening and lengthening contractions involves both supraspinal and spinal mechanisms, the association with differences in performance cannot be determined without more careful attention to the details of the task. [source] Comparative effects of resistance training on peak isometric torque, muscle hypertrophy, voluntary activation and surface EMG between young and elderly womenCLINICAL PHYSIOLOGY AND FUNCTIONAL IMAGING, Issue 2 2007Jack Cannon Summary We compared the effect of a 10-week resistance training program on peak isometric torque, muscle hypertrophy, voluntary activation and electromyogram signal amplitude (EMG) of the knee extensors between young and elderly women. Nine young women (YW; range 20,30 years) and eight elderly women (EW; 64,78 years) performed three sets of ten repetitions at 75% 1 repetition maximum for the bilateral leg extension and bilateral leg curl 3 days per week for 10 weeks. Peak isometric torque, EMG and voluntary activation were assessed before, during, and after the training period, while knee extensor lean muscle cross-sectional area (LCSA) and lean muscle volume (LMV) were assessed before and after the training period only. Similar increases in peak isometric torque (16% and 18%), LCSA (13% and 12%), LMV (10% and 9%) and EMG (19% and 21%) were observed between YW and EW, respectively, at the completion of training (P<0·05), while the increase in voluntary activation in YW (1·9%) and EW (2·1%) was not significant (P>0·05). These findings provide evidence to indicate that participation in regular resistance exercise can have significant neuromuscular benefits in women independent of age. The lack of change in voluntary activation following resistance training in both age groups despite the increase in EMG may be related to differences between measurements in their ability to detect resistance training-induced changes in motor unit activity. However, it is possible that neural adaptation did not occur and that the increase in EMG was due to peripheral adaptations. [source] | |||||||||||||