Fatigability of the thenar muscles using electrical nerve stimulation with fixed stimuli count, while varying the frequency and duty cycle.

Our aim was to compare three electrical stimulation protocols (P20, P30 and P40), with the same number of stimuli, but different stimulation frequencies (20, 30 and 40 Hz, respectively) and duty cycles [1.2:1.2 s (continuous), 0.8:1.2 s (intermittent) and 0.6:1.2 s (intermittent), respectively). Twitch force and the peak-to-peak M-wave amplitude of the thenar muscles were measured before, during and after each protocol at 1-40 Hz in random order. Twelve healthy adults (23-41 years old) were examined for each protocol in random order and in separate sessions. P20 elicited the highest mean force, and P40 the lowest decrease in percent force at the end of the protocol. Force evoked at 1 and 10 Hz decreased less after P40, compared with P20 and P30. The M-wave amplitude was significantly reduced throughout all protocols, with the largest decrease observed during P30. Although an increase in frequency typically induced earlier and greater decrement in force, this was compensated or even reversed by increasing the interval between each stimulation train, while keeping the number of pulses per stimulation cycle constant. The lesser decrease in M-wave amplitude during P40 compared with P20 indicates that longer between-train intervals may help maintaining the integrity of neuromuscular propagation.

European consensus on the concepts and measurement of the pathophysiological neuromuscular responses to passive muscle stretch.

BACKGROUND AND PURPOSE: To support clinical decision-making in central neurological disorders, a physical examination is used to assess responses to passive muscle stretch. However, what exactly is being assessed is expressed and interpreted in different ways. A clear diagnostic framework is lacking. Therefore, the aim was to arrive at unambiguous terminology about the concepts and measurement around pathophysiological neuromuscular response to passive muscle stretch. METHODS: During two consensus meetings, 37 experts from 12 European countries filled online questionnaires based on a Delphi approach, followed by plenary discussion after rounds. Consensus was reached for agreement ≥75%. RESULTS: The term hyper-resistance should be used to describe the phenomenon of impaired neuromuscular response during passive stretch, instead of for example 'spasticity' or 'hypertonia'. From there, it is essential to distinguish non-neural (tissue-related) from neural (central nervous system related) contributions to hyper-resistance. Tissue contributions are elasticity, viscosity and muscle shortening. Neural contributions are velocity dependent stretch hyperreflexia and non-velocity dependent involuntary background activation. The term 'spasticity' should only be used next to stretch hyperreflexia, and 'stiffness' next to passive tissue contributions. When joint angle, moment and electromyography are recorded, components of hyper-resistance within the framework can be quantitatively assessed. CONCLUSIONS: A conceptual framework of pathophysiological responses to passive muscle stretch is defined. This framework can be used in clinical assessment of hyper-resistance and will improve communication between clinicians. Components within the framework are defined by objective parameters from instrumented assessment. These parameters need experimental validation in order to develop treatment algorithms based on the aetiology of the clinical phenomena.

Landing from different heights: Biomechanical and neuromuscular strategies in trained gymnasts and untrained prepubescent girls.

The purpose of this study was to examine the biomechanics of the lower limb, during landing in female prepubertal gymnasts and prepubertal untrained girls, aged 9-12years. Ten healthy participants were included in each group and performed five landings from 20, 40, and 60cm. Kinematics, ground reaction forces (GRF) and electromyogram (EMG) from the lateral gastrocnemius, tibialis anterior, and vastus lateralis are presented. Gymnasts had higher vertical GRF and shorter braking phase during landing. Compared to untrained girls, gymnasts exhibited for all examined drop heights more knee flexion before and at ground contact, but less knee flexion at maximum knee flexion position. Especially when increasing drop heights the gymnasts activated their examined muscles earlier, and generally they had higher pre- and post landing EMG amplitudes normalized to the peak EMG at 60cm drop height. Furthermore, gymnasts had lower antagonist EMG for the tibialis anterior compared to untrained girls, especially when landing from higher heights. It is concluded that the landing strategy preferred by gymnasts is influenced by long-term and specialized training and induces a stiffer landing pattern. This could have implications in injury prevention, which requires further investigation.

Tendon vibration during submaximal isometric strength and postural tasks.

To establish the effect of tendon vibration (TV) on steady submaximal strength and static balance control, 29 women performed isometric plantarflexions at 10, 20, 30 and 50% of MVC and postural tasks of increasing difficulty (Normal Quiet Stance, NQS, Sharpened Tandem Stance and One Leg Stance) with vibrators (80 Hz) applied to the Achilles tendon. Both tasks were performed under four conditions (10 s each): eyes open, eyes closed, eyes closed with TV, eyes open with TV. During the isometric tasks, the application of TV increased the plantarflexion torque at 20 and 30% of MVC, accompanied by a greater (p < 0.001) activation of agonists (Medial Gastrocnemius, MGAS and Soleus, SOL), as well as a greater (p < 0.05) coactivation of the antagonist (tibialis anterior, TA). Non-significant differences were observed at 10 and 50% of MVC. During posture, TV resulted in a backward displacement of the Centre of Pressure only for NQS and an increase (p < 0.01) in EMG activity of the ankle muscles in all balance tasks. The MGAS contraction during TV application was confirmed (by ultrasonography) at ten participants by an increase of the pennation angle and decrease of its fascicle length (p < 0.05). The vibration-induced muscle excitation and accompanying increase in motor output, probably due to excitatory Ia afferent input, was confirmed during strength and postural tasks. However, motor output attenuates when the magnitude of central drive to the ankle muscles increases.

Submaximal fatigue and recovery in boys and men.

We examined the effects of a sustained submaximal isometric contraction on fatigue and recovery rates in untrained prepubescent boys and men. Fifteen prepubescent boys and 15 men executed an isometric plantar flexion at 20% of their maximal voluntary contraction for 10 min. During the fatigue protocol, surface electromyogram of the soleus, medial gastrocnemius, and tibialis anterior muscles were obtained. Following the fatigue protocol, maximal voluntary contraction data were also obtained every 3 min throughout a 15-min recovery period. During the fatigue protocol, agonist and antagonist surface electromyogram increased gradually to a similar extent in both groups. Following fatigue, torque and surface electromyogram during a maximal voluntary contraction decreased compared to pre-fatigue values and recovered in a similar manner in both groups. However, boys showed faster recovery in torque and surface electromyogram during the third minute of recovery period. It is concluded that a low-intensity sustained isometric fatigue protocol induces similar fatigue levels in boys and men. However, there is evidence that boys can recover faster than men.

Fatigue differences between adults and prepubertal males.

The purpose of the present study was to investigate the differences in neuromuscular activation of agonist and antagonist muscles between men and prepubertal boys during a maximal isokinetic fatigue test. Ten prepubertal boys (mean +/- SD age: 10.5 +/- 0.6 years) and fourteen adults (age: 24.3 +/- 2.5 years) executed 25 consecutive maximal isokinetic knee extensions at 60 deg . s (-1). Peak torque and the electromyogram (EMG) of the vastus lateralis, vastus medialis and biceps femoris muscles were recorded. During the fatigue protocol, the prepubertal boys were able to produce higher torque than the adults, when expressed as percent of their maximal value, indicating that adults were more fatigable. The agonist activity, especially for the vastus lateralis muscle, increased in both groups during the first 10 knee extensions, and then decreased more in adults. The antagonist activity of biceps femoris muscle in adults remained constant throughout the fatigue task, whereas the children showed, on average, an increased biceps femoris antagonistic activation, especially during the first 10 and last 5 knee extensions. These results suggest that adults are more fatigable than children during a maximal isokinetic fatigue protocol, probably due to an increased inhibition or reduced facilitation of their agonist drive.

Internal rotation gait in spastic diplegia--critical considerations for the femoral derotation osteotomy.

The purpose of this study was to assess under- and overcorrection following femoral derotation osteotomy (FDO) in spastic diplegic children with functionally compromising internally rotated gait, especially with respect to asymmetry. A total of 30 children with spastic diplegia and internally rotated gait underwent multilevel surgery including FDO and were assessed pre- and 1 year postoperatively by clinical examination and instrumented gait analysis. The amount of derotation was determined intra-operatively based on the neutral midpoint between passive internal and external hip rotation and was controlled with K-wires. Sixteen patients showed an asymmetry in mean hip rotation in stance of more than 10 degrees preoperatively. Accordingly, all legs were classified as the more or the less involved side. Improvement parameters were established for the evaluation of over- and undercorrection. For the clinical midpoint, no significant difference in change and improvement was found between the sides. The mean hip rotation in stance improved significantly in the more involved side with few incidences of mal-correction (13%). The less involved side, however, got worse due to a high occurrence of over- and undercorrections (59%). Hence the clinical midpoint as indication criterion does not give sufficient information about the functional alignment of the distal femur during gait, especially in legs with mild involvement. The study underlines the importance of gait analysis for indication in addition to the clinical midpoint. Taking into account the precision of gait analysis data and derotation amount, we suggest FDO to be carried out if the mean internal hip rotation in stance exceeds 15 degrees .

Electromyographic patterns in children with cerebral palsy: do they change after surgery?

The purpose of this study was to investigate the changes in electromyographic (EMG) patterns after multilevel surgical treatment in children with spastic cerebral palsy. Children with diplegia (n=18) and hemiplegia (n=16) aging from 6 to 16 years participated in the study. Twenty healthy children within the same age span are presented as reference. Gait analysis and surface electromyograms of seven major lower limb muscles were assessed before and 1-5 years after the multilevel surgery. The most frequent procedures were equinus correction, distal rectus femoris transfer, femoral derotation osteotomy and hamstrings lengthening. The results showed that the EMG pattern of the soleus, lateral gastrocnemius and tibialis anterior muscles became closer to normal after the surgery, while no differences were detected between diplegic and hemiplegic patients. Furthermore, a subgroup of 10 patients showed an increase in medial hamstrings activation during preswing that decreased postoperatively. These findings indicate that changes in EMG patterns should not be ruled out after surgical treatment, although the extent of these changes is limited compared to changes in the kinematics. Abnormal muscle activation before the operation can be related to a compensatory response in some patients and this can be manipulated after surgery.

Changes in the reflex excitability during and after a sustained, low-intensity muscle contraction.

The purpose of the study was to evaluate the soleus H-reflex amplitude during and after a low-intensity isometric contraction. Twelve healthy, untrained subjects performed a 10-minute isometric plantar flexion at 20% of their maximum voluntary contraction torque output. The electromyogram, H-reflex, and maximum M-wave (Mmax) of the soleus muscle was recorded during and 10 minutes after the end of the contraction. The results indicated that the H-reflex increased significantly (mean +/- SEM: 44.7 +/- 16.6%, p < 0.05), but when the fatigue protocol was over, the H-reflex was depressed for the first 3 minutes, relative to the H-reflex that was recorded before fatigue, when the muscle was relaxed. The Mmax did not change significantly during the whole experiment. Furthermore, the stimulation frequency (0.1 vs. 0.3 Hz) did not have any significant effect on the H-reflex modulation. The results of the current study suggest that the reflex excitability is increased as fatigue develops, whereas this increase turns to depression for the first minutes of the recovery phase. The functional significance of these changes and the neural mechanisms which might be responsible are discussed.

Agonist and antagonist muscle activation during maximal and submaximal isokinetic fatigue tests of the knee extensors.

The purpose of this study was to examine the differences in electromyographic activity of agonist and antagonist knee musculature between a maximal and a submaximal isokinetic fatigue protocol. Fourteen healthy males (age: 24.3+/-2.5 years) performed 25 maximal (MIFP) and 60 submaximal (SIFP) isokinetic concentric efforts of the knee extensors at 60 degrees s(-1), across a 90 degrees range of motion. The two protocols were performed a week apart. The EMG activity of vastus medialis (VM), vastus lateralis (VL) and biceps femoris (BF) were recorded using surface electrodes. The peak torque (PT) and average EMG (aEMG) were expressed as percentages of pre-fatigue maximal value. One-way analysis of variance indicated a significant (p<0.05) decline of PT during the maximal (45.7%) and submaximal (46.8%) protocols. During the maximal test, the VM and VL aEMG initially increased and then decreased. In contrast, VM and VL aEMG continuously increased during submaximal testing (p<0.05). The antagonist (BF) aEMG remained constant during maximal test but it increased significantly and then declined during the submaximal testing. The above results indicate that agonist and antagonist activity depends on the intensity of the selected isokinetic fatigue test.

Electromyographic evaluation of the sound and involved side during gait of spastic hemiplegic children with cerebral palsy.

The purpose of this study was to investigate the surface electromyogram (EMG) of the lower limbs of hemiplegic children with spastic cerebral palsy during gait. The EMG of seven muscles was analyzed for 17 children (5-12 years old). The EMG of the involved side was decreased for the examined shank muscles and increased for the hamstrings, during certain phases of the gait cycle, compared with the sound side. The rectus femoris of the involved side showed prolonged activation during the swing phase. The enhanced activation of the hamstrings may be beneficial/compensatory, preventing knee hyperextension. The presence of equinus foot obstructs the foot clearance and hence the prolonged rectus femoris EMG activity during the swing phase may contribute to shorten the lower limb by flexing the hip. Possible compensatory mechanisms of the proximal muscles of the involved lower limb that can be measured with instrumented gait analysis should be taken into account before the decision for a specific treatment.

The effect of the ankle joint angle in the level of soleus Ia afferent presynaptic inhibition.

The factors that are responsible for the relationship between motoneuron excitability and muscle length may have both mechanical and/or neurophysiologic origins. The aim of the study was to investigate the changes in the level of presynaptic inhibition, as measured with a soleus H-reflex conditioning protocol, and muscle length. Ten healthy volunteers were measured at three different ankle angles: 30 degrees plantar flexion, neutral position (0 degrees) and 15 degrees dorsiflexion. At each position the soleus H-reflex and the maximum M-wave were measured while the limb was relaxed. The H-reflex was conditioned by a stimulation of the common peroneal nerve, 100 ms prior to the tibial nerve stimulation. The results revealed that the level of presynaptic inhibition was higher at the neutral position in comparison to the dorsiflexed or plantarflexed positions. Additionally, the HMAX/MMAX ratio was significantly decreased when the joint position was set at dorsiflexion. Further, there was a significant correlation, independent of ankle joint angle, between presynaptic inhibition levels and the HMAX/MMAX ratio. The above findings support the concept that peripheral feedback from passive, static modifications in the joint angle and consequently in muscle length, can modify the input/output threshold of the motoneurons on a presynaptic level.

Electromyographic changes of agonist and antagonist calf muscles during maximum isometric induced fatigue.

The purpose of this study was to examine electromyographic changes of the agonist and antagonist muscles during fatigue. Nine healthy, untrained subjects exerted a maximum voluntary heel lifting contraction with their dominant limb. The EMG activity over the soleus and the tibialis anterior muscles was recorded during the contraction. The results showed that the torque output during heel lifting and the soleus EMG activity decreased, whereas the tibialis anterior EMG revealed a small but non-significant decrease. However, the ratio of the tibialis anterior to the soleus EMG increased significantly at the end of the fatigue protocol, a fact that reveals that the decrease rate of the antagonist's activity was significantly lower than the decrease rate of the agonist activity. It is concluded that during a maximal fatigue protocol, both the agonist and antagonist muscle activity may decline, however, the slower rate of antagonist's activity decrease relative to the agonist's activity is a finding that requires further investigation. This finding may reflect a higher level of agonist and antagonist muscle co-activation and probably a relatively higher opposing torque from the antagonist muscles at the end of the fatigue session.