Effects of voluntary contraction on the soleus H-reflex of different amplitudes in healthy young adults and in the elderly.

A number of H-reflex studies used a moderate steady voluntary contraction in an attempt to keep the motoneuron pool excitability relatively constant. However, it is not clear whether the voluntary muscle activation itself represents a confounding factor for the elderly, as a few ongoing mechanisms of reflex modulation might be compromised. Further, it is well-known that the amount of either inhibition or facilitation from a given conditioning depends on the size of the test H-reflex. The present study aimed at evaluating the effects of voluntary contraction over a wide range of reflex amplitudes. A significant reflex facilitation during an isometric voluntary contraction of the soleus muscle (15% of the maximal voluntary isometric contraction-MVC) was found for both young adults and the elderly (p < 0.05), regardless of their test reflex amplitudes (considering the ascending limb of the H-reflex recruitment curve-RC). No significant difference was detected in the level of reflex facilitation between groups for all the amplitude parameters extracted from the RC. Simulations with a computational model of the motoneuron pool driven by stationary descending commands yielded qualitatively similar amount of reflex facilitation, as compared to human experiments. Both the experimental and modeling results suggest that possible age-related differences in spinal cord mechanisms do not significantly influence the reflex modulation during a moderate voluntary muscle activation. Therefore, a background voluntary contraction of the ankle extensors (e.g., similar to the one necessary to maintain upright stance) can be used in experiments designed to compare the RCs of both populations. Finally, in an attempt to elucidate the controversy around changes in the direct motor response (M-wave) during contraction, the maximum M-wave (Mmax) was compared between groups and conditions. It was found that the Mmax significantly increases (p < 0.05) during contraction and decreases (p < 0.05) with age arguably due to muscle fiber shortening and motoneuron loss, respectively.

Modulation of spinal cord excitability following remote limb ischemic preconditioning in healthy young men.

Remote limb ischemic preconditioning (RIPC) has shown to improve dynamic postural control in humans. However, studies on the underlying adaptations of spinal cord networks have never been performed. The present work addresses this issue by investigating parameters from the soleus H-reflex recruitment curve (RC), presynaptic mechanisms of reflex modulation (presynaptic inhibition-PSI, and post activation depression-PAD), and the excursion of the center of pressure (CP) recorded during 1 min in upright stance over a compliant surface. A sham ischemic protocol (partial obstruction of blood flow) was applied to the contralateral thigh along four consecutive days. The same procedure was repeated with full obstruction (RIPC) three days after ending the sham protocol. Data were collected before and after both sham and RIPC protocols. The follow-up data were collected five days after the last ischemic intervention. Significant reduction was detected for both the fast oscillations of the CP (higher frequency components) and the parameter estimated from the RC corresponding to the high amplitude H-reflexes (p < 0.05). Even though the magnitude of effects was similar, it was washed out within three days after sham, but persisted for at least five days after RIPC. No significant differences were found for PSI and PAD levels across conditions. These findings indicate that RIPC leads to enduring changes in spinal cord excitability for the latest reflexively recruited motoneurons, along with improvement in balance control. However, these adaptations were not mediated by the presynaptic mechanisms currently assessed.

Kinesiotaping enhances the rate of force development but not the neuromuscular efficiency of physically active young men.

INTRODUCTION: Investigations on the effects of KT on human performance have been increasing in the last few years. However, there is a paucity of studies investigating its effects on neuromuscular efficiency (NME) and rate of force development (RFD). OBJECTIVE: To evaluate the NME and RFD of the soleus and gastrocnemius muscles in physically active individuals under KT application. METHOD: Twenty young males (79.7±8.2kg; 1.78±0.05m; 24.7±4.4years) performed three conditions in a randomized order: (1) Baseline (BL, no tape); (2) Activation (ACTIKT, tape for muscle activation); and (3) Inhibition (INHIKT, tape for muscle inhibition). The tape was applied along the lateral and medial border of gastrocnemius with 30% tension for 48h. Peak torque (PT), RFD and NME were measured at BL and 48h after ACTIKT and INHIKT by performing a maximum isometric contraction. RESULTS: The RFD was significantly higher in ACTIKT compared to BL at 0-30 (P=0.010), 0-50 (P=0.008) and 0-100ms (P=0.007). The PT and NME did not differ among conditions (P>0.05). CONCLUSION: KT applied for muscle activation yielded a higher RFD during the initial phase of the muscle contraction. However, KT has no enhancement effect on NME and peak torque.

Enhanced D1 and D2 inhibitions induced by low-frequency trains of conditioning stimuli: differential effects on H- and T-reflexes and possible mechanisms.

Mechanically evoked reflexes have been postulated to be less sensitive to presynaptic inhibition (PSI) than the H-reflex. This has implications on investigations of spinal cord neurophysiology that are based on the T-reflex. Preceding studies have shown an enhanced effect of PSI on the H-reflex when a train of ~10 conditioning stimuli at 1 Hz was applied to the nerve of the antagonist muscle. The main questions to be addressed in the present study are if indeed T-reflexes are less sensitive to PSI and whether (and to what extent and by what possible mechanisms) the effect of low frequency conditioning, found previously for the H-reflex, can be reproduced on T-reflexes from the soleus muscle. We explored two different conditioning-to-test (C-T) intervals: 15 and 100 ms (corresponding to D1 and D2 inhibitions, respectively). Test stimuli consisted of either electrical pulses applied to the posterior tibial nerve to elicit H-reflexes or mechanical percussion to the Achilles tendon to elicit T-reflexes. The 1 Hz train of conditioning electrical stimuli delivered to the common peroneal nerve induced a stronger effect of PSI as compared to a single conditioning pulse, for both reflexes (T and H), regardless of C-T-intervals. Moreover, the conditioning train of pulses (with respect to a single conditioning pulse) was proportionally more effective for T-reflexes as compared to H-reflexes (irrespective of the C-T interval), which might be associated with the differential contingent of Ia afferents activated by mechanical and electrical test stimuli. A conceivable explanation for the enhanced PSI effect in response to a train of stimuli is the occurrence of homosynaptic depression at synapses on inhibitory interneurons interposed within the PSI pathway. The present results add to the discussion of the sensitivity of the stretch reflex pathway to PSI and its functional role.

Absence of effects of contralateral group I muscle afferents on presynaptic inhibition of Ia terminals in humans and cats.

Crossed effects from group I afferents on reflex excitability and their mechanisms of action are not yet well understood. The current view is that the influence is weak and takes place indirectly via oligosynaptic pathways. We examined possible contralateral effects from group I afferents on presynaptic inhibition of Ia terminals in humans and cats. In resting and seated human subjects the soleus (SO) H-reflex was conditioned by an electrical stimulus to the ipsilateral common peroneal nerve (CPN) to assess the level of presynaptic inhibition (PSI_control). A brief conditioning vibratory stimulus was applied to the triceps surae tendon at the contralateral side (to activate preferentially Ia muscle afferents). The amplitude of the resulting H-reflex response (PSI_conditioned) was compared to the H-reflex under PSI_control, i.e., without the vibration. The interstimulus interval between the brief vibratory stimulus and the electrical shock to the CPN was -60 to 60 ms. The H-reflex conditioned by both stimuli did not differ from that conditioned exclusively by the ipsilateral CPN stimulation. In anesthetized cats, bilateral monosynaptic reflexes (MSRs) in the left and right L(7) ventral roots were recorded simultaneously. Conditioning stimulation applied to the contralateral group I posterior biceps and semitendinosus (PBSt) afferents at different time intervals (0-120 ms) did not have an effect on the ipsilateral gastrocnemius/soleus (GS) MSR. An additional experimental paradigm in the cat using contralateral tendon vibration, similar to that conducted in humans, was also performed. No significant differences between GS-MSRs conditioned by ipsilateral PBSt stimulus alone and those conditioned by both ipsilateral PBSt stimulus and contralateral tendon vibration were detected. The present results strongly suggest an absence of effects from contralateral group I fibers on the presynaptic mechanism of MSR modulation in relaxed humans and anesthetized cats.

Electromyography and kinematic changes of gait cycle at different cadences in diabetic neuropathic individuals.

INTRODUCTION: Changes in gait cadence caused by challenging situations in daily life might induce higher demand for strength and propulsion in diabetic neuropathic (DN) subjects. METHODS: Forty-six subjects (healthy and DN) walked at two cadences (self-selected and 25% higher). Kinematic and electromyographic data were obtained from lower limbs and compared across the gait cycle. RESULTS: DN subjects showed a delayed peak in plantarflexor activity along the whole cycle (irrespective of cadence) compared with healthy subjects. However, during the imposed cadence, DN individuals showed reduced ankle range of motion along the entire cycle compared with the self-selected condition and healthy individuals walking at both cadences (P = 0.002). CONCLUSIONS: These findings suggest that when diabetic individuals face a new challenging situation that induces a higher demand for muscle strength and propulsion, the necessary range of motion and neuromuscular control around distal joints are insufficient.

A method to estimate EMG crosstalk between two muscles based on the silent period following an H-reflex.

The crosstalk phenomenon consists in recording the volume-conducted electromyographic activity of muscles other than that under study. This interference may impair the correct interpretation of the results in a variety of experiments. A new protocol is presented here for crosstalk assessment between two muscles based on changes in their electrical activity following a reflex discharge in one of the muscles in response to nerve stimulation. A reflex compound muscle action potential (H-reflex) was used to induce a silent period in the muscle that causes the crosstalk, called here the remote muscle. The rationale is that if the activity recorded in the target muscle is influenced by a distant source (the remote muscle) a silent period observed in the electromyogram (EMG) of the remote muscle would coincide with a decrease in the EMG activity of the target muscle. The new crosstalk index is evaluated based on the root mean square (RMS) values of the EMGs obtained in two distinct periods (background EMG and silent period) of both the remote and the target muscles. In the present work the application focused on the estimation of the degree of crosstalk from the soleus muscle to the tibialis anterior muscle during quiet stance. However, the technique may be extended to other pairs of muscles provided a silent period may be evoked in one of them.

Postural control during kneeling.

Postural control was studied when the subject was kneeling with erect trunk in a quiet posture and compared to that obtained during quiet standing. The analysis was based on the center of pressure motion in the sagittal plane (CPx), both in the time and in the frequency domains. One could assume that postural control during kneeling would be poorer than in standing because it is a less natural posture. This could cause a higher CPx variability. The power spectral density (PSD) of the CPx obtained from the experimental data in the kneeling position (KN) showed a significant decrease at frequencies below 0.3 Hz compared to upright (UP) (P < 0.01), which indicates less sway in KN. Conversely, there was an increase in fast postural oscillations (above 0.7 Hz) during KN compared to UP (P < 0.05). The root mean square (RMS) of the CPx was higher for UP (P < 0.01) while the mean velocity (MV) was higher during KN (P < 0.05). Lack of vision had a significant effect on the PSD and the parameters estimated from the CPx in both positions. We also sought to verify whether the changes in the PSD of the CPx found between the UP and KN positions were exclusively due to biomechanical factors (e.g., lowered center of gravity), or also reflected changes in the neural processes involved in the control of balance. To reach this goal, besides the experimental approach, a simple feedback model (a PID neural system, with added neural noise and controlling an inverted pendulum) was used to simulate postural sway in both conditions (in KN the pendulum was shortened, the mass and the moment of inertia were decreased). A parameter optimization method was used to fit the CPx power spectrum given by the model to that obtained experimentally. The results indicated that the changed anthropometric parameters in KN would indeed cause a large decrease in the power spectrum at low frequencies. However, the model fitting also showed that there were considerable changes also in the neural subsystem when the subject went from standing to kneeling. There was a lowering of the proportional and derivative gains and an increase in the neural noise power. Additional increases in the neural noise power were found also when the subject closed his eyes.

Control of upright stance over inclined surfaces.

The present work investigated the control of upright posture on inclined surfaces (14 degrees). Such conditions could, for example, change the contributions of muscle spindles resulting in alterations in postural sway. Subjects stood in quiet stance over a force platform positioned in one of three different fixed positions: horizontal (H), toes-up (ankle dorsi-flexion, D) and toes-down (ankle plantar-flexion, P). The experiments were done in the presence and also in the absence of vision. The analysis of the resulting sway was based on the power spectrum of the center of pressure displacement in the anterior-posterior direction (CP_ap). Analysis of the electromyogram (EMG) of the leg muscles and evaluation of the level of presynaptic inhibition (PSI) of the soleus (SO) Ia afferents complemented the study. The results showed that the spectrum of the CP_ap changed with the inclination of the surface of support. In condition D a higher instability was found as reflected by the higher spectral amplitudes at lower frequencies (below 0.3 Hz). On the other hand, the CP_ap of subjects in condition P contained increased amplitudes at high frequencies (above 0.3 Hz) and smaller amplitudes at low frequencies. The modifications found in the CP_ap power spectra when standing over an inclined surface may indicate changes in both short-term and long-term systems of postural control. These results do not seem to be associated with changes in group Ia feedback gain since no changes in the level of PSI were found among the three standing conditions. The SO EMG increased in condition P but did not change in condition D. On the other hand, the tibialis anterior had a tendency towards increased bursting activity in condition D. Eye closure caused an increase in the power of the oscillations at all spectral frequencies in the three standing conditions (H, P or D) and also a change in the profile of the CP_ap power spectrum. This may suggest a nonlinearity in the postural control system. The control of the slow component of the postural sway was more dependent on vision when the subject was in condition D, probably in association with the biomechanical constraints of standing on a toes-up ramp. A conclusion of this work was that, depending on the postural demand (direction of the ramp of support), the ensuing proprioceptive and biomechanical changes affect differentially the fast and slow mechanisms of balance control.