Reciprocal inhibition and coactivation are strategies of the central nervous system used to perform various daily tasks. In automatic postural responses (APR), coactivation is widely investigated in the ankle joint muscles, however reciprocal inhibition, although clear in manipulative motor actions, has not been investigated in the context of APRs. The aim was to identify whether reciprocal inhibition can be observed as a strategy in the recruitment of gastrocnemius Medialis (GM), Soleus (So) and Tibialis Anterior (TA) muscles in low- and high-velocity forward and backward perturbations. We applied two balance perturbations with a low and a high velocity of displacement of the movable platform in forward and backward conditions and we evaluated the magnitude and latency time of TA, GM and So activation latency, measured by electromyography (EMG). In forward perturbations, coactivation of the three muscles was observed, with greater activation amplitude of the GM and lesser amplitude of the So and TA muscles. For backward, the pattern of response observed was activation of the TA muscle, a decrease in the EMG signal, which characterizes reciprocal inhibition of the GM muscle and maintenance of the basal state of the So muscle. This result indicates that backward perturbations are more challenging.

Evaluating reciprocal inhibition of the thigh muscles is important to investigate the neural circuits of locomotor behaviors. However, measurements of reciprocal inhibition of thigh muscles using spinal reflex, such as H-reflex, have never been systematically established owing to methodological limitations. The present study aimed to clarify the existence of reciprocal inhibition in the thigh muscles using transcutaneous spinal cord stimulation (tSCS). Twenty able-bodied male individuals were enrolled. We evoked spinal reflex from the biceps femoris muscle (BF) by tSCS on the lumber posterior root. We examined whether the tSCS-evoked BF reflex was reciprocally inhibited by the following conditionings: (1) single-pulse electrical stimulation on the femoral nerve innervating the rectus femoris muscle (RF) at various inter-stimulus intervals in the resting condition; (2) voluntary contraction of the RF; and (3) vibration stimulus on the RF. The BF reflex was significantly inhibited when the conditioning electrical stimulation was delivered at 10 and 20 ms prior to tSCS, during voluntary contraction of the RF, and during vibration on the RF. These data suggested a piece of evidence of the existence of reciprocal inhibition from the RF to the BF muscle in humans and highlighted the utility of methods for evaluating reciprocal inhibition of the thigh muscles using tSCS.

Introduction Cerebral palsy (CP) is a neurodevelopmental condition that results from an injury to a developing brain. Children with CP fail to execute precise, well-coordinated movements, and excessive muscular co-contraction or co-activation is a prominent attribute of CP. The normal reciprocal relationship between agonists and antagonists during voluntary movements is altered in patients with CP. H-reflex, which is often regarded as the electrical equivalent of the spinal stretch reflex, can be used to examine the overall reflex arc, including the Ia sensory afferent strength and the spinal motoneuron excitability state. Furthermore, neuromodulatory influence of vibration on H-reflex has been found, which has been increasingly investigated to ascertain its potential use as an intervention in patients with increased spinal reflex excitability. Our goal was to identify the brain mechanism underlying the motor deficits by studying Soleus H-reflex changes during voluntary movement (dorsiflexion) and also to determine the role of vibration in H-reflex modulation in children with spastic CP. Methods Soleus H-reflex was recorded in 12 children with spastic CP (10-16 years) and 15 age-matched controls. Recordings were obtained at rest, during dorsiflexion, and during vibratory stimulation for each subject. H-responses (Hmax amplitudes and Hmax-to-Mmax ratio) were compared among the controls and the cases (CP), for the experiments performed, by the Wilcoxon signed-rank test. The recruitment curves depicting the distribution of mean H-response amplitudes with stimulus intensity increment, for dorsiflexion and vibration were compared among controls and cases by the two-sample Kolmogorov-Smirnov (KS) test. p-value <0.05 was considered as statistically significant. Results Hmax amplitudes and the Hmax-to-Mmax ratio increased (15 % and 12.2 % increment, respectively) from the resting values in the children with CP (p<0.05), while controls exhibited a decrease (reduction of 62% and 57 %, respectively) during dorsiflexion (p<0.05). Vibratory stimulation produced a decreasing trend in H-response measures in both the groups. There was about 15 % and 16 % reduction respectively among children with CP while that of 24 % and 21 % respectively among the controls. The differences in the recruitment curves (distribution of average H-response amplitudes with stimulation intensity) recorded during dorsiflexion and vibration experiments among controls compared with those with CP were found to be statistically significant by the two-sample KS test (p<0.0001). Conclusion The failure of H-reflex suppression during voluntary antagonist muscle activation suggests the presence of impaired reciprocal inhibition in spastic CP. The relatively modest H-response reduction caused by vibratory stimulation in children with CP provides limited evidence of vibratory regulation of the H-reflex in CP. More research into the mechanisms driving motor abnormalities in children with CP is needed, which could aid in therapy planning.

OBJECTIVE: Isometric training and pre-activation are proven to enhance acceleration performance. However, traditional strength training exercises do not mirror the acceleration-specific activation patterns of the gluteal muscles, characterized by ipsilateral hip extension during contralateral hip flexion. Therefore, the aim of the study was to determine gluteal muscle activity of acceleration-specific exercises compared to traditional strength training exercises.
METHODS: In a cross-sectional study design, the peak electromyographic activity of two acceleration-specific exercises was investigated and compared to two traditional strength training exercises each for the gluteus maximus and medius. Twenty-four participants from various athletic backgrounds (13 males, 11 females, 26 years, 178 cm, 77 kg) performed four gluteus maximus [half-kneeling glute squeeze (HKGS), resisted knee split (RKS), hip thrust (HT), split squat (SS)] and four gluteus medius [resisted prone hip abduction (RPHA), isometric clam (IC), side-plank with leg abduction (SP), resisted side-stepping (RSS)] exercises in a randomized order.
RESULTS: The RKS (p = 0.011, d = 0.96) and the HKGS (p = 0.064, d = 0.68) elicited higher peak gluteus maximus activity than the SS with large and moderate effects, respectively. No significant differences (p > 0.05) were found between the HT, RKS and HKGS. The RPHA elicited significantly higher gluteus medius activity with a large effect compared to RSS (p < 0.001, d = 1.41) and a moderate effect relative to the SP (p = 0.002, d = 0.78).
CONCLUSIONS: The acceleration-specific exercises effectively activate the gluteal muscles for pre-activation and strength training purposes and might help improve horizontal acceleration due to their direct coordinative transfer.

In early infancy, rats randomly alternate between the sleeping and waking states-from postnatal day 2-10 (P2-P10), sleep and wake bouts are both exponentially distributed with increasing means, while from P10-P21 sleep and wake bout means continue to increase, though there is a striking qualitative shift in the distribution of wake bouts from exponential to power law. The behavioral states of sleep and wakefulness correspond to the activity of sleep-active and wake-active neuronal brainstem populations, with reciprocal inhibition between the two ensuring that only one population is active at a time. The locus coeruleus (LC) forms a third component of this circuit that rises in prominence during the P10-P21 period, as experimental evidence shows that an as-of-yet undeciphered interaction of the LC with sleep-active and wake-active populations is responsible for the transformation of the wake bout distribution from exponential to power law. Interestingly, the LC undergoes remarkable physiological changes during the P10-P21 period-gap junctions within the LC are pruned and network-wide oscillatory synchrony declines and vanishes. In this work, we discuss a series of models of sleep-active, wake-active, and the LC populations, and we use these models to postulate the nature of the interaction between these three populations and how these interactions explain empirical observations of sleep and wake bout dynamics. We hypothesize a circuit in which there is reciprocal excitation between the LC and wake-active population with inhibition from the sleep-active population to the LC that suppresses the LC during sleep bouts. During the P2-P10 period, we argue that a noise-based switching mechanism between the sleep-active and wake-active populations provides a simple and natural way to account for exponential bout distributions, and that the locked oscillatory state of the LC prevents it from impacting bout distributions. From P10-P21, we use our models to postulate that, as the LC gradually shifts from a state of synchronized oscillations to a state of continuous firing, reciprocal excitation between the LC and the wake-active population is able to gradually transform the wake bout distribution from exponential to power law.

This study aimed to evaluate the effects of the participant\'s attention target during repetitive passive movement (RPM) intervention on reciprocal inhibition (RI) and joint movement function. Twenty healthy adults participated in two experiments involving four attention conditions [control (forward attention with no RPM), forward attention (during RPM), monitor attention (monitor counting task during RPM), ankle joint attention (ankle movement counting task during RPM)] during 10-min RPM interventions on the ankle joint. Counting tasks were included to ensure the participant\'s attention remained on the target during the intervention. In Experiment 1, RI was measured before, immediately after, and 5, 10, 15, 20, and 30 min after the RPM intervention. In Experiment 2, we evaluated ankle joint movement function at the same time points before and after RPM intervention. The maximum ankle dorsiflexion movement (from 30° plantar flexion to 10° dorsiflexion) was measured, reflecting RI. In Experiment 1, the RI function reciprocal Ia inhibition was enhanced for 10 min after RPM under all attention conditions (excluding the control condition. D1 inhibition was enhanced for 20 min after RPM in the forward and monitor attention conditions and 30 min after RPM in the ankle joint attention condition. In Experiment 2, the joint movement function decreased under the forward and monitor attention conditions but improved under the ankle joint attention condition. This study is the first to demonstrate that the participant\'s attention target affected the intervention effect of the RI enhancement method, which has implications for improving the intervention effect of rehabilitation.

Static stretching and proprioceptive neuromuscular facilitation stretching techniques can modulate specific neural mechanisms to improve the range of motion. However, the effects of modulation of these neural pathways on changes in the range of motion with static stretching remain unclear. Patterned electrical stimulation of the sensory nerve induces plastic changes in reciprocal Ia inhibition. The present study examined the effects of patterned electrical stimulation and static stretching on a range of motion and passive torque in plantarflexion muscles. The subjects were 14 young men (age 20.8 ± 1.3 years). The effects of patterned electrical stimulation (10 pulses at 100 Hz every 1.5 s) or uniform electrical stimulation (one pulse every 150 ms) to the common peroneal nerve for 20 min on reciprocal Ia inhibition of the Hoffman reflex (H-reflex) were examined. Reciprocal Ia inhibition was evaluated as short-latency suppression of the soleus H-reflex by conditioning stimulation of the common peroneal nerve. Then, the effects of transcutaneous electrical nerve stimulation (patterned electrical stimulation or uniform electrical stimulation) or prolonged resting (without electrical stimulation) and static 3-min stretching on the maximal dorsiflexion angle and passive torque were investigated. The passive ankle dorsiflexion test was performed on an isokinetic dynamometer. Stretch tolerance and stiffness of the muscle-tendon unit were evaluated by the peak and slope of passive torques, respectively. Patterned electrical stimulation significantly increased reciprocal Ia inhibition of soleus H-reflex amplitude (9.7 ± 6.1%), but uniform electrical stimulation decreased it significantly (19.5 ± 8.8%). The maximal dorsiflexion angle was significantly changed by patterned electrical stimulation (4.0 ± 1.4°), uniform electrical stimulation (3.8 ± 2.3°), and stretching without electrical stimulation (2.1 ± 3.3°). The increase in stretch tolerance was significantly greater after patterned electrical stimulation and uniform electrical stimulation than after stretching without electrical stimulation. Stiffness of the muscle-tendon unit was significantly decreased by patterned electrical stimulation, uniform electrical stimulation, and stretching without electrical stimulation. Transcutaneous electrical nerve stimulation and static stretching improve stretch tolerance regardless of the degree of reciprocal Ia inhibition.

A well-coordinated facilitation-inhibition control of motor neuronal persistent inward currents (PICs) via diffuse neuromodulation and local inhibition is essential to ensure motor units discharge at required times and frequencies. Present best estimates indicate that PICs are reduced in older adults; however, it is not yet known whether PIC facilitation-inhibition control is also altered with ageing. We investigated the responses of PICs to (i) a remote handgrip contraction, which is believed to diffusely increase serotonergic input onto motor neurones, and (ii) tendon vibration of the antagonist muscle, which elicits reciprocal inhibition, in young and older adults. High-density surface electromyograms were collected from soleus and tibialis anterior of 18 young and 26 older adults during triangular-shaped plantar and dorsiflexion contractions to 20% (handgrip experiments) and 30% (vibration experiments) of maximum torque (rise-decline rate of 2%/s). A paired-motor-unit analysis was used to calculate ∆F, which is assumed to be proportional to PIC strength. ΔF increased in both soleus (0.55 peaks per second (pps), 16.0%) and tibialis anterior (0.42 pps, 11.4%) after the handgrip contraction independent of age. Although antagonist tendon vibration reduced ΔF in soleus (0.28 pps, 12.6%) independent of age, less reduction was observed in older (0.42 pps, 10.7%) than young adults (0.72 pps, 17.8%) in tibialis anterior. Our data indicate a preserved ability of older adults to amplify PICs following a remote handgrip contraction, during which increased serotonergic input onto the motor neurones is expected, in both lower leg muscles. However, PIC deactivation in response to reciprocal inhibition was impaired with ageing in tibialis anterior despite being preserved in soleus. KEY POINTS: Motor neuronal persistent inward currents (PICs) are facilitated via diffuse neuromodulation and deactivated by local inhibition to ensure motor units discharge at required times and frequencies, allowing normal motor behaviour. PIC amplitudes appear to be reduced with ageing; however, it is not known whether PIC facilitation-inhibition control is also altered. Remote handgrip contraction, which should diffusely increase serotonergic input onto motor neurones, facilitated PICs similarly in both soleus and tibialis anterior of young and older adults. Antagonist tendon vibration, which induces reciprocal inhibition, reduced PICs in soleus in both young and older adults but had less effect in tibialis anterior in older adults. Data from lower-threshold motor units during low-force contractions suggest that PIC facilitation is preserved with ageing in soleus and tibialis anterior. However, the effect of reciprocal inhibition on the contribution of PICs to motor neurone discharge seems reduced in tibialis anterior but preserved in soleus.

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To assess the relationship between spinal reflexes and motor function in sub-acute (SAS) and chronic stroke (CS) patients.
Twelve SAS and 16 CS patients underwent electrophysiological assessment of heteronymous facilitation (HF), heteronymous inhibition (HI), disynaptic reciprocal inhibition (DRI), and D1 inhibition (D1). The Fugl-Meyer Assessment Lower Extremity (FMA-LE) and modified Ashworth scale (MAS) were assessed. The relationship between spinal reflexes and motor function was examined in a cross-sectional manner. SAS patients were also longitudinally evaluated before and after intensive rehabilitation for approximately 2 months.
SAS patients with triceps surae muscle spasticity (MAS ≥ 1) showed higher HF values (p = 0.03) than those without spasticity. SAS patients with quadriceps muscle spasticity showed higher HF values (p < 0.01); patients with hamstring muscle spasticity showed higher DRI value (disinhibition) (p < 0.01) than those without spasticity. CS patients showed no significant correlation between spinal reflexes and motor function. The longitudinal study revealed a significant correlation between increase in D1 inhibition and FMA-LE improvement in SAS patients (r = 0.69).
The association between impaired spinal reflexes varies with the stage of stroke; HF and DRI may be spasticity indicators in SAS patients.
Spinal reflexes as potential biomarkers may facilitate tailor-made rehabilitation of stroke patients.