Leg Muscle Responses Research Articles

Responses to brief perturbations of stance: EMG, midline cortical, and subcortical changes.

We studied simultaneous EMG and midline EEG responses, including over the cerebellum, in 10 standing subjects (35 ± 15 yr; 5 females, 5 males). Recordings were made following repeated taps to the sternum, stimuli known to evoke short-latency EMG responses in leg muscles, consistent with postural reflexes. EEG power had relatively more high-frequency components (>30 Hz) when recorded from electrodes over the cerebellum (Iz and SIz) compared with other midline electrodes. We confirmed a previous report using a similar stimulus that evoked short-latency potentials over the cerebellum. We showed clear midline-evoked EEG potentials occurring at short latency over the cerebellum (P23, N31, N42, and P54) and frontally (N28 and N57) before the previously described perturbation-evoked potential (P1/N1/P2). The P23 response correlated with the subsequent EMG response in the tibialis anterior muscles (r = 0.72, P = 0.018), confirming and extending previous observations. We did not find a correlation with the N1 amplitude. We conclude that early activity occurs from electrodes over the inion in response to a brief tap to the sternum. This is likely to represent cerebellar activity and it appears to modulate short-latency postural EMG responses.NEW & NOTEWORTHY We studied the effects of a brief tap to the sternum in human subjects, known to evoke short-latency postural responses. Using an extended EEG recording system, we showed early evoked responses over the midline cerebellum, including the P23 potential, which correlated with the EMG responses in tibialis anterior, consistent with a cerebellar role in postural reflexes. The stimulus also evoked later EEG responses, including the perturbation potential.

Enhanced selectivity of transcutaneous spinal cord stimulation by multielectrode configuration.

Transcutaneous spinal cord stimulation (tSCS) has been gaining momentum as a non-invasive rehabilitation approach to restore movement to paralyzed muscles after spinal cord injury (SCI). However, its low selectivity limits the types of movements that can be enabled and, thus, its potential applications in rehabilitation. In this cross-over study design, we investigated whether muscle recruitment selectivity of individual muscles could be enhanced by multielectrode configurations of tSCS in 16 neurologically intact individuals. We hypothesized that due to the segmental innervation of lower limb muscles, we could identify muscle-specific optimal stimulation locations that would enable improved recruitment selectivity over conventional tSCS. We elicited leg muscle responses by delivering biphasic pulses of electrical stimulation to the lumbosacral enlargement using conventional and multielectrode tSCS. Analysis of recruitment curve responses confirmed that multielectrode configurations could improve the rostrocaudal and lateral selectivity of tSCS. To investigate whether motor responses elicited by spatially selective tSCS were mediated by posterior root-muscle reflexes, each stimulation event was a paired pulse with a conditioning-test interval of 33.3 ms. Muscle responses to the second stimulation pulse were significantly suppressed, a characteristic of post-activation depression suggesting that spatially selective tSCS recruits proprioceptive fibers that reflexively activate muscle specific-motor neurons in the spinal cord. Moreover, the combination of leg muscle recruitment probability and segmental innervation maps revealed a stereotypical spinal activation map in congruence with each electrode's position. Improvements in muscle recruitment selectivity could be essential for the effective translation into stimulation protocols that selectively enhance single-joint movements in neurorehabilitation.&#xD.

Responses of stance leg muscles induced by support surface translation during gait

This study determined the presence of the muscle responses to the support surface translation in the stance leg during gait and examined the effect of the direction and time point of the translation and that of the cognitive process on the responses. The rectus femoris (RF), biceps femoris (BF), soleus (SOL), and tibialis anterior (TA) muscles in the stance leg were tested. There was no significant effect of cognitive process on the electromyographic (EMG) activity induced by the translation of the support surface. In all muscles except the SOL, the EMG amplitude increased 0–300 ​ms after the support surface translation at the initial stance (IS) or middle stance (MS) of the tested leg. This means that the EMG activity in the leg muscles other than the SOL occurs after the support surface translation at the IS or MS no matter the direction of the translation. The EMG amplitude was not changed after the translation at the late stance, indicating that the translation does not influence the EMG amplitude at the double limb support phase with the tested leg behind the other. In the SOL, the EMG amplitude increased after the backward translation at the IS and after the forward translation at the MS, but decreased after the forward translation at the IS, indicating that the support surface translation-induced change in the EMG amplitude of the SOL is dependent on its direction. The change in the EMG amplitude of the TA and RF induced by the forward translation was greatest when the translation was given at the IS. In the SOL, the decrease in the EMG amplitude after the forward translation and the increase in the amplitude after the backward translation were greatest at the IS. Taken together, the change in the EMG amplitude induced by the support surface translation is greatest when the translation is given at the IS. The increase in the EMG amplitude in the TA and RF after the forward translation was greater than that after the backward translation at the IS, indicating that the EMG activity of the frontal leg muscles after the forward translation is greater when the translation is given at the IS.

Effect of Parkinson's Disease on Cardio-postural Coupling During Orthostatic Challenge.

Cardiac baroreflex and leg muscles activation are two important mechanisms for blood pressure regulation, failure of which could result in syncope and falls. Parkinson’s disease is known to be associated with cardiac baroreflex impairment and skeletal muscle dysfunction contributing to falls. However, the mechanical effect of leg muscles contractions on blood pressure (muscle-pump) and the baroreflex-like responses of leg muscles to blood pressure changes is yet to be comprehensively investigated. In this study, we examined the involvement of the cardiac baroreflex and this hypothesized reflex muscle-pump function (cardio-postural coupling) to maintain blood pressure in Parkinson’s patients and healthy controls during an orthostatic challenge induced via a head-up tilt test. We also studied the mechanical effect of the heart and leg muscles contractions on blood pressure. We recorded electrocardiogram, blood pressure and electromyogram from 21 patients with Parkinson’s disease and 18 age-matched healthy controls during supine, head-up tilt at 70°, and standing positions with eyes open. The interaction and bidirectional causalities between the cardiovascular and musculoskeletal signals were studied using wavelet transform coherence and convergent cross mapping techniques, respectively. Parkinson’s patients displayed an impaired cardiac baroreflex and a reduced mechanical effect of the heart on blood pressure during supine, tilt and standing positions. However, the effectiveness of the cardiac baroreflex decreased in both Parkinson’s patients and healthy controls during standing as compared to supine. In addition, Parkinson’s patients demonstrated cardio-postural coupling impairment along with a mechanical muscle pump dysfunction which both could lead to dizziness and falls. Moreover, the cardiac baroreflex had a limited effect on blood pressure during standing while lower limb muscles continued to contract and maintain blood pressure via the muscle-pump mechanism. The study findings highlighted altered bidirectional coupling between heart rate and blood pressure, as well as between muscle activity and blood pressure in Parkinson’s disease. The outcomes of this study could assist in the development of appropriate physical exercise programs to reduce falls in Parkinson’s disease by monitoring the cardiac baroreflex and cardio-postural coupling effect on maintaining blood pressure.

Collic evoked potentials, myogenic potentials (CEMPs) and postural responses produced by brief 100 Hz vibration of the sternocleidomastoid muscle

We present an initial report using 5 subjects, of short and long latency collic evoked responses following a half cycle of 100 Hz vibration (5 ms) applied to the sternocleidomastoid (SCM) tendon. These were detected in EEG and extraocular and leg muscles and compared with vestibular-dependent responses from direct mastoid stimulation. The responses from the extraocular recording site are likely to be evoked myogenic potentials, thus “collic evoked myogenic potentials” (CEMPs). An n19/p24 presumed ocular CEMP (oCEMP) was followed by a P22/N28 response over the posterior fossa, referred to as a collic cerebellar evoked potential (CoCEP), with responses in leg muscles starting around 55 ms. In contrast to their vestibular analogues, the oCEMP and CoCEP were predominantly ipsilateral to the side of stimulation, consistent with a double-crossed projection. In addition, their thresholds were just above the threshold of vibrotactile sensation, implying a low threshold, oligo-synaptic projection of SCM afferents to both extraocular and cerebellar targets. Following these short latency responses, SCM tendon stimulation evoked prolonged EMG responses in postural muscles of the legs, consistent with a role in the afferent limb of a short latency, spino-bulbar-spinal postural response to sternal perturbations. These collic evoked responses are likely to be of value in understanding the functions of cervical muscle afferents and have clinical value, for example in monitoring compensation after vestibular loss.

Tensiomyographic Responses to Warm-Up Protocols in Collegiate Male Soccer Athletes.

The mechanical properties of knee flexors and extensors in 15 collegiate male soccer players following different warm-up protocols [small-sided games (SSG), dynamic (DYN), and plyometric (PLY)] were evaluated. Tensiomyography (TMG) was used to assess contraction time (Tc), delay time (Td) and maximal displacement (Dm) of the rectus femoris (RF) and biceps femoris (BF) of both legs before and after each warm-up, while countermovement jump height variables, 20 m sprint, t-test and sit-and-reach were measured following the warm-ups. TMG was analyzed using a three-way [condition × time × leg] ANOVA, while performance variables were analyzed with a repeated measures ANOVA. Main effects of time were observed for BF-Tc (p = 0.035), RF-Td (p < 0.001), and BF-Td, (p = 0.008), and a main effect of condition was seen for RF-Tc (p = 0.038). Moreover, participants’ 20 m sprint improved following SSG (p = 0.021) compared to DYN and PLY. Sit-and-reach was greater following PLY (p = 0.021). No significant interactions were noted for the measured TMG variables. Warm-up-specific improvements were demonstrated in sprint speed and flexibility following SSG and PLY, respectively. The present study revealed changes in certain TMG measures following the warm-ups that suggest enhanced response of lower leg muscles regardless of specific activities used.

Increasing muscle activity correlations during spontaneous movements in the first six months of life

Spontaneous muscle activity in the first months of life is an important prerequisite to developing voluntary motor skills and to adapting sensorimotor circuits and muscle tone to body and environmental changes. Even though high variability is a characteristic of early development, several studies have reported significant correlations of limb movements. These assessments were typically made based on kinematics, while the analysis of lower and upper limb muscle activity may provide additional information about maturation of the neuromuscular control. To this end, we examined the electromyographic activity of 12 muscles of the upper and lower limbs in full-term healthy infants (n = 40) aged from 1 week to six months. An increase of ipsilateral and contralateral limb muscle activity correlations with age was found in both flexors and extensors and may reflect a progressive emergence of elements of coordinative neuromuscular behaviour. Correlations between arm and leg muscle responses also increased during passive leg movements. Overall, the findings are consistent with maturation of physiologically relevant neuromuscular network connections during the course of transition from spontaneous-like to voluntary goal-directed movements during early development.

Association between Oxygen Consumption and Surface Electromyographic Amplitude and Its Variation within Individual Calf Muscles during Walking at Various Speeds.

Surface electromyography (EMG) has been used to estimate muscle work and physiological burden of the whole body during human movements. However, there are spatial variations in surface EMG responses within individual muscles. The aim of this study was to investigate the relation between oxygen consumption and surface EMG responses of lower leg muscles during walking at various speeds and to quantify its spatial variation within an individual muscle. Nine young males walked on a treadmill at four speeds: preferred minus 1 km/h, preferred, preferred plus 1 km/h, and preferred plus 2 km/h, and the metabolic response was measured based on the expired gas. High-density surface EMG of the tibialis anterior (TA), medial gastrocnemius (MG), lateral gastrocnemius, and soleus muscles was performed using 64 two-dimensional electrode grids. Correlation coefficients between oxygen consumption and the surface EMG amplitude were calculated across the gait speeds for each channel in the electrode grid and for individual muscles. Mean correlation coefficients across electrodes were 0.69–0.87 for the four individual muscles, and the spatial variation of correlation between the surface EMG amplitude and oxygen consumption within an electrode grid was significantly greater in MG muscle than in TA muscle (Quartile deviations: 0.24 for MG and 0.02 for TA, p < 0.05). These results suggest that the physiological burden of the whole body during gait at various speeds can be estimated from the surface EMG amplitude of calf muscles, but we need to note its spatial distribution within the MG muscle.

Mechanical response of porcine hind leg muscles under dynamic tensile loading

A modified split Hopkinson tension bar (SHTB) apparatus was used to investigate the dynamic tensile mechanical response of porcine muscles. A hollow aluminum alloy transmission bar and a semiconductor strain gauge were used to enhance the weak signal from porcine muscles. A ring-shaped copper pulse shaper was used to achieve stress equilibrium and constant approximate strain rates in the specimens. The thin muscle specimen, fixed by 3D-printed clamps, was warped around the bar ends to minimize the radial inertial effect during tensile loading. The quasi-static tests at strain rates of 0.1 s∧-1 were also conducted on a universal material testing machine to investigate the strain rate dependence. The true stress-strain curves of porcine muscle tissues along the fiber direction were determined at approximate strain rates of 800 s∧-1, 2000 s∧-1, and 3000 s∧-1. The experimental results show that the porcine muscle exhibits nonlinear, rate-sensitive, and orthotropic behavior. The Mooney–Rivlin model with two material constants was sufficient to represent the tensile response of porcine muscles at each strain rate. The rate-dependent Fields–Backofen model can describe the high strain rate response of the porcine muscles.

Ancestral persistence of vestibulospinal reflexes in axial muscles in humans.

Most studies addressing the role of vestibulospinal reflexes in balance maintenance have mainly focused on responses in the lower limbs, while limited attention has been paid to the output in trunk and back muscles. To address this issue, we tested whether electromyographic (EMG) responses to galvanic vestibular stimulations (GVS) were modulated similarly in back and leg muscles, in situations where the leg muscle responses to GVS are known to be attenuated. Body sway and surface EMG signals were recorded in the paraspinal and limb muscles of humans (n = 19) under three complementary conditions. During treadmill locomotion, EMG responses in the lower limbs were observed only during stance, whereas responses in trunk muscles were observed during all phases of the locomotor cycle. During upright standing, a slight head contact abolished the responses in the lower limbs, while the responses remained present in back muscles. Similarly, during parabolic flight-induced microgravity, EMG responses in lower limb muscles were suppressed but remained in axial muscles despite the abolished gravitational otolithic drive. Our results suggest a differentiated control of axial and appendicular muscles when a perturbation is detected by vestibular inputs. The persistence and low modulation of axial muscle responses suggests that a hard-wired reflex is functionally efficient to maintain posture. By contrast, the ankle responses to GVS occur only in balance tasks when proprioceptive feedback is congruent. This study using GVS in microgravity is the first to present an approach delineating feedforward vestibular control in unconstrained environment.NEW & NOTEWORTHY This study addresses the extent of conservation of trunk muscle control in humans. Results show that galvanic vestibular stimulation-evoked vestibular responses in trunk muscles remain strong in conditions where leg muscle responses are downmodulated (walking, standing, microgravity). This suggests a phylogenetically conserved blueprint of sensorimotor organization, with strongly hardwired vestibulospinal inputs to axial motoneurons and a higher degree of flexibility in the later emerging limb control system.

Preferential activation of spinal sensorimotor networks via lateralized transcutaneous spinal stimulation in neurologically intact humans.

Transcutaneous spinal stimulation (TSS), a noninvasive technique to modulate sensorimotor circuitry within the spinal cord, has been shown to enable a wide range of functions that were thought to be permanently impaired in humans with spinal cord injury. However, the extent to which TSS can be used to target specific mediolateral spinal cord circuitry remains undefined. We tested the hypothesis that TSS applied unilaterally to the skin ~2 cm lateral to the midline of the lumbosacral spine selectively activates ipsilateral spinal sensorimotor circuitry, resulting in ipsilateral activation of downstream lower extremity neuromusculature. TSS cathodes and anodes were positioned lateral from the midline of the spine in 15 healthy subjects while supine, and the timing of TSS pulses was synchronized to recordings of lower extremity muscle activity and force. At motor threshold, left and right TSS-evoked muscle activity was significantly higher in the ipsilateral leg compared with contralateral recordings from the same muscles. Similarly, we observed a significant increase in force production in the ipsilateral leg compared with the contralateral leg. Delivery of paired TSS pulses, during which an initial stimulus was applied to one side of the spinal cord and 50 ms later a second stimulus was applied to the contralateral side, revealed that ipsilateral leg muscle responses decreased following the initial stimulus, whereas contralateral muscle responses did not decrease, indicating side-specific activation of lateral spinal sensorimotor circuitry. Our results indicate TSS can selectively engage ipsilateral neuromusculature via lumbosacral sensorimotor networks responsible for lower extremity function in healthy humans.NEW & NOTEWORTHY We demonstrate the selectivity of transcutaneous spinal stimulation (TSS), which has been shown to enable function in humans with chronic paralysis. Specifically, we demonstrate that TSS applied to locations lateral to the spinal cord can selectively activate ipsilateral spinal sensorimotor networks. We quantified lumbosacral spinal network activity by recording lower extremity muscle electromyography and force. Our results suggest lumbosacral TSS engages side-specific spinal sensorimotor networks associated with ipsilateral lower extremity function in humans.

A computer simulation study on the influences of loss and damage of primary muscle spindle afferents on soleus muscle stretch reflex.

Sensory loss is detrimental to sensorimotor control. Several studies have reported that loss and/or damage of primary (Ia) muscle spindle afferents significantly influence the stretch reflex responses of leg and foot muscles. However, a systematic experimental evaluation on how the impairment of Ia muscle spindle afferents affects the stretch reflex is difficult due to technical and ethical issues. In the present study, the aim was to use computer simulations of a multiscale neuromusculoskeletal model to investigate how changes in: i) the number of Ia afferents, ii) the synaptic conductance between Ia sensory fibers and spinal motor neurons (MNs), and iii) the conduction velocities (CVs) of Ia afferents, would influence the stretch reflex of a leg muscle (soleus). Simulation results showed that both anatomical and functional loss of Ia afferents exerted an influence on the amplitude of short-latency stretch reflex response (M1) and the late phase of medium-latency response (M2). Additionally, changes in CVs of Ia afferents mainly influenced the latency of M1 and the amplitude of M2. Our findings provide conceptual evidence that a combination of anatomical and functional loss, as well as changes in CVs of Ia afferents due to demyelination, can explain the stretch reflex responses observed in peripheral neuropathies.

Axial reflexes are present in older subjects and may contribute to balance responses.

We studied the response to axial taps (mini-perturbations) of a group of 13 healthy older subjects (mean age 63 ± 12years, 7 females, 6 males), 12 of whom were also studied using larger applied (macro-) perturbations requiring active postural responses. The mini-perturbation consisted of a brief impulsive force produced by a mini-shaker applied to the trunk at the level of the shoulders and anteriorly at the upper sternum which was perceived as a tap. Acceleration, force platform, and EMG measurements were made. The average peak accelerations for the mini-perturbations were 108mG (anterior) and - 78.9mG (posterior). Responses overall were very similar to those previously reported for younger subjects: the perturbation evoked short latency responses in leg muscles, modulated by degree and direction of lean, and were largest for the muscle most relevant for the postural correction. The increases in the amplitude for the main agonist were greater than the increase in tonic activity. With both anterior and posterior lean, co-contraction responses were present. The size of the EMG response to the mini-perturbations correlated with the corresponding earliest EMG responses (0-100, 100-200ms intervals) to the larger postural perturbations, timing which corresponds to balance responses. The balance responses evoked by the larger imposed postural perturbations may, therefore, receive a contribution through the reflex pathway mediating the axial tap responses, whose efferent limb appears to be the reticulospinal tract.

Common neural structures activated by epidural and transcutaneous lumbar spinal cord stimulation: Elicitation of posterior root-muscle reflexes.

Epidural electrical stimulation of the lumbar spinal cord is currently regaining momentum as a neuromodulation intervention in spinal cord injury (SCI) to modify dysregulated sensorimotor functions and augment residual motor capacity. There is ample evidence that it engages spinal circuits through the electrical stimulation of large-to-medium diameter afferent fibers within lumbar and upper sacral posterior roots. Recent pilot studies suggested that the surface electrode-based method of transcutaneous spinal cord stimulation (SCS) may produce similar neuromodulatory effects as caused by epidural SCS. Neurophysiological and computer modeling studies proposed that this noninvasive technique stimulates posterior-root fibers as well, likely activating similar input structures to the spinal cord as epidural stimulation. Here, we add a yet missing piece of evidence substantiating this assumption. We conducted in-depth analyses and direct comparisons of the electromyographic (EMG) characteristics of short-latency responses in multiple leg muscles to both stimulation techniques derived from ten individuals with SCI each. Post-activation depression of responses evoked by paired pulses applied either epidurally or transcutaneously confirmed the reflex nature of the responses. The muscle responses to both techniques had the same latencies, EMG peak-to-peak amplitudes, and waveforms, except for smaller responses with shorter onset latencies in the triceps surae muscle group and shorter offsets of the responses in the biceps femoris muscle during epidural stimulation. Responses obtained in three subjects tested with both methods at different time points had near-identical waveforms per muscle group as well as same onset latencies. The present results strongly corroborate the activation of common neural input structures to the lumbar spinal cord—predominantly primary afferent fibers within multiple posterior roots—by both techniques and add to unraveling the basic mechanisms underlying electrical SCS.

Different fatigue-resistant leg muscles and EMG response during whole-body vibration

The purpose of this study was to determine the effects of static whole-body vibration (WBV) on the Electromyograhic (EMG) responses of leg muscles, which are fatigue-resistant in different manner. The study population was divided into two groups according to the values obtained by the Fatigue Index [Group I: Less Fatigue Resistant (LFR), n=11; Group II: More Fatigue Resistant (MFR), n=11]. The repeated electromyographic (EMG) activities of four leg muscles were analyzed the following determinants: (1) frequency (30 Hz, 35 Hz and 40 Hz); (2) stance position (static squat position); (3) amplitude (2 mm and 4 mm) and (4) knee flexion angle (120°), (5) vertical vibration platform. Vibration data were analyzed using Minitab 16 (Minitab Ltd, State College, PA, USA). The significance level was set at p<.05. The study results showed that static WBV stimuli given at different frequencies and amplitudes resulted in a significant increase (p<.05) in compared, the LFR group showed significantly (1) higher rates of quadriceps femoris and hamstring muscle fatigue (p<.05), (2) higher levels of knee extensor and flexor torque (p<.05) and (3) higher percentage increases in EMG activation at higher frequencies (max at 40 Hz) and amplitudes (4 mm) (p<.05). The present study can be used for the optimal prescription of vibration exercise and can serve to guide the development of training programs.

A fuzzy controller for movement stabilization using afferent control: Controller synthesis and simulation

Stimulation of spinal sensorimotor circuits can improve motor control in animal models and humans with spinal cord injury (SCI). More recent evidence suggests that the stimulation increases the level of excitability in the spinal circuits, activates central pattern generators, and it is also able to recruit distinctive afferent pathways connected to specific sensorimotor circuits. In addition, the stimulation generates well-defined responses in leg muscles after each pulse. The problem is that in most of the neuromodulation devices, electrical stimulation parameters are regulated manually and stay constant during movement. Such a technique is likely suboptimal to intercede maximum therapeutic effects in patients. Therefore, in this article, a fuzzy controller has been designed to control limb kinematics during locomotion using the afferent control in a neuromechanical model without supraspinal drive simulating post-SCI situation. The proposed controller automatically tunes the weights of group Ia afferent inputs of the spinal cord to reset the phase appropriately during the reaction to an external perturbation. The kinematic motion data and weights of group Ia afferent inputs were the input and output of the controller, respectively. Simulation results showed the acceptable performance of the controller to establish adaptive locomotion against the perturbing forces based on the phase resetting of the walking rhythm.

Calibration of the Leg Muscle Responses Elicited by Predictable Perturbations of Stance and the Effect of Vision.

Motor adaptation due to task practice implies a gradual shift from deliberate control of behavior to automatic processing, which is less resource- and effort-demanding. This is true both for deliberate aiming movements and for more stereotyped movements such as locomotion and equilibrium maintenance. Balance control under persisting critical conditions would require large conscious and motor effort in the absence of gradual modification of the behavior. We defined time-course of kinematic and muscle features of the process of adaptation to repeated, predictable perturbations of balance eliciting both reflex and anticipatory responses. Fifty-nine sinusoidal (10 cm, 0.6 Hz) platform displacement cycles were administered to 10 subjects eyes-closed (EC) and eyes-open (EO). Head and Center of Mass (CoM) position, ankle angle and Tibialis Anterior (TA) and Soleus (Sol) EMG were assessed. EMG bursts were classified as reflex or anticipatory based on the relationship between burst amplitude and ankle angular velocity. Muscle activity decreased over time, to a much larger extent for TA than Sol. The attenuation was larger for the reflex than the anticipatory responses. Regardless of muscle activity attenuation, latency of muscle bursts and peak-to-peak CoM displacement did not change across perturbation cycles. Vision more than doubled speed and the amount of EMG adaptation particularly for TA activity, rapidly enhanced body segment coordination, and crucially reduced head displacement. The findings give new insight on the mode of amplitude- and time-modulation of motor output during adaptation in a balancing task, advocate a protocol for assessing flexibility of balance strategies, and provide a reference for addressing balance problems in patients with movement disorders.

Growth responses of breast and leg muscles to essential amino acids in broiler chicks

The first three essential amino acids (EAA) for broilers including methionine (Met), lysine (Lys) and threonine (Thr) may greatly influence the growth of chick muscles at early stages of life. In order to survey the potential effects of those EAA on growth muscles, a rotatable three-variable central composite design (CCD) was conducted to track the interrelationships of dietary digestible Met (dMet), Lys (dLys) and Thr (dThr) for optimization of processing yields in broiler chicks using response surface methodology. A total of 60 floor pens of six birds each were assigned to 15 dietary treatments based on CCD containing five levels of dMet (0.416% to 0.584% of diet), dLys (0.881% to 1.319% of diet) and dThr (0.532% to 0.868% of diet) from 3 to 16 days of age. Experimental treatments significantly affected breast mass (BM) and leg mass (LM) of the birds (P<0.05) in which the main effect of dLys on BM was threefold higher than the main effect of dThr, and interaction effect between dMet and dLys was observed on BM (P<0.05). However, in the case of LM, the main effect of dThr was higher than the main effects of dMet and dLys and highest interaction effect exist between dThr and dMet (P<0.05). The second-order models for BM and LM were fitted by least squares regression. Canonical analysis revealed that the stationary points for carcass components were saddle points, thus ridge analysis was performed for getting optimal values of each EAA. Ridge analyses of BM and LM models showed that the maximum BM point may be obtained with 0.58%, 1.05% and 0.76% of dMet, dLys and dThr, respectively, in diet, and maximum LM point may be achieved with 0.58%, 1.09% and 0.70% of dMet, dLys and dThr, respectively, in diet. The resultant ideal ratios of dMet and dThr to dLys were 55% and 72% for BM; 53% and 64% for LM. Moreover, sensitivity analysis showed that the most important amino acids in BM and LM models were Lys and Thr, respectively. In conclusion, providing these three amino acid for BM optimization may warrant LM optimization and higher ideal ratios of dMet and dThr for breast muscle may indicate the higher importance of these EAA in this muscle than those in thigh muscle.

Rebuttal from Brian C. Horslen, Christopher J. Dakin, J. Timothy Inglis, Jean-Sébastien Blouin and Mark G. Carpenter.

The topic of debate is whether fear influences vestibular-evoked balance responses. Opposing arguments from Reynolds et al. (2015) rely on the reported lack of changes in the early ( 800 ms) responses. The evidence drawn from the lack of early response change is unconvincing due to an increased likelihood of type II error caused by a small sample size and poor resolution resulting from the following factors: (i) use of centre of mass position and velocity estimates to detect vestibular-evoked responses in leg muscles, especially as tandem-stance requires multijoint control; (ii) the limited number of stimulations; and (iii) small early responses due to low (1 mA) stimulus intensity (Ali et al. 2003). Likewise, interpretation of late responses is confounded by the potential influence of fear on reactions to non-vestibular sensory feedback related to the vestibular perturbation (Day & Guerraz, 2007) and conscious strategies (Reynolds, 2010) within this time period. We disagree with Reynolds et al. (2015) that fear-related increases in gain and coupling of ground reaction forces (GRFs) to stochastic vestibular stimulation (SVS) are irrelevant to balance. Early responses are clearly distinguishable using GRFs (Marsden et al. 2002), with less mechanical filtering of the neuromuscular response affecting GRFs than kinematics (Forbes et al. 2015). In our study, SVS was engineered to highlight changes in early balance responses to vestibular stimulation that are not confounded by later perturbation-induced sensorimotor responses (Horslen et al. 2014). The significant SVS gain changes observed between 2 and 16 Hz coincides with natural head frequencies recorded during gait and whole-body perturbations (for review, see Forbes et al. 2015). After establishing threat-related effects on early vestibular-evoked balance responses, we confirmed these effects in GRFs and EMG from posturally engaged muscles using an SVS stimulus that included the 0–2 Hz bandwidth and with vestibular-evoked myogenic potentials (see Horslen et al. 2015). The notion that we have confused statistical and functional significance is false; we view evidence of a true effect, beyond chance or bias, as a necessary prerequisite to considering functional significance. Furthermore, functional significance should not be discounted because of small absolute changes, which would be expected given the small virtual rotational velocity induced by the stimulus (Peters et al. 2015) and are consistent with magnitudes reported elsewhere (Reynolds, 2010; Mian & Day, 2014). In contrast, we believe that the relative change in gain of early vestibular-evoked responses provides more meaningful insight into functional significance. Thus, fear-related increases in gain (81–231%) are non-trivial and functionally significant, particularly when translated to real-life head accelerations.

Precueing time but not direction of postural perturbation induces early muscular activation: Comparison between young and elderly individuals

In this study, we evaluated the effect of precueing characteristics of an impending perturbation to upright stance on reactive responses of distal leg muscles. Young and older individuals were compared in a task of recovering stable upright stance following rotation of the supporting platform to induce anterior or posterior body sway. Directions of the supporting platform rotation were randomized across trials. Immediately before postural perturbation participants were cued about direction and/or time of platform rotation, or performed the task under directional and temporal uncertainty of the impending perturbation. Results showed that precueing time of perturbation led to earlier muscular activation onset, while precueing perturbation direction did not modulate either latency or magnitude of muscular activation. Those effects were similar between age groups. Our findings suggest that awareness of the perturbation time favored shorter response latencies in both the young and older individuals.