Age-Related Differences in Motor Coordination during Simultaneous Leg Flexion and Finger Extension: Influence of Temporal Pressure

Although the effect of temporal pressure on spatio-temporal aspects of motor coordination and posture is well established in young adults, there is a clear lack of data on elderly subjects. This work examined the aging-related effects of temporal pressure on movement synchronization and dynamic stability. Sixteen young and eleven elderly subjects performed series of simultaneous rapid leg flexions in an erect posture paired with ipsilateral index-finger extensions, minimizing the difference between heel and finger movement onsets. This task was repeated ten times under two temporal conditions (self-initiated [SI] vs. reaction-time [RT]). Results showed that, first, temporal pressure modified movement synchronization; the finger extension preceded swing heel-off in RT, and inversely in SI. Synchronization error and associated standard deviation were significantly greater in elderly than in young adults in SI only, i.e. in the condition where proprioception is thought to be crucial for temporal coordination. Secondly, both groups developed a significantly shorter mediolateral (ML) anticipatory postural adjustment duration in RT (high temporal pressure) than in SI. In both groups, this shortening was compensated by an increase in the anticipatory peak of centre-of-gravity (CoG) acceleration towards the stance-leg so that ML dynamic stability at foot-off, quantified with the “extrapolated centre-of-mass”, remained unchanged across temporal conditions. This increased CoG acceleration was associated with an increased anticipatory peak of ML centre-of-pressure shift towards the swing-leg in young adults only. This suggested that the ability to accelerate the CoG with the centre-of-pressure shift was degraded in elderly, probably due to weakness in the lower limb muscles. Dynamic stability at foot-off was also degraded in elderly, with a consequent increased risk of ML imbalance and falling. The present study provides new insights into the ability of elderly adults to deal with temporal pressure constraints in adapting whole-body coordination of postural and focal components of paired movement.     

Gait Initiation in Children with Rett Syndrome

Rett syndrome is an X-linked neurodevelopmental condition mainly characterized by loss of spoken language and a regression of purposeful hand use, with the development of distinctive hand stereotypies, and gait abnormalities. Gait initiation is the transition from quiet stance to steady-state condition of walking. The associated motor program seems to be centrally mediated and includes preparatory adjustments prior to any apparent voluntary movement of the lower limbs. Anticipatory postural adjustments contribute to postural stability and to create the propulsive forces necessary to reach steady-state gait at a predefined velocity and may be indicative of the effectiveness of the feedforward control of gait. In this study, we examined anticipatory postural adjustments associated with gait initiation in eleven girls with Rett syndrome and ten healthy subjects. Muscle activity (tibialis anterior and soleus muscles), ground reaction forces and body kinematic were recorded. Children with Rett syndrome showed a distinctive impairment in temporal organization of all phases of the anticipatory postural adjustments. The lack of appropriate temporal scaling resulted in a diminished impulse to move forward, documented by an impairment in several parameters describing the efficiency of gait start: length and velocity of the first step, magnitude and orientation of centre of pressure-centre of mass vector at the instant of (swing-)toe off. These findings were related to an abnormal muscular activation pattern mainly characterized by a disruption of the synergistic activity of antagonistic pairs of postural muscles. This study showed that girls with Rett syndrome lack accurate tuning of feedforward control of gait.     

Controlling propulsive forces in gait initiation in transfemoral amputees

During prosthetic gait initiation, transfemoral (TF) amputees control the spatial and temporal parameters that modulate the propulsive forces, the positions of the center of pressure (CoP), and the center of mass (CoM). Whether their sound leg or the prosthetic leg is leading, the TF amputees reach the same end velocity. We wondered how the CoM velocity build up is influenced by the differences in propulsive components in the legs and how the trajectory of the CoP differs from the CoP trajectory in able bodied (AB) subjects. Seven TF subjects and eight AB subjects were tested on a force plate and on an 8 m long walkway. On the force plate, they initiated gait two times with their sound leg and two times with their prosthetic leg. Force measurement data were used to calculate the CoM velocity curves in horizontal and vertical directions. Gait initiated on the walkway was used to determine the leg preference. We hypothesized that because of the differences in propulsive components, the motions of the CoP and the CoM have to be different, as ankle muscles are used to help generate horizontal ground reaction force components. Also, due to the absence of an active ankle function in the prosthetic leg, the vertical CoM velocity during gait initiation may be different when leading with the prosthetic leg compared to when leading with the sound leg. The data showed that whether the TF subjects initiated a gait with their prosthetic leg or with their sound leg, their horizontal end velocity was equal. The subjects compensated the loss of propulsive force under the prosthesis with the sound leg, both when the prosthetic leg was leading and when the sound leg was leading. In the vertical CoM velocity, a tendency for differences between the two conditions was found. When initiating gait with the sound leg, the downward vertical CoM velocity at the end of the gait initiation was higher compared to when leading with the prosthetic leg. Our subjects used a gait initiation strategy that depended mainly on the active ankle function of the sound leg; therefore, they changed the relative durations of the gait initiation anticipatory postural adjustment phase and the step execution phase. Both legs were controlled in one single system of gait propulsion. The shape of the CoP trajectories, the applied forces, and the CoM velocity curves are described in this paper.     

Age and Muscle-Dependent Variations in Corticospinal Excitability during Standing Tasks

In this study, we investigated how modulation in corticospinal excitability elicited in the context of standing tasks varies as a function of age and between muscles. Changes in motor evoked potentials (MEPs) recorded in tibialis anterior (TA) and gastrocnemius lateralis (GL) were monitored while participants (young, n = 10; seniors, n = 11) either quietly stood (QS) or performed a heel raise (HR) task. In the later condition, transcranial magnetic stimulation (TMS) pulses were delivered at three specific time points during the task: 1) 250 ms before the “go” cue (preparatory (PREP) phase), 2) 100 ms before the heel rise (anticipatory postural adjustment (APA) phase), and 3) 200 ms after heel rise (execution (EXEC) phase). In each task and each phase, variations in MEP characteristics were analysed for age and muscle-dependent effects. Variations in silent period (SP) duration were also examined for certain phases (APA and EXEC). Our analysis revealed no major difference during QS, as participants exhibited very similar patterns of modulation in both TA and GL, irrespective of their age group. During the HR task, young adults exhibited a differential modulation in the PREP phase with enhanced responses in TA relative to GL, which was not seen in seniors. Finally, besides differences in MEP latency, age had little influence on MEP modulation during the APA and EXEC phases, where amplitude was largely a function of background muscle activity associated with each phase (i.e., APA: TA; EXEC: GL). No age or muscle effects were detected for SP measurements. Overall, our results revealed no major differences between young adults and healthy seniors in the ability to modulate corticospinal facilitation destined to ankle muscles during standing tasks, with maybe the exception of the ability to prime muscle synergies in the preparatory phase of action.     

Improved compensatory postural adjustments of the deep abdominals following exercise in people with chronic low back pain

The purpose of this study was to determine if 8 weeks of exercise affects motor control in people with chronic low back pain (CLBP), measured by anticipatory (APAs) and compensatory postural adjustments (CPAs). APAs and CPAs were measured prior to and following 8 weeks in two groups of people with CLBP: an exercise group (n = 12) who attended three exercise sessions per week for 8 weeks; and a non-exercise control group (n = 12) who were advised to continue their usual activities for the duration of the study. APAs and CPAs were recorded during unilateral arm flexion, bilaterally from rectus abdominis (RA), transverse abdominis/internal oblique (TA/IO), and erector spinae (ES) via surface electromyography. Analysis of muscle onsets and APA amplitudes suggests APAs did not change for either group. Ipsi-lateral TA/IO CPAs increased for the exercise group and ipsi-lateral TA/IO CPAs decreased for the control group. Only exercise promoted a pattern of TA/IO activity during CPAs similar to healthy individuals, suggesting improved control of rotational torques. These results show motor control improvement following exercise in people with CLBP, highlighted by improved side specific control of TA/IO.     

Contractile properties of human skeletal muscle in childhood and adolescence

Using a combination of single maximal stimuli and maximum voluntary contractions, a comparison has been made of muscle properties in pre- and post-pubertal male subjects. In the dorsiflexor and plantarflexor muscles of the ankle, the twitch and maximum voluntary torques were approximately twice as large in the older subjects; the mean height and mean weight increased by factors of 1.20 and 1.86 respectively. The only other muscle parameter that changed, as a function of age, was the contraction time of the ankle dorsiflexors; the mean value was significantly longer in the older subjects. In the younger subjects, there were already clear differences between the dorsiflexor and plantarflexor muscles, the former developing smaller torques and having shorter contraction and half-relaxation times, greater post-activation potentiation and more susceptibility to fatigue. Even in the youngest subject, motor unit activation was complete in the ankle dorsiflexors; although this was not always true of the plantarflexors, the difference between the two subject groups was not significant.     

Postural maintenance during movement: Simulations of a two joint model

Voluntary movements of the upper body are accompanied by anticipatory postural adjustments to the lower body in a standing subject. The long-standing hypothesis is that these anticipatory adjustments serve to counteract the perturbation to the body's center of gravity caused by the voluntary arm movement. This paper presents model simulations investigating the possible roles of anticipatory postural activity that accompanies a rapid, upward arm swing. The model encorporates two (idealized) antagonistic muscle pairs controlling the movements of a double-joint system, with a shoulder joint between the arm and stiff body links, and an ankle joint between the stiff body-leg segment and the ground. Each muscle is represented by a nonlinear viscoelastic element and also includes proprioceptive feedback. Four inputs to the model define the motor control signals for muscle force generation in both the arm and the postural muscle pairs. The neurological component of the model describes consequences of alternate strategies for cocontractions, stretch reflex activity, and anticipatory and synchronous postural activities (or combinations thereof). Simulations with this model show that: (1) none of the postural maintenance schemes considered in these simulations (including varying anticipation) could suppress the initial backward thrust on the body link; (2) the more important destabilizing perturbation is a subsequent forward sway that, left uncountered by postural activity, would eventually leave the body to fall flat on its face; and (3) anticipatory silencing of the postural extensor followed by a brief period of extensor activation (descending control) and synchronous reflex activity (feedback control) appears to be the most likely postural stabilizing strategy that inhibits the continuous forward sway and is consistent with the experimental evidence.     

Coactivation during gait as an adaptive behavior after stroke

The aims of the present study were to quantify the impairment in ankle coactivation on the paretic and non-paretic sides of subjects with hemiparesis and to examine the relationship of ankle coactivation with postural instability, motor deficit of the paretic lower extremity and locomotor performance. Electromyography of the medial gastrocnemius (MG) and tibialis anterior (TA) muscles were recorded bilaterally during gait in 30 subjects (62.1±9.9 years) who had suffered a recent stroke (<6 months) as well as on one side of 17 healthy controls (59.3±9.1 years) walking at very slow speed. Ankle muscle coactivation was calculated by dividing the time of overlap between MG and TA signals (threshold of 20 μV) by the duration of the gait phases of interest: stance, swing, first and second double support sub-phases and single support sub-phase. The time spent in single support and the peak plantarflexor moment of force on the paretic side were used to measure, respectively, postural stability and dynamic strength of the paretic plantarflexors. The subjects with hemiparesis demonstrated less coactivation on the paretic side during the single support sub-phase (p<0.01) and more coactivation during first and second double support sub-phases on the non-paretic side (p<0.001) compared to control values. The patients with coactivation patterns that differed the most from controls were the patients with the more severe impairments and disabilities. While the reduced coactivation on the paretic side may contribute to poor postural stability and poor locomotor performance, the presence of excessive coactivation on the non-paretic side when both limbs were in ground contact may be an adaptation to help maintain postural stability during gait.     

Standing on wedges modifies side-specific postural control in the presence of lateral external perturbations

Standing on wedges changes the position in the ankle joints and affects postural stability in the medial-lateral direction. The objective of the study was to investigate the role of wedges and external lateral perturbations on anticipatory (APA) and compensatory postural adjustments (CPA). Ten healthy young participants were exposed to perturbations applied to the lateral part of their right shoulder when standing on a planar surface, on a medial or lateral wedges. Bilateral electromyographic activity of dorsal and ventral postural muscles and the center of pressure (COP) displacement were recorded and analyzed during the APA and CPA phases. When exposed to the lateral perturbation, reciprocal activation of shank muscles was seen on the side of the perturbation while co-contraction of shank muscles was seen on the contralateral side during the APA and CPA phases. Standing on a wedge was associated with decreased magnitudes of co-contraction and reciprocal activation of shank muscles. The COP displacements were smaller in the APA phase and larger in the CPA phase while standing on wedges compared to standing on the planar surface. The outcome of the study provides a basis for future investigations of incorporating wedges in balance re-training paradigms for the elderly or individuals with neurological impairment.     

Anticipatory postural adjustments depend on final equilibrium and task complexity in vertical high jump movements.

Anticipatory postural adjustments (APA) associated with high vertical jumping were studied in five normal men. The dependence of APA on final equilibrium and task complexity was studied by analyzing the characteristics of single versus repetitive jumps. Subjects performed series of 25 single jumps and 25 repetitive jumps on a force platform, in response to a Go signal. Surface EMG from the Soleus (SO), Tibialis anterior (TA), Vastus lateralis (VL) and Biceps femoris (BF) muscles were recorded simultaneously with the three components of the ground reaction forces.The existence of APA associated with both single and repetitive jumps was evidenced by a backward shift of the center of pressure, an SO deactivation, and a TA activation which preceded the onset of movement by hundreds of milliseconds. In repetitive jumps, SO deactivation occurred later than in single jump, whereas the center of pressure shift started earlier, lasted longer and had a greater amplitude. Moreover, repetitive jumps were initiated later with respect to the Go signal. These data show that the central nervous system anticipates not only the equilibrium disturbance caused by an initial movement but that caused by a whole sequence of movements. They also show that the corresponding APA are based heavily on a single functional ankle synergy.     

Effect of hip and ankle muscle fatigue on unipedal postural control

Fatigue and deficits in postural control may predispose musculoskeletal injury. The purpose of this study was to examine the effects of fatigue at the hip and ankle during frontal plane movements on postural control during single-leg stance. Thirteen healthy volunteers completed two testing sessions 1 week apart consisting of isokinetic fatigue of the frontal plane movers of either the ankle or hip with measures of static unipedal postural control taken before and after fatigue. Postural control was assessed during three 30-s trials of unilateral stance with eyes open before and after the fatigue protocol at each testing session. Mean center of pressure (COP) excursion velocity in the sagittal and frontal planes was compared between pre- and post-fatigue across the two joints. Fatigue of the hip musculature led to postural control impairments in the frontal and sagittal planes, while fatigue of the ankle musculature did not significantly impair postural control in either plane. Our results suggest that there is a greater effect of localized fatigue of the frontal plane movers of the hip compared to the ankle on maintenance of a postural control in single-leg stance.     

Activation timing of soleus and tibialis anterior muscles during sit-to-stand and stand-to-sit in post-stroke vs. healthy subjects

Introduction. Sit-to-stand (SitTS) and stand-to-sit (StandTS) are very important functional tasks that become compromised in stroke patients. As in other voluntary movements, they require an adequate postural control (PC) involving the generation of anticipatory postural adjustments (APAs). In order to give clues for more efficient and directed rehabilitation programs, a deeper knowledge about APAs during challenging and daily life movements is essential.

Purpose. To analyze the activation timing of tibialis anterior (TA) and soleus (SOL) muscles during SitTS and StandTS in healthy subjects and in post-stroke patients.

Methods. Two groups participated in this study: one composed of ten healthy subjects and the other by ten subjects with a history of stroke and increased H-reflex. Electromyographic activity (EMGa) of SOL and TA was analyzed during SitTS and StandTS in the ipsilateral (IPSI) and the contralateral (CONTRA) limb to the side lesion in stroke subjects, and in one limb in healthy subjects. A force plate was used to identify the movement onset.

Results. In both sequences, in the stroke group SOL activation timing occurred prior to movement onset, contrary to the pattern observed in the healthy subjects. Statistically significant differences were found in SOL activation timings between each lower limb of the stroke and healthy groups, but no significant differences were found between the IPSI and the CONTRA limb. The TA activation timing seems to be delayed in the CONTRA limb when compared to the healthy subjects and showed a better organization of TA timing activation in StandTS when compared to SitTS.

Conclusion. Compared to healthy subjects, APAs seem to be altered in both limbs of the post-stroke subjects, with the SOL activation timing being anticipated in both SitTS and StandTS.     

Simultaneous Postural Adjustments (SPA) scrutinized using the Lissajous method

The goal of this research was to study the postural adjustments that occur during the course of a voluntary movement (Simultaneous Postural Adjustments: SPA). A pointing task performed at maximal velocity was considered and upper limb kinematics and body kinetics were recorded. A 2-DOF model was elaborated that distinguishes between the body segments that are mobilized in order to perform the pointing movement. These segments are the right upper limb (termed the “focal” component) and the rest of the body (termed the “postural” component). This model allowed for the calculation of both sub-systems? kinetics and a comparison of the resultant reaction (RoSh) with the corresponding action (AoSh) at the shoulder level. The analysis was based on the ellipsoidal shape of their relationship. The ellipse computation (“Lissajous ellipse”) allowed the time lag to be estimated. The results showed that the kinetics of the postural component preceded that of the focal ones and that the time lag during the SPA was not statistically different from the APA duration (dAPA). In addition, the kinetics of the postural component were found to be opposed to the perturbation induced by the pointing movement, but only during part of the SPA time interval. It was concluded that the postural component plays a dual role during the movement, which consists of postural stabilization and propulsive action, with one prevailing over the other depending on the time-instant of movement evolution. This new evidence in healthy subjects is helpful to further specify differences associated with motor impairments.     

Biomechanical study of the programming of anticipatory postural adjustments associated with voluntary movement

The present research concerns anticipatory postural adjustments (APA), with the purpose of determining whether they are preprogrammed and of specifying their biomechanical finality. The experimental situation allowed us to distinguish between the voluntary movement itself (an upper limb elevation) and the postural adjustments associated with it. To this aim, the upper limb kinematics, evaluated from an accelerometer fixed at wrist level, were compared to the whole body dynamics, recorded by means of a force platform. Movements, executed in series of five, were studied according to three conditions: bilateral flexions (BF) and unilateral flexions (UF), with (IUF) and without (OUF) an additional inertia, of the stretched upper limb(s). Six right handed adults were tested twice. Results showed that the ground reaction resultant forces as well as the ground reaction resultant moment about the vertical axis presented reproducible variations before and after the onset of upper limb acceleration. The biomechanical organization of APA corresponded, for the three experimental conditions, to an upward and forward acceleration of the body center of gravity, and also, for UF, to a resultant moment directed towards the contralateral side. The duration of APA varied with the characteristics of the forthcoming voluntary movement, increasing significantly from BF to OUF and from OUF to IUF. It is concluded that APA correspond to dynamic phenomena which are centrally preprogrammed. The inertia forces associated with APA may, when the time comes, balance the inertia forces due to the movement of the mobile limb therefore counteracting the disturbance to postural equilibrium.(ABSTRACT TRUNCATED AT 250 WORDS)     

The effect of short-term changes in body mass distribution on feed-forward postural control

It was recently shown that short-term changes in the whole body mass and associated changes in the vertical position of the center of mass (COM) modify anticipatory postural adjustments (APAs) [Li X, Aruin AS. The effect of short-term changes in the body mass on anticipatory postural adjustments. Exp Brain Res 2007;181:333–46]. In this study, we investigated whether changes in the body mass distribution and related changes in the anterior–posterior COM position affect APA generation. Fourteen subjects were instructed to catch a 2.2 kg load with their arms extended while standing with no additional weight or while carrying a 9.08 kg weight. Adding weight to a backpack, front pack or belly pocket was associated with an increase of the whole body mass, but it also involved changes in the anterior–posterior (A/P) and vertical positions of the COM. Electromyographic activity of leg and trunk muscles, body kinematics, and ground reaction forces were recorded and quantified within the typical time intervals of APAs. APAs were modified in conditions with changed body mass distribution: increased magnitude of anticipatory EMG activity in leg and trunk muscles, as well as co-activation of leg muscles and decreased anticipatory displacement of the COM in the vertical direction, were seen in conditions with increased body mass. Changes in the COM position induced in both A/P and vertical directions were associated with increased anticipatory EMG activity. In addition, they were linked to a co-activation of muscles at the ankle joints and significant changes in the center of pressure (COP) position. Modifications of the COM position induced in the A/P direction were related to increased anticipatory EMG activity in the leg and trunk muscles. At the same time, no significant differences in anticipatory EMG activity or displacement of COP were observed when changes of COM position were induced in the vertical direction. The study outcome suggests that the CNS uses different strategies while generating APAs in conditions with changes in the COM position induced in the anterior–posterior and vertical directions.     

Effect of wearing a dorsiflexion assist orthosis on mobility, perceived fatigue and exertion during the six-minute walk test in people with multiple sclerosis: a randomised cross-over protocol

ABSTRACT: BACKGROUND: Fatigue in combination with gait and balance impairments can severely limit daily activities in people with multiple sclerosis (PWMS). Generalised fatigue has a major impact on walking ability, with moderately disabled PWMS experiencing difficulty in walking extended distances. Localised motor fatigue in the ankle dorsiflexors can lead to foot drop, further reducing functional ambulation. The aim of this study is to evaluate the effect of a simple dynamic dorsiflexion assist orthosis on walking-induced fatigue, gait, balance and functional mobility in PWMS. METHODS: A randomised cross-over trial will be conducted with 40 community dwelling PWMS with mild to moderate mobility disability. Participants will initially be screened for disease severity, balance, strength, depression and fatigue at the South Australian Motion Analysis Centre. On two non-consecutive occasions, within two weeks, participants will undergo either the 6-minute walk test (6MWT) or the 6MWT while wearing a dorsiflexion ankle orthosis (with a randomised condition order). Distance walked, perceived exertion, perceived fatigue and the physiological cost of walking (the primary outcome measures) will be compared between the two walking conditions. Additional pre- and post-6MWT assessments for the two conditions will include tests of strength, reaction time, gait and balance. DISCUSSION: This study will increase our understanding of motor fatigue on gait and balance control inPWMS and elucidate the effect of a Dynamic Ankle Orthosis on fatigue-related balance and gait in PWMS. It will also examine relationships between mobility and balance performance with perceived fatigue levels in this group. Trial Registration Number ACTRN12612000218897.     

Spinal Mechanisms May Provide a Combination of Intermittent and Continuous Control of Human Posture: Predictions from a Biologically Based Neuromusculoskeletal Model

Several models have been employed to study human postural control during upright quiet stance. Most have adopted an inverted pendulum approximation to the standing human and theoretical models to account for the neural feedback necessary to keep balance. The present study adds to the previous efforts in focusing more closely on modelling the physiological mechanisms of important elements associated with the control of human posture. This paper studies neuromuscular mechanisms behind upright stance control by means of a biologically based large-scale neuromusculoskeletal (NMS) model. It encompasses: i) conductance-based spinal neuron models (motor neurons and interneurons); ii) muscle proprioceptor models (spindle and Golgi tendon organ) providing sensory afferent feedback; iii) Hill-type muscle models of the leg plantar and dorsiflexors; and iv) an inverted pendulum model for the body biomechanics during upright stance. The motor neuron pools are driven by stochastic spike trains. Simulation results showed that the neuromechanical outputs generated by the NMS model resemble experimental data from subjects standing on a stable surface. Interesting findings were that: i) an intermittent pattern of muscle activation emerged from this posture control model for two of the leg muscles (Medial and Lateral Gastrocnemius); and ii) the Soleus muscle was mostly activated in a continuous manner. These results suggest that the spinal cord anatomy and neurophysiology (e.g., motor unit types, synaptic connectivities, ordered recruitment), along with the modulation of afferent activity, may account for the mixture of intermittent and continuous control that has been a subject of debate in recent studies on postural control. Another finding was the occurrence of the so-called “paradoxical” behaviour of muscle fibre lengths as a function of postural sway. The simulations confirmed previous conjectures that reciprocal inhibition is possibly contributing to this effect, but on the other hand showed that this effect may arise without any anticipatory neural control mechanism.     

Mechanisms of Motor Adaptation in Reactive Balance Control

Balance control must be rapidly modified to provide stability in the face of environmental challenges. Although changes in reactive balance over repeated perturbations have been observed previously, only anticipatory postural adjustments preceding voluntary movements have been studied in the framework of motor adaptation and learning theory. Here, we hypothesized that adaptation occurs in task-level balance control during responses to perturbations due to central changes in the control of both anticipatory and reactive components of balance. Our adaptation paradigm consisted of a Training set of forward support-surface perturbations, a Reversal set of novel countermanding perturbations that reversed direction, and a Washout set identical to the Training set. Adaptation was characterized by a change in a motor variable from the beginning to the end of each set, the presence of aftereffects at the beginning of the Washout set when the novel perturbations were removed, and a return of the variable at the end of the Washout to a level comparable to the end of the Training set. Task-level balance performance was characterized by peak center of mass (CoM) excursion and velocity, which showed adaptive changes with repetitive trials. Only small changes in anticipatory postural control, characterized by body lean and background muscle activity were observed. Adaptation was found in the evoked long-latency muscular response, and also in the sensorimotor transformation mediating that response. Finally, in each set, temporal patterns of muscle activity converged towards an optimum predicted by a trade-off between maximizing motor performance and minimizing muscle activity. Our results suggest that adaptation in balance, as well as other motor tasks, is mediated by altering central sensitivity to perturbations and may be driven by energetic considerations.     

Properties of twitch motor units of the ankle extensor muscles in the bullfrog Rana catesbeiana

A study of the mechanical properties of the twitch motor units in the ankle extensor muscles of bullfrogs was undertaken to expand our view of the diversity of motor unit properties among vertebrates. Two muscles were chosen that represent a wide range of extensor function: the plantaris longus (PL) is a large muscle providing most of the force for ankle extension in hopping and swimming, and the tibialis posticus (TP) is relatively small and may act as an ankle stabilizer or be primarily postural in function. Both muscles have highly fatigable motor units, but also some (especially in TP) low or non-fatigable ones. Mean tetanic tensions of motor units in both muscles are relatively large as compared with those of mammals but are especially large in PL, No clear correlations were found between contraction times and either motor unit tetanic tensions or fatigability, nor did the motor units fall into clearly defined types based on any functional parameters. Overall contraction and relaxation times are slow compared with those of mammals and are somewhat slow compared to those of other frogs; unlike results from earlier studies, the large units of PL are slower than the small units of TP. This results in PL units reaching fused tetani at lower stimulus frequencies. The twitch/tetanus and force/frequency ratios in PL motor units are much larger than those of TP, giving PL units greater relative forces at lower stimulus rates. These results are discussed in the context of motor unit function. © 1994 Wiley-Liss, Inc.     

Early postural adjustments in preparation to whole-body voluntary sway

We studied postural adjustments associated with a quick voluntary postural sway under two conditions, self-paced and simple reaction-time. Standing subjects were required to produce quick discrete shifts of the center of pressure (COP) forward. About 400-500 ms prior to the instructed COP shift, there were deviations of the COP in the opposite direction (backwards) accompanied by changes in the activation levels of several postural muscles. Under the reaction-time conditions, the timing of those early postural adjustments did not change (repeated measures MANOVA: p > 0.05) while its magnitude increased significantly (confirmed by repeated measures MANOVA: p < 0.05). These observations are opposite to those reported for anticipatory postural adjustments under simple reaction time conditions (a significant change in the timing without major changes in the magnitude). We conclude that there are two types of feed-forward postural adjustments. Early postural adjustments prepare the body for the planned action and/or expected perturbation. Some of these preparatory actions may be mechanically necessary. Later, anticipatory postural adjustments generate net forces and moments of force acting against those associated with the expected perturbation. Both types of adjustments fit well the referent configuration hypothesis, which offers a unified view on movement-posture control.