Tibialis anterior muscle fascicle dynamics adequately represent postural sway during standing balance

Three Hertz postural leg tremor impairs posture maintenance in multiple system atrophy-cerebellar type

BACKGROUND

Three-Hz postural leg tremor has recently been identified as highly prevalent in patients with the cerebellar type of multiple system atrophy, but its impact on posture maintenance remains poorly understood.

PATIENTS AND METHODS

Thirty-seven patients with spinocerebellar ataxia and 58 others with cerebellar type of multiple system atrophy were given Synapsys posturography examinations. Fifty-three healthy controls were also tested. Low, medium, and high-frequency sway were recorded to compute energy values. Frequency shift and postural strategy predominance were evaluated from the postural sway distributions, mainly from the proportions of higher frequency values among the total values. The trajectories of postural sway components were evaluated with the generalized additive mixed model. Distributions of the components and their relationships with falls and tremors were assessed through repeated measures correlation analysis.

RESULTS

As the test difficulty increased, the standard controls showed slight increases in the energy values at every frequency. Distributions of the higher frequency (>0.5 Hz) values increased escalatingly with test difficulty, illustrating frequency shifts and hip strategy predominance. Medium and high-frequency values were strongly and positively inter-correlated in normal stances, but this was not observed among the spinocerebellar ataxia or multiple system atrophy patients. Unlike normal stances, the proportion of medium frequency values was negatively related to the total value among the spinocerebellar ataxia and multiple system atrophy patients, implying a failure of frequency shift in response to perturbation. Medium frequency proportions were also inversely correlated with tremors among the multiple system atrophy patients.

CONCLUSIONS

The observed synchronized changes in medium and high-frequency postural sway indicate that they constitute a complete hip strategy for posture control. The strategy was rigid in those with spinocerebellar ataxia but completely disrupted in those with multiple system atrophy. Three Hertz postural leg tremors destabilize the ankle joints and interfere with postural adjustment among those with multiple system atrophy.

History-dependent muscle resistance to stretch remains high after small, posturally relevant pre-movements

The contributions of intrinsic muscle fiber resistance during mechanical perturbations to standing and other postural behaviors are unclear. Muscle short-range stiffness is known to vary depending on the current level and history of the muscle's activation, as well as the muscle's recent movement history; this property has been referred to as history dependence or muscle thixotropy. However, we currently lack sufficient data about the degree to which muscle stiffness is modulated across posturally relevant characteristics of muscle stretch and activation. We characterized the history dependence of muscle's resistance to stretch in single, permeabilized, activated, muscle fibers in posturally relevant stretch conditions and activation levels. We used a classic paired muscle stretch paradigm, varying the amplitude of a 'conditioning' triangular stretch-shorten cycle followed by a 'test' ramp-and-hold imposed after a variable inter-stretch interval. We tested low (<15%), intermediate (15-50%) and high (>50%) muscle fiber activation levels, evaluating short-range stiffness and total impulse in the test stretch. Muscle fiber resistance to stretch remained high at conditioning amplitudes of <1% optimal fiber length, L0, and inter-stretch intervals of >1 s, characteristic of healthy standing postural sway. An ∼70% attenuation of muscle resistance to stretch was reached at conditioning amplitudes of >3% L0 and inter-stretch intervals of <0.1 s, characteristic of larger, faster postural sway in balance-impaired individuals. The thixotropic changes cannot be predicted solely on muscle force at the time of stretch. Consistent with the disruption of muscle cross-bridges, muscle resistance to stretch during behavior can be substantially attenuated if the prior motion is large enough and/or frequent enough.

Mental fatigue impairs physical performance but not the neural drive to the muscle: a preliminary analysis

Mental fatigue (MF) does not only affect cognitive but also physical performance. This study aimed to explore the effects of MF on muscle endurance, rate of perceived exertion (RPE), and motor units' activity. Ten healthy males participated in a randomised crossover study. The subjects attended two identical experimental sessions separated by 3 days with the only difference of a cognitive task (incongruent Stroop task [ST]) and a control condition (watching a documentary). Perceived MF and motivation were measured for each session at baseline and after each cognitive task. Four contractions at 20% of maximal voluntary contraction (MVIC) were performed at baseline, after each cognitive and after muscle endurance task while measuring motor units by high-density surface electromyography. Muscle endurance until failure at 50% of MVIC was measured after each cognitive task and the RPE was measured right after failure. ST significantly increased MF (p = 0.001) reduced the motivation (p = 0.008) for the subsequent physical task and also impaired physical performance (p = 0.044). However, estimates of common synaptic inputs and motor unit discharge rates as well as RPE were not affected by MF (p > 0.11). In conclusion, MF impairs muscle endurance and motivation for the physical task but not the neural drive to the muscle at any frequency bands. Although it is physiologically possible for mentally fatigued subjects to generate an optimal neuromuscular function, the altered motivation seems to limit physical performance. Preliminarily, our results suggest that the corticospinal pathways are not affected by MF.

Corticospinal activity during a single-leg stance in people with chronic ankle instability

PURPOSE

The aim of the study was to determine whether corticospinal excitability and inhibition of the tibialis anterior during single-leg standing differs among individuals with chronic ankle instability (CAI), lateral ankle sprain copers, and healthy controls.

METHODS

Twenty-three participants with CAI, 23 lateral ankle sprain copers, and 24 healthy control participants volunteered. Active motor threshold (AMT), normalized motor-evoked potential (MEP), and cortical silent period (CSP) were evaluated by transcranial magnetic stimulation while participants performed a single-leg standing task.

RESULTS

Participants with CAI had significantly longer CSP at 100% of AMT and lower normalized MEP at 120% of AMT compared to lateral ankle sprain copers (CSP_100%: p = 0.003; MEP_120%: p = 0.044) and controls (CSP_100%: p = 0.041; MEP_120%: p = 0.006).

CONCLUSION

This investigation demonstrate altered corticospinal excitability and inhibition of the tibialis anterior during single-leg standing in participants with CAI. Further research is needed to examine the effects of corticospinal maladaptations to motor control of the tibial anterior on postural control performance in those with CAI.

Anatomical Variations of the Tibialis Anterior Tendon Insertion: An Updated and Comprehensive Review

The tibialis anterior muscle originates on the lateral condyle of the tibia, on the upper two-thirds of the lateral surface of this bone, on the anterior surface of the interosseous membrane and on the deep surface of the fascia cruris. The distal attachment is typically at the medial cuneiform and first metatarsal. However, the tibialis anterior tendon can vary morphologically in both adults and fetuses. Different authors have created new classification systems for it. The main aim of this review is to present condensed information about the tibialis anterior tendon based on the available literature. Another aim is to compare classification systems and the results of previous studies.

Comparison of muscle activity, strength, and balance, before and after a 6-month training using the FIFA11+ program (part 2)

BACKGROUND

Sports injury prevention training programs have been reported to be effective in decreasing the incidence of injuries. The aim of this study was to evaluate the effects of a 6-month training period, using part 2 of the FIFA11+ program (the Fédération Internationale de Football), on the activation and strength of core and lower limb muscles and on static and dynamic balance performance.

STUDY DESIGN

Case series; level of evidence, 4.

METHODS

Eight college male soccer players, aged mean 20.4 ± 0.5 years old, completed the FIFA11+ program at least three times per week for 6 months. The following variables were measured both before and after the 6-month training program: activities of more than 30 muscles (core and lower limb muscles), measured using the standardized uptake values of 18F-fluorodeoxyglucose on positron emission tomography; isokinetic strength of the knee flexor and extensor and hip abductor muscles, measured at 60° s^-1; static balance over a 60-s period, measured using a gravicorder; and dynamic balance, measured using the star excursion balance test.

RESULTS

Training improved the activity levels of core (obliquus externus abdominis and erector spinae) and lower limb (tibialis anterior) muscles (p ≤ 0.03), corrected the between-limb difference in the activation of the semimembranosus and improved dynamic balance, with a greater training effect on the nondominant limb (p ≤ 0.02). Training also improved the knee flexor force of the nondominant lower limb (p = 0.02).

CONCLUSION

Routine performance of the FIFA11+ program can improve the activation of core and lower limb muscles, with a concomitant improvement in dynamic balance. These improvements could be beneficial in lowering the risk of sports-related injuries.

Differences in estimated persistent inward currents between ankle flexors and extensors in humans

Persistent inward currents (PICs) are responsible for amplifying motoneuronal synaptic inputs and contribute to generating normal motoneuron activation. Delta-F (ΔF) is a well-established method that estimates PICs in humans indirectly from firing patterns of individual motor units. Traditionally, motor unit firing patterns are obtained by manually decomposing electromyography (EMG) signals recorded through intramuscular electrodes (iEMG). A previous iEMG study has shown that in humans the elbow extensors have higher ΔF than the elbow flexors. In this study, EMG signals were collected from the ankle extensors and flexors using high-density surface array electrodes during isometric sitting and standing at 10-30% maximum voluntary contraction. The signals were then decomposed into individual motor unit firings. We hypothesized that comparable to the upper limb, the lower limb extensor muscles (soleus) would have higher ΔF than the lower limb flexor muscles [tibialis anterior (TA)]. Contrary to our expectations, ΔF was higher in the TA than the soleus during sitting and standing despite the difference in cohort of participants and body positions. The TA also had significantly higher maximum discharge rate than the soleus while there was no difference in rate increase. When only the unit pairs with similar maximum discharge rates were compared, ∆F was still higher in the TA than the soleus. Future studies will focus on investigating the functional significance of the findings.NEW & NOTEWORTHY With the use of high-density surface array electrodes and convolutive blind source separation algorithm, thousands of motor units were decomposed from the soleus and tibialis anterior muscles. Persistent inward currents were estimated under seated and standing conditions via delta-F (∆F) calculation, and the results showed that unlike the upper limb, the flexor has higher ∆F than the extensor in the lower limb. Future studies will focus on functional significance of the findings.

Analysis of center of mass acceleration and muscle activation in hemiplegic paralysis during quiet standing

Hemiplegic paralysis after stroke may augment postural instability and decrease the balance control ability for standing. The center of mass acceleration (COMacc) is considered to be an effective indicator of postural stability for standing balance control. However, it is less studied how the COMacc could be affected by the muscle activities on lower-limbs in post-stroke hemiplegic patients. This study aimed to examine the effects of hemiplegic paralysis in post-stroke individuals on the amplitude and structural variabilities of COMacc and surface electromyography (sEMG) signals during quiet standing. Eleven post-stroke hemiplegic patients and the same number of gender- and age-matched healthy volunteers participated in the experiment. The sEMG signals of tibialis anterior (TA) and lateral gastrocnemius (LG) muscles of the both limbs, and the COMacc in the anterior-posterior direction with and without visual feedback (VF vs. NVF) were recorded simultaneously during quiet standing. The sEMG and COMacc were analyzed using root mean square (RMS) or standard deviation (SD), and a modified detrended fluctuation analysis based on empirical mode decomposition (EMD-DFA). Results showed that the SD and the scale exponent α of EMD-DFA of the COMacc from the patients were significantly higher than the values from the controls under both VF (p < 0.01) and NVF (p < 0.001) conditions. The RMSs of TA and LG on the non-paretic limbs were significantly higher than those on paretic limbs (p < 0.05) for both the patients and controls (p < 0.05). The TA of both the paretic and non-paretic limbs of the patients showed augmented α values than the TA of the controls (p < 0.05). The α of the TA and LG of non-paretic limbs, and the α of COMacc were significantly increased after removing visual feedback in patients (p < 0.05). These results suggested an increased amplitude variability but decreased structural variability of COMacc, associated with asymmetric muscle contraction between the paretic and the non-paretic limbs in hemiplegic paralysis, revealing a deficiency in integration of sensorimotor information and a loss of flexibility of postural control due to stroke.

Corticospinal excitability of tibialis anterior and soleus differs during passive ankle movement

The purpose of this study was to assess corticospinal excitability of soleus (SOL) and tibialis anterior (TA) at a segmental level during passive ankle movement. Four experimental components were performed to assess the effects of passive ankle movement and muscle length on corticospinal excitability (MEP/M_max) at different muscle lengths, subcortical excitability at the level of lumbar spinal segments (LEP/M_max), intracortical inhibition (SICI) and facilitation (ICF), and H-reflex in SOL and TA. In addition, the degree of fascicle length changes between SOL and TA was assessed in a subpopulation during passive ankle movement. Fascicles shortened and lengthened with joint movement during passive shortening and lengthening of SOL and TA to a similar degree (p < 0.001). Resting motor threshold was greater in SOL compared to TA (p ≤ 0.014). MEP/M_max was facilitated in TA during passive shortening relative to the static position (p ≤ 0.023) and passive lengthening (p ≤ 0.001), but remained similar during passive ankle movement in SOL (p ≥ 0.497), regardless of muscle length at the point of stimulus (p = 0.922). LEP/M_max (SOL: p = 0.075, TA: p = 0.071), SICI (SOL: p = 0.427, TA: p = 0.540), and ICF (SOL: p = 0.177, TA: p = 0.777) remained similar during passive ankle movement. H-reflex was not different across conditions in TA (p = 0.258), but was reduced during passive lengthening compared to shortening in SOL (p = 0.048). These results suggest a differential modulation of corticospinal excitability between plantar and dorsiflexors during passive movement. The corticospinal behaviour observed might be mediated by an increase in corticospinal drive as a result of reduced afferent input during muscle shortening and appears to be flexor-biased.

Age-related changes in leg proprioception: implications for postural control

In addition to being a prerequisite for many activities of daily living, the ability to maintain steady upright standing is a relevant model to study sensorimotor integrative function. Upright standing requires managing multimodal sensory inputs to produce finely tuned motor output that can be adjusted to accommodate changes in standing conditions and environment. The sensory information used for postural control mainly arises from the vestibular system of the inner ear, vision, and proprioception. Proprioception (sense of body position and movement) encompasses signals from mechanoreceptors (proprioceptors) located in muscles, tendons, and joint capsules. There is general agreement that proprioception signals from leg muscles provide the primary source of information for postural control. This is because of their exquisite sensitivity to detect body sway during unperturbed upright standing that mainly results from variations in leg muscle length induced by rotations around the ankle joint. However, aging is associated with alterations of muscle spindles and their neural pathways, which induce a decrease in the sensitivity, acuity, and integration of the proprioceptive signal. These alterations promote changes in postural control that reduce its efficiency and thereby may have deleterious consequences for the functional independence of an individual. This narrative review provides an overview of how aging alters the proprioceptive signal from the legs and presents compelling evidence that these changes modify the neural control of upright standing.

Tibialis anterior tendinous tissue plays a key role in energy absorption during human walking

The elastic tendinous tissues of distal lower limb muscles can improve the economy of walking and running, amplify the power generated by a muscle and absorb energy. This paper explores the behaviour of the tibialis anterior (TA) muscle and its tendinous tissue during gait, as it absorbs energy during contact and controls foot position during swing. Simultaneous measurements of ultrasound, surface electromyography and 3D motion capture with musculoskeletal modelling from 12 healthy participants were recorded as they walked at preferred and fast walking speeds. We quantified the length changes and velocities of the TA muscle-tendon unit (MTU) and its fascicles across the stride at each speed. Fascicle length changes and velocities were relatively consistent across speeds, although the magnitude of fascicle length change differed between the deep and superficial regions. At contact, when the TA is actively generating force, the fascicles remained relatively isometric as the MTU actively lengthened, presumably stretching the TA tendinous tissue and absorbing energy. This potentially protects the muscle fibres from damage during weight acceptance and allows energy to be returned to the system later in the stride. During early swing, the fascicles and MTU both actively shortened to dorsiflex the foot, clearing the toes from the ground; however, at the fast walking velocity, the majority of shortening occurred through tendinous tissue recoil, highlighting its role in accelerating ankle dorsiflexion to power rapid foot clearance in swing.

Significance of epimuscular myofascial force transmission under passive muscle conditions

In the past 20 yr, force transmission via connective tissue linkages at the muscle belly surface, called epimuscular myofascial force transmission, has been studied extensively. In this article, the effects of epimuscular linkages under passive muscle conditions are reviewed. Several animal studies that included direct (invasive) measurements of force transmission have shown that different connective tissue structures serve as an epimuscular pathway and that these tissues have sufficient stiffness, especially at supraphysiological muscle lengths and relative positions, to transmit substantial passive forces (up to 15% of active optimal force). Exact values of lumped tissue stiffness for different connective tissue structures have not yet been estimated. Experiments using various imaging techniques (ultrasound, MRI, shear wave elastography) have yielded some, but weak, evidence of epimuscular myofascial force transmission for passive muscles in humans. At this point, the functional consequences of epimuscular pathways for muscle and joint mechanics in the intact body are still unknown. Potentially, however, these pathways may affect sensory feedback and, thereby, neuromuscular control. In addition, altered epimuscular force transmission in pathological conditions may also contribute to changes in passive range of joint motion.

A reduction in compliance or activation level reduces residual force depression in human tibialis anterior

AIM

We investigated if residual force depression (rFD) is present during voluntary fixed-end contractions of human tibialis anterior (TA) and whether reducing TA's activation level after active shortening could reduce rFD.

METHODS

Ten participants performed fixed-end dorsiflexion contractions to a low, moderate or high level while electromyography (EMG), dorsiflexion force and TA ultrasound images were recorded. Contractions were force- or EMG-matched and after the low or high contraction level was attained, participants respectively increased or decreased their force/EMG to a moderate level. Participants also performed moderate level contractions while the TA muscle-tendon unit (MTU) was lengthened during the force/EMG rise to the reference MTU length.

RESULTS

Equivalent fascicle shortening over moderate and low to moderate level contractions did not alter EMG (P = 0.45) or dorsiflexion force (P = 0.47) at the moderate level. Greater initial fascicle shortening magnitudes (1.7 mm; P ≤ 0.01) to the high contraction level did not alter EMG (P = 0.45) or dorsiflexion force (P = 0.30) at the subsequent moderate level compared with moderate level contractions. TA MTU lengthening during the initial force/EMG rise reduced TA fascicle shortening (-2.5 mm; P ≤ 0.01), which reduced EMG (-3.9% MVC; P < 0.01) and increased dorsiflexion force (3.7% MVC; P < 0.01) at the moderate level compared with fixed-end moderate level contractions.

CONCLUSION

rFD is present during fixed-end dorsiflexion contractions because fascicles actively shorten as force/EMG increases and rFD can be reduced by reducing the effective MTU compliance. A reduction in muscle activation level also reduces rFD by potentially triggering residual force enhancement-related mechanisms as force drops and some fascicles actively lengthen.

Motor learning in a complex balance task and associated neuroplasticity: a comparison between endurance athletes and nonathletes

Studies suggested that motor expertise is associated with functional and structural brain alterations, which positively affect sensorimotor performance and learning capabilities. The purpose of the present study was to unravel differences in motor skill learning and associated functional neuroplasticity between endurance athletes (EA) and nonathletes (NA). For this purpose, participants had to perform a multimodal balance task (MBT) training on 2 sessions, which were separated by 1 wk. Before and after MBT training, a static balance task (SBT) had to be performed. MBT-induced functional neuroplasticity and neuromuscular alterations were assessed by means of functional near-infrared spectroscopy (fNIRS) and electromyography (EMG) during SBT performance. We hypothesized that EA would showed superior initial SBT performance and stronger MBT-induced improvements in SBT learning rates compared with NA. On a cortical level, we hypothesized that MBT training would lead to differential learning-dependent functional changes in motor-related brain regions [such as primary motor cortex (M1)] during SBT performance. In fact, EA showed superior initial SBT performance, whereas learning rates did not differ between groups. On a cortical level, fNIRS recordings (time × group interaction) revealed a stronger MBT-induced decrease in left M1 and inferior parietal lobe (IPL) for deoxygenated hemoglobin in EA. Even more interesting, learning rates were correlated with fNIRS changes in right M1/IPL. On the basis of these findings, we provide novel evidence for superior MBT training-induced functional neuroplasticity in highly trained athletes. Future studies should investigate these effects in different sports disciplines to strengthen previous work on experience-dependent neuroplasticity.NEW & NOTEWORTHY Motor expertise is associated with functional/structural brain plasticity. How such neuroplastic reorganization translates into altered motor learning processes remains elusive. We investigated endurance athletes (EA) and nonathletes (NA) in a multimodal balance task (MBT). EA showed superior static balance performance (SBT), whereas MBT-induced SBT improvements did not differ between groups. Functional near-infrared spectroscopy recordings revealed a differential MBT training-induced decrease of deoxygenated hemoglobin in left primary motor cortex and inferior parietal lobe between groups.

Muscle spindles in human tibialis anterior encode muscle fascicle length changes

Muscle spindles provide exquisitely sensitive proprioceptive information regarding joint position and movement. Through passively driven length changes in the muscle-tendon unit (MTU), muscle spindles detect joint rotations because of their in-parallel mechanical linkage to muscle fascicles. In human microneurography studies, muscle fascicles are assumed to follow the MTU and, as such, fascicle length is not measured in such studies. However, under certain mechanical conditions, compliant structures can act to decouple the fascicles, and, therefore, the spindles, from the MTU. Such decoupling may reduce the fidelity by which muscle spindles encode joint position and movement. The aim of the present study was to measure, for the first time, both the changes in firing of single muscle spindle afferents and changes in muscle fascicle length in vivo from the tibialis anterior muscle (TA) during passive rotations about the ankle. Unitary recordings were made from 15 muscle spindle afferents supplying TA via a microelectrode inserted into the common peroneal nerve. Ultrasonography was used to measure the length of an individual fascicle of TA. We saw a strong correlation between fascicle length and firing rate during passive ankle rotations of varying rates (0.1-0.5 Hz) and amplitudes (1-9°). In particular, we saw responses observed at relatively small changes in muscle length that highlight the sensitivity of the TA muscle to small length changes. This study is the first to measure spindle firing and fascicle dynamics in vivo and provides an experimental basis for further understanding the link between fascicle length, MTU length, and spindle firing patterns.NEW & NOTEWORTHY Muscle spindles are exquisitely sensitive to changes in muscle length, but recordings from human muscle spindle afferents are usually correlated with joint angle rather than muscle fascicle length. In this study, we monitored both muscle fascicle length and spindle firing from the human tibialis anterior muscle in vivo. Our findings are the first to measure these signals in vivo and provide an experimental basis for exploring this link further.

Doublet potentiation in the triceps surae is limited by series compliance and dynamic fascicle behavior

Activation of skeletal muscle twice in quick succession results in nonlinear force summation (i.e., doublet potentiation). The force contributed by a second activation is typically of augmented amplitude, longer in duration, and generated at a greater rate. The purpose of this study was to examine force summation in a muscle attached to a compliant tendon, where considerable internal shortening occurs during a fixed-end contraction. The triceps surae of 21 ( Experiment 1) and 9 ( Experiment 2) young adults were maximally activated with doublet stimulation of different interstimulus intervals (ISIs) (5-100 ms) at several muscle lengths. Ultrasound images acquired from lateral gastrocnemius and soleus muscles allowed quantification of dynamic fascicle behavior. Force summation was muscle length dependent. Force augmentation was limited to a short muscle length. Lateral gastrocnemius and soleus fascicles underwent large amounts of active shortening and achieved high velocities in response to doublet stimulation, dynamics unfavorable for force production. Summation amplitude and the sensitivity of summation to ISI were dramatically depressed in the triceps surae after comparison to muscles with less fixed-end compliance. We propose that the internal shortening permitted by high series compliance limited force augmentation by offsetting and/or interfering with activation and cross-bridge processes driving augmentation. High series compliance may also reduce the sensitivity of the summated response to ISI, an assertion supported by predictions from a Hill-type muscle model. These muscles may exhibit greater force augmentation during more accustomed stretch-shorten tasks (i.e., hopping), where the compliance of the Achilles tendon actually enables near-isometric fascicle behavior.

Evidence for beta corticomuscular coherence during human standing balance: Effects of stance width, vision, and support surface

The role of the cerebral cortex in maintaining human standing balance remains unclear. Beta corticomuscular coherence (CMC) provides a measure of communication between the sensory-motor cortex and muscle, but past literature has not demonstrated significant beta CMC during human stance. This study evaluated the effects of stance width, vision, and surface compliance on beta CMC during human stance using methods to enhance sensitivity to CMC. Ten healthy, young adults stood for three 60-s trials in each of a wide or narrow stance width while on a firm surface and in narrow stance on a foam surface, each with eyes open or closed. Beta CMC was calculated between contralateral electroencephalographic and electromyographic recordings. Electromyography was recorded from bilateral tibialis anterior and gastrocnemius lateralis muscles. CMC magnitude was defined as the average integrated area of coherence spectrum above a significance threshold. Measures of center-of-pressure (COP) sway were derived from force plates under the subjects' feet. Results of CMC from four muscles across six stance conditions (a total of 24 combinations) demonstrated significant average CMC magnitude from every subject in 20 combinations and significant average CMC magnitude in nine of 10 subjects in the remaining four combinations. The CMC magnitude was significantly larger in the wide-stance condition than in the narrow-stance condition with eyes open. No significant differences were detected when comparing eyes-open to eyes-closed conditions or when comparing firm- to foam-surface conditions. Correlations between CMC magnitude and COP sway elicited some significant relationships, but there was no consistent direction or pattern of correlation based on muscle or stance condition. Results demonstrate that significant beta CMC is evident during human standing balance, and that beta CMC is responsive to changes in mechanical, but not visual or surface, conditions.

Comparison of the electromyographic activity of the tibialis anterior and gastrocnemius in stroke patients and healthy subjects during squat exercise

[Purpose] The purpose of this study was to compare the EMG activity of the tibialis anterior (TA) and gastrocnemius (GCM) during the downward, maintenance, and upward phases of the squat exercise and during passive ankle dorsiflexion range of motion between stroke patients and healthy subjects. [Subjects] Fifteen hemiplegic (8 males, 7 females) and 15 healthy subjects (4 males, 11 females) volunteered for this study. [Methods] All subjects performed a double-leg squat exercise with the knee joint flexed to 30°. Surface electromyography (EMG) signals were recorded from the TA and GCM on the paretic or nondominant side. Passive ankle dorsiflexion range of motion (DF PROM) was measured using a goniometer in the knee-extended prone position. [Results] In the downward and maintenance phases, TA activity was significantly higher in stroke patients compared with healthy subjects. In the upward phase, GCM activity was significantly lower in stroke patients compared with healthy subjects. Ankle DF PROM was significantly lower in stroke patients compared with healthy subjects. [Conclusion] The observed EMG patterns should be taken into consideration to inform and enhance therapy for stroke patients.

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.

The control of upright stance is a challenging task since the objective is to maintain the equilibrium of an intrinsically unstable biomechanical system. Somatosensory information is used by the central nervous system to modulate muscle contraction, which prevents the body from falling. While the visual and vestibular systems also provide important additional sensory information, a human being with only somatosensory inputs is able to maintain an upright stance. In this study, we used a biologically-based large-scale neuromusculoskeletal model driven only by somatosensory feedback to investigate human postural control from a neurophysiological point of view. No neural structures above the spinal cord were included in the model. The results showed that the model based on a spinal control of posture can reproduce several neuromechanical outcomes previously reported in the literature, including an intermittent muscle activation. Since this intermittent muscular recruitment is an emergent property of this spinal-like controller, we argue that the so-called intermittent control of upright stance might be produced by an interplay between spinal cord properties and modulated sensory inflow.

Reciprocal activation of gastrocnemius and soleus motor units is associated with fascicle length change during knee flexion

While medial gastrocnemius (MG) and soleus (SOL) are considered synergists, they are anatomically exclusive in that SOL crosses only the ankle, while MG crosses both the knee and ankle. Due to the force-length properties of both active and passive structures, activation of SOL and MG must be constantly regulated to provide the required joint torques for any planned movement. As such, the aim of this study was to investigate the neural regulation of MG and SOL when independently changing their length by changing only the knee joint angle, thus exclusively altering the length of MG fibers. MG and SOL motor units (MU) were recorded intramuscularly along with ultrasound imaging of MG and SOL fascicle lengths, while moving the knee through 60° of rotation and maintaining a low level of voluntary plantar flexor torque. The results showed a reciprocal activation of MG and SOL as the knee was moved into flexion and extension. A clear reduction in MG MU firing rates occurred as the knee was flexed (MG fascicles shortening), with de-recruitment of most MG MU occurring at close to full knee flexion. A concomitant increase in SOL MU activity was observed while no change in the length of its fascicles was found. The opposite effects were found when the knee was moved into extension. A strong correlation (ICC = 0.78) was found between the fascicle length at which MG MUs were de-recruited and subsequently re-recruited. This was stronger than the relationship of de-recruitment and re-recruitment with knee angle (ICC = 0.52), indicating that in this instance, muscle fascicle length rather than joint angle is more influential in regulating MG recruitment. Such a reciprocal arrangement like the one presented here for SOL and MG is essential for human voluntary movements such as walking or cycling.