Characteristics of the Force Applied to a Pedal During Human Pushing Efforts: Emergent Linearity

The force seated humans exert on a translationally fixed pedal (foot force) may be directed at any angle because the fixed distance between the seat and the pedal axis kinematically constrains the lower limb. The authors' objective in the present work was to characterize such force. Participants (N = 7) generated force with their lower limb by pushing against the pedal in the most comfortable manner. Pushing efforts were repeated randomly 3 times at each of 97 sagittal-plane pedal axis positions and 10 additional times in 9 of those positions (2,895 total pushes). In 87% of the pushes, the measured sagittal-plane force exerted on the pedal by the foot changed magnitude and direction through time, such that the path of the head of the force vector traced a straight line. The linearity of the foot force paths reflected directional invariance in the changes of the foot force vector as the magnitude of the vector increased. The orientation of those linear force paths varied with limb posture in a similar manner across participants. The authors conclude mat the emergent linearity of the force path is consistent with minimization of path length in foot force space. Alternatively, the linearity of the force paths suggests a motor control strategy that simplifies the control to a monoparametric form.     

Directional Variability of the Isometric Force Vector Produced by the Human Hand in Multijoint Planar Tasks

Numerous studies have examined control of force magnitude, but relatively little research has considered force direction control. The subjects applied isometric forces to a handle and the authors compared within-trial variability when force is produced in different directions. The standard deviation of the force parallel to the prescribed direction of force production increased linearly with the targeted force level, as did the standard deviation of the force perpendicular to the instructed direction. In contrast, the standard deviation of the angle of force production decreased with increased force level. In the 4 (of 8) instructed force directions where the endpoint force was generated due to a joint torque in only 1 joint (either the shoulder or elbow) the principal component axes in force space were well aligned with the prescribed direction of force production. In the other directions, the variance was approximately equal along the 2 force axes. The variance explained by the first principal component was significantly larger in torque space compared to the force space, and mostly corresponded to positive correlation between the joint torques. Such coordinated changes suggest that the torque variability was mainly due to the variability of the common drive to the muscles serving 2 joints, although this statement needs to be supported by direct studies of muscle activation in the future.     

Sensori-motor integration during stance: Time adaptation of control mechanisms on adding or removing vision

Sudden addition or removal of visual information can be particularly critical to balance control. The promptness of adaptation of stance control mechanisms is quantified by the latency at which body oscillation and postural muscle activity vary after a shift in visual condition. In the present study, volunteers stood on a force platform with feet parallel or in tandem. Shifts in visual condition were produced by electronic spectacles. Ground reaction force (center of foot pressure, CoP) and EMG of leg postural muscles were acquired, and latency of CoP and EMG changes estimated by t-tests on the averaged traces. Time-to-reach steady-state was estimated by means of an exponential model. On allowing or occluding vision, decrements and increments in CoP position and oscillation occurred within about 2 s. These were preceded by changes in muscle activity, regardless of visual-shift direction, foot position or front or rear leg in tandem. These time intervals were longer than simple reaction-time responses. The time course of recovery to steady-state was about 3 s, shorter for oscillation than position. The capacity of modifying balance control at very short intervals both during quiet standing and under more critical balance conditions speaks in favor of a necessary coupling between vision, postural reference, and postural muscle activity, and of the swiftness of this sensory reweighing process.     

The Organization of Multisegmental Pulls Made by Standing Humans: II. Submaximal Pulls

Previously, an autonomous oscillator model with three parameters was derived that describes the relationship between anterior-posterior center of mass motions and pulling force for near-maximal, bimanual pulls made by standing subjects (Michaels, Lee, & Pai, 1993). The present study evaluated the extent to which a full range of pulling forces could be fit by the model and how the model's three parameters changed with intended pulling force. How much variation in force each parameter could contribute was determined by simulating the model. Qualitative and quantitative analyses of pulls made by 6 well-practiced subjects at 5%, 10%, 20%, 40%, 60%, 80%, and 95% of their maximum pulling force revealed that the model holds well, except for the least forceful pulls of some subjects. Two parameters appeared to be controlled; one, related to the position of the center of pressure, varied most among less forceful pulls; the second, related to the position of the center of mass at the time of handle-force onset, varied most among more forceful pulls. How these parameters might be set is discussed.     

Perception of large change in distribution of heel pressure during backward leaning

We investigated the perception of the large change in distribution of heel pressure during backward leaning. Subjects were 12 healthy adults who reported perceiving a large change in distribution of heel pressure by a handheld switch while leaning voluntarily backward on a sole pressure analyzer and on a heel force plate. The large change was indicated at the center of heel pressure. Morphological features of the foot were measured on an X-ray film. The position of heel pressure center and the morphological locations were represented as relative distance (%) from the hindmost point of the heel, where foot length represented 100%. Center of heel pressure changed largely during backward leaning, and the position at which large change occurred was the same as that of the peak of the distribution. Large change in distribution of heel pressure was perceived at a position 1.3% posterior from that at which the large change actually occurred. The correlation between perceived and actual positions was significant (r = .91). Significant correlations were found between position of a large change of center and locations of heel pressure of both the lateral process of the calcaneal tuberosity and the top of the talar trochlea (r = .86; r = .71, respectively). The results indicate that subjects accurately perceive large changes in distribution of heel pressure and that the morphological features of the foot contribute to these changes.     

Perceived standing position after reduction of foot-pressure sensation by cooling the sole

We investigated the influence of the reduction of foot-pressure sensation by cooling the sole of the foot, at 1 degree C for 30 or 40 minutes, on the perception of standing position varied in the anteroposterior direction. The subjects were 16 healthy undergraduates. Firstly, for 4 of the subjects, cooling the sole of the foot decreased sensory information from the mechanoreceptors in the sole, by testing for an increase in the threshold for two-point discrepancy discrimination on the sole of the foot and for the disappearance of postural change with vibration to the sole. Next, the perception of standing position was measured by reproduction of a given standing reference position involving forward or backward leaning under both normal and cooled conditions of the feet. Standing position was varied in relation to the location of the center of foot pressure, defined as distance from the heel in percentage of the length of the foot. The reference positions, representing various locations of the center of foot pressure, were set at 10% increments from 20% to 80% of the length of the foot. With eyes closed, the subject first experienced the reference position and then attempted to reproduce it. The mean location of the center of foot pressure in the quiet standing posture was 45.7%. At the 40%, 50%, and 60% reference positions, those closest to quiet standing, absolute errors of reproduction were significantly larger than at other reference positions in both the normal and the cooled conditions. They were significantly larger in the cooled than in the normal condition. The 50% and 60% reference positions were reproduced significantly further forward in the cooled than in the normal condition. These results may be explained as due to an absence of marked changes in sensory information from both muscular activity and foot pressure when moving to reference positions close to the quiet standing posture.     

Standing balance of children with developmental coordination disorder under altered sensory conditions

Maintenance of standing balance requires that sensory inputs be organized with the motor system. Current data regarding the influence of sensory inputs on standing balance in children with developmental coordination disorder (DCD) are limited. This study compared the influence of sensory organization and each sensory input on the standing stability between a group of 20 children, 4-6 years old, with DCD and an age- and gender-matched control group of 20 children. Three types of visual inputs (eyes open, eyes closed, or unreliable vision) and two types of somatosensory inputs (fixed or compliant foot support) were varied factorially to yield six sensory conditions. Standing stability was measured with a Kistler force plate for 30s and expressed as the center of pressure sway area. The results showed that the standing stability of the children with DCD was significantly poorer than that of the control children under all sensory conditions, especially when the somatosensory input was unreliable (compliant foot support) compared to when it was reliable (fixed foot support). The effectiveness of an individual sensory system, when it was the dominant source of sensory input, did not significantly differ between the groups. The results suggest that children with DCD experience more difficulty coping with altered sensory inputs, and that such difficulty is more likely due to a deficit in sensory organization rather than compromised effectiveness of individual sensory systems.     

Effect of localized muscle fatigue on vertical ground reaction forces and ankle joint motion during running

The purpose of this study was to examine the changes in the vertical ground reaction force (VGRF) and ankle joint motion during the first 50% of the stance phase of running following fatiguing exercise of either the dorsiflexors or the invertors of the foot. VGRFs, sagittal and rearfoot kinematic data were collected from 11 female recreational runners running at 2.9 m/second on a treadmill prior to and following localized muscle fatigue of either the invertors or dorsiflexors of the right foot. Loading rate of the impact peak force significantly increased following fatiguing exercise of the dorsiflexors, while the peak magnitudes of the impact and push-off forces remained unchanged. There were significant decreases in dorsiflexion at heel contact, but no significant difference in any rearfoot motion parameters tested following dorsiflexor fatigue. Following fatiguing exercise of the invertors, impact peak magnitude, push-off peak magnitude and the rate of decline of the impact peak force significantly decreased; there was no change in the loading rate of the impact peak force. Invertor fatigue also resulted in a less inverted foot position at heel contact, but there were no significant differences in any other kinematic parameters tested. The results demonstrate that localized muscle fatigue of either the invertors or dorsiflexors can have a significant effect on the loading rates, peak magnitudes and ankle joint motion seen during running. These changes, due to localized muscle fatigue, may play a role in many common lower extremity running injuries.     

Effect of Sensory Deprivation on Maximal Force Abilities from Local to Non-local Digits

Abstract

The present study investigates the effect of sensory deprivation of the index and middle finger on motor function of all digits during maximal voluntary force production tasks. A total of 27 subjects performed maximal isometric pressing tasks by using different instructed finger combinations. Subjects completed the same tasks in two visits: a control visit when they had normal sensory feedback in all fingers, and an anesthesia visit when digital nerve blocks were performed on their right index and middle fingers. We evaluated three aspects of motor adaptation on both local (anesthetized) and non-local (non-anesthetized) digits during maximal force production: (1) task-relevant and overall force magnitude, (2) force directional application, and (3) digital individuation and force sharing. Our results indicate that selective digital anesthesia resulted in decreased maximal force magnitude, changed direction of force production, and significant changes extended to non-local digits. The motor weakness and inefficiency revealed in the non-local digits implies that sensory information from each digit can be shared across the digits to assist motor execution within the same hand.     

Premovement posture and focal movement velocity affects on postural responses accompanying rapid arm movement

Coordination of intentional upper limb movement concurrent with supporting postural activity was investigated in adult males under varying task conditions. Seven subjects performed a 60 deg rapid elbow flexion (focal movement) to a target in movement times of 170, 195, or 220 ms while standing. Measurement of center of pressure via a force platform revealed that subjects adopted individual premovement postural preferences such that locus of center of pressure resided in one predominant quadrant of the foot. Each premovement postural preference was accompanied by one most common postural muscle onset sequence as indicated by bilateral EMG analysis of rectus femoris and biceps femoris. In addition, onset times for postural muscles exhibiting anticipatory postural activity occurred earlier relative to biceps branchii as focal movement velocity increased. The finding that each premovement postural condition was accompanied by one particular postural muscle onset sequence suggested that postural synergies were flexibly organized with respect to onset sequence.     

Characteristics of finger force production during one- and two-hand tasks

Relations among finger forces were studied during one-hand and two-hand isometric maximal force production tasks in right- and left-handers. We particularly focused on the phenomena of force deficit during one-hand multi-finger tasks and of bilateral force deficit during two-hand tasks. Ten healthy subjects (five of them left-handed) performed maximal voluntary force production tasks with different finger combinations involving fingers of one of the hands or of both hands together. In one-hand tasks, finger enslaving (forces produced by fingers that were not instructed to produce force) was larger in the dominant hand, while force deficit (drop in individual finger peak force during multi-finger tasks) showed no differences between the hands. An additional drop in finger forces was seen in two-hand tests (bilateral deficit). The magnitude of the bilateral deficit for a hand was larger for tasks involving fewer fingers within the hand and more fingers in the other hand, with a ceiling effect. Smaller bilateral deficit was seen in tasks involving symmetrical finger combinations. In two-hand tasks that could potentially lead to the generation of large total moments in the frontal plane, the hand that was expected to generate larger moments showed larger bilateral deficit, so that the magnitude of the total moment was reduced. These observations suggest that force deficit within a hand and bilateral deficit have different origins but their effects are combined at a certain level of the multi-finger control hierarchy. Bilateral deficit may display task dependence reflecting, in particular, the principle of minimization of secondary moments. A double-representation, mirror-image hypothesis is suggested to provide a neurophysiological basis for the observed patterns of bilateral deficit.     

Grip Force-Load Force Coupling Is Influenced by Altered Visual Feedback about Object Kinematics

Abstract

Recent evidence suggests that visual feedback influences the adjustment of grip force to the changing load force exerted by a grasped object as it is manipulated. The current project investigated how visual feedback of object kinematics affects the coupling of grip force to load force by scaling the apparent displacements of the object viewed in virtual reality. Participants moved the object to manually track a moving virtual target. The predictability of the changing load force exerted by the object was also manipulated by altering the nature of target trajectories (and therefore the nature of object motions). When apparent object displacements increased in magnitude, grip force became more tightly coupled to load force over time. Furthermore, when load force variations were less predictable, the magnitude of apparent object displacements affected the relative degree of continuous versus intermittent coupling of grip force to load force. These findings show that visual feedback of object motion affects the ongoing dynamical coupling between grip force control and load force experienced during manipulation of a grasped object.     

The representation of predictive force control and internal forward models: evidence from lesion studies and brain imaging

Force control on the basis of prediction avoids time delays from sensory feedback during motor performance. Thus, self-produced loads arising from gravitational and inertial forces during object manipulation can be compensated for by simultaneous anticipatory changes in grip force. It has been suggested that internal forward models predict the consequences of our movements, so that grip force can be programmed in anticipation of movement-induced loads. The cerebellum has been proposed as the anatomical correlate of such internal models. Here, we present behavioural data from patients with cerebellar damage and data from brain imaging in healthy subjects further elucidating the role of the cerebellum in predictive force control. Patients with cerebellar damage exhibited clear deficits in the coupling between grip force and load. A positron-emission-tomography (PET) paradigm that separated the process of the grip force/load coupling from the isolated production of similar grip forces and loads was developed. Interaction and conjunction analyses revealed a strong activation peak in the ipsilateral posterior cerebellum particularly devoted to the predictive coupling between grip force and load. Both approaches clearly demonstrate that the cerebellum plays a major role in force prediction that cannot be compensated for by other sensorimotor structures in case of cerebellar disease. However, evidence suggests that also extra-cerebellar structures may significantly contribute to predictive force control: (1) grip force/load coupling may also be impaired after cerebral and peripheral sensorimotor lesions, (2) a coupling-related activation outside the cerebellum was observed in our PET study, and (3) the scaling of the grip force level and the dynamic grip force coupling are dissociable aspects of grip force control.     

Asymmetrical stabilization and mobilization exploited during static single leg stance and goal directed kicking

The motor control properties of the right and left legs are dependent on the stabilization and mobilization features of the motor tasks. The current investigation examined the right and left leg control differences – interlateral asymmetries – during static single leg stance and dynamic goal directed kicking with an emphasis of the asymmetrical stabilization and mobilization components of movements. Ten young, healthy, right-leg preferred individuals with minimal kicking experience completed both tests on each limb. During static single leg stance, participants were requested to stand as still as possible with one leg in contact with a force platform. Interlateral asymmetries of the standing leg were quantified using postural variability measures of the center of pressure (COP) standard deviation in the anterior-posterior (SD-COP_AP) and medial-lateral (SD-COP_ML) directions, resultant COP length and velocity, and 95% COP elliptical area. During dynamic goal directed kicking, participants stood on two adjacent force platforms in a side-by-side foot position and kicked a soccer ball toward three different directions as soon as they received an auditory cue of kicking. Three targets were located −30°, 0° or 30° in front and 3.05 m away from the participants’ midline. Participants kicked the ball toward the targets with each of their feet. The vertical ground reaction force (vGRF) of the kicking leg was used to define the preparation (from above two standard deviations of vGRF baseline to toe-off) and swing (from toe-off to toe-return) phases of dynamic kicking. To determine the presence of interlateral asymmetries during dynamic kicking, the magnitude and timing of the anticipatory postural adjustments (APA) during the preparation phase of kicking were quantified using the lateral net COP (COPnet-ML) time series derived from both force platforms. Postural variability measures of the support leg and the kinematic joint range of motion (JROM) trajectories of the kicking leg were also used to examined interlateral asymmetries. During static stance, no between-leg significance was identified for all dependent measures of COP variability suggesting symmetrical stabilization. During the preparation phase of kicking, both right and left leg kicking exhibited a similar level of APA magnitude, although the left leg kicking was shown to reach its maximum APA magnitude earlier than the right leg. In the support leg role, the right leg showed greater COP variability in the ML direction as compared to the left support leg and greater COP variability was observed when kicking in the ipsilateral direction compared to the center and contralateral directions. For mobilization control, the left kicking leg showed greater JROM displacements at the distal (knee and ankle) joints and reduced JROM primarily with hip frontal plane movements compared to the right kicking leg. The reported interlateral asymmetries during kicking may reflect a behavioral adaptation that results in differential stabilization between the right and left legs. Overall, the findings suggest that novel tasks, such as dynamic goal directed kicking, appear to be more sensitive than static balance in identifying interlateral asymmetries.     

The limits of perceived magnitude: Comparison among individuals and among perceptual continua

Two models of perceived magnitude that focus on the concept of dynamic range (DR) for sensory systems are summarized and compared. The first asserts that, whereas the size of DR varies physically from one perceptual continuum to another, all are subjectively equal. The other asserts that, for any given continuum, whereas the size of DR varies over individual observers, the subjective range does not.A test of the second model is reported for perceived effort for a range of loads on a bicycle ergometer. Both DR and judgmental (magnitude estimation) range estimates were obtained for 30 subjects. The former exhibited the expected variability over subjects, but there was no support for the predicted invariance of the latter. However, it is concluded, first, that the finding constitutes disconfirming evidence for the second model only if it is assumed that magnitude estimates are proportional to perceived magnitude, an issue which itself remains in dispute, and second, that the testability of the second model is doubtful unless a measure can be identified that is a stateable function of perceived magnitude.     

Inter-limb coupling in individuals with transtibial amputation during bilateral stance is direction dependent

We investigated the control of upright standing in individuals with unilateral transtibial amputation (TTA) by assessing the inter-limb coupling and the coupling between the center of pressure beneath both limbs combined (COP_NET) and the center of pressure (COP) beneath the prosthetic limb and the intact limb. Twenty-one adults with TTA and eighteen unimpaired adults completed 90 s of standing on two parallel force plates. The inter-limb coupling and the coupling between the COP beneath each limb and the COP_NET were assessed by quantifying the synchronization of the COP signals. This included the number of epochs with synchronized signals, the total duration of signal synchronization and the relative phase and deviation phase between the signals. Additionally, magnitude and temporal characteristics of the COP displacements were quantified. Individuals with TTA exhibited looser inter-limb coupling in the anterior-posterior direction, characterized by more shifts between epochs with synchronized signals, shorter total duration of signal synchronization, less in-phase coordination patterns and a higher deviation phase between the two limbs, compared to unimpaired individuals. This coincided with a larger and more irregular postural sway in the TTA group. No group difference was observed in the mediolateral direction. The coupling between the COP_NET and the COP beneath the individual limbs was similarly direction dependent, and tighter for the intact side, suggesting that an intact limb-driven strategy was utilized.     

The 'extrapolated center of mass' concept suggests a simple control of balance in walking

Next to position x and velocity v of the whole body center of mass (CoM) the 'extrapolated center of mass' (XcoM) can be introduced: xi = chi + nu/omega 0, where omega 0 is a constant related to stature. Based on the inverted pendulum model of balance, the XcoM enables to formulate the requirements for stable walking in a relatively simple form. In a very simple walking model, with the effects of foot roll-over neglected, the trajectory of the XcoM is a succession of straight lines, directed in the line from center of pressure (CoP) to the XcoM at the time of foot contact. The CoM follows the XcoM in a more sinusoidal trajectory. A simple rule is sufficient for stable walking: at foot placement the CoP should be placed at a certain distance behind and outward of the XcoM at the time of foot contact. In practice this means that a disturbance which results in a CoM velocity change Deltav can be compensated by a change in foot position (CoP) equal to Deltav/omega 0 in the same direction. Similar simple rules could be formulated for starting and stopping and for making a turn.     

Effects of plantar cutaneo-muscular and tendon vibration on posture and balance during quiet and perturbed stance

Modulation of lower limb somatosensory information by tendon or plantar vibration produces directionally specific, vibration-induced falling reactions that depend on the tendon or the region of the sole that is vibrated. This study characterized the effects of different patterns of plantar cutaneo-muscular vibration and bilateral Achilles tendon vibration (ATV) on the postural strategies observed during quiet and perturbed stance. Twelve healthy young participants stood barefooted, with their vision blocked, on two sets of plantar vibrators placed on two AMTI force plates embedded in a moveable support surface. Two other vibrators were positioned over the Achilles tendons. Participants were randomly exposed to different patterns of plantar cutaneo-muscular and ATV. Tilts of the support surface in the toes-up (TU) and toes-down (TD) directions were given 5-8 s after the beginning of vibration. Body kinematics in 3D and ground reaction forces were recorded. Bilateral ATV applied with or without rearfoot vibration (RFV) during quiet stance resulted in a whole-body backward leaning accompanied by an increase in trunk extension and hip and knee flexion. RFV alone produced a forward whole-body tilt with increased flexion in trunk, hip, and ankle. When stance was perturbed by TU tilts, the center of mass (CoM) and center of pressure (CoP) displacements were larger in the presence of RFV or ATV and associated with increased peak trunk flexion. TD tilts with or without ATV resulted in no significant difference in CoM and CoP displacements, while larger trunk extension and smaller distal angular displacements were observed during ATV. RFV altered the magnitude of the balance reactions, as observed by an increase in CoP displacements and variable response in trunk displacement. Significant interactions between ATV and RFV were obtained for the peak angular excursions for both directions of perturbations, where ATV either enhanced (for TU tilts) or attenuated (for TD tilts) the influence of RFV. Manipulating somatosensory information from the plantar cutaneo-muscular and muscle spindle Ia afferents thus results in altered and widespread postural responses, as shown by profound changes in body kinematics and CoM and CoP displacements. This suggests that the CNS uses plantar cutaneo-muscular and ankle spindle afferent inputs to build an appropriate reference of verticality that influences the control of equilibrium during quiet and perturbed stance.     

The discovery and comparison of symbolic magnitudes

Humans and other primates are able to make relative magnitude comparisons, both with perceptual stimuli and with symbolic inputs that convey magnitude information. Although numerous models of magnitude comparison have been proposed, the basic question of how symbolic magnitudes (e.g., size or intelligence of animals) are derived and represented in memory has received little attention. We argue that symbolic magnitudes often will not correspond directly to elementary features of individual concepts. Rather, magnitudes may be formed in working memory based on computations over more basic features stored in long-term memory. We present a model of how magnitudes can be acquired and compared based on BARTlet, a representationally simpler version of Bayesian Analogy with Relational Transformations (BART; Lu, Chen, & Holyoak, 2012). BARTlet operates on distributions of magnitude variables created by applying dimension-specific weights (learned with the aid of empirical priors derived from pre-categorical comparisons) to more primitive features of objects. The resulting magnitude distributions, formed and maintained in working memory, are sensitive to contextual influences such as the range of stimuli and polarity of the question. By incorporating psychological reference points that control the precision of magnitudes in working memory and applying the tools of signal detection theory, BARTlet is able to account for a wide range of empirical phenomena involving magnitude comparisons, including the symbolic distance effect and the semantic congruity effect. We discuss the role of reference points in cognitive and social decision-making, and implications for the evolution of relational representations.