Vibration influences haptic perception of surface compliance during walking

Background

The haptic perception of ground compliance is used for stable regulation of dynamic posture and the control of locomotion in diverse natural environments. Although rarely investigated in relation to walking, vibrotactile sensory channels are known to be active in the discrimination of material properties of objects and surfaces through touch. This study investigated how the perception of ground surface compliance is altered by plantar vibration feedback.

Methodology/Principal Findings

Subjects walked in shoes over a rigid floor plate that provided plantar vibration feedback, and responded indicating how compliant it felt, either in subjective magnitude or via pairwise comparisons. In one experiment, the compliance of the floor plate was also varied. Results showed that perceived compliance of the plate increased monotonically with vibration feedback intensity, and depended to a lesser extent on the temporal or frequency distribution of the feedback. When both plate stiffness (inverse compliance) and vibration amplitude were manipulated, the effect persisted, with both factors contributing to compliance perception. A significant influence of vibration was observed even for amplitudes close to psychophysical detection thresholds.

Conclusions/Significance

These findings reveal that vibrotactile sensory channels are highly salient to the perception of surface compliance, and suggest that correlations between vibrotactile sensory information and motor activity may be of broader significance for the control of human locomotion than has been previously acknowledged.     

Activity of upper limb muscles during human walking

The EMG activity of upper limb muscles during human gait has rarely been studied previously. It was examined in 20 normal volunteers in four conditions: walking on a treadmill (1) with unrestrained natural arm swing (Normal), (2) while volitionally holding the arms still (Held), (3) with the arms immobilized (Bound), and (4) with the arms swinging in phase with the ipsilateral legs, i.e. opposite-to-normal phasing (Anti-Normal). Normal arm swing involved weak rhythmical lengthening and shortening contractions of arm and shoulder muscles. Phasic muscle activity was needed to keep the unrestricted arms still during walking (Held), indicating a passive component of arm swing. An active component, possibly programmed centrally, existed as well, because some EMG signals persisted when the arms were immobilized during walking (Bound). Anti-Normal gait involved stronger EMG activity than Normal walking and was uneconomical. The present results indicate that normal arm swing has both passive and active components.     

In vivo behaviour of human muscle tendon during walking

In the present study we investigated in vivo length changes in the fascicles and tendon of the human gastrocnemius medialis (GM) muscle during walking. The experimental protocol involved real-time ultrasound scanning of the GM muscle, recording of the electrical activity of the muscle, measurement of knee- and ankle-joint rotations, and measurement of ground reaction forces in six men during walking at 3 km h(-1) on a treadmill. Fascicular lengths were measured from the sonographs recorded. Musculotendon complex length changes were estimated from anatomical and joint kinematic data. Tendon length changes were obtained combining the musculotendon complex and fascicular length-change data. The fascicles followed a different length-change pattern from those of the musculotendon complex and tendon throughout the step cycle. Two important features emerged: (i) the muscle contracted near-isometrically in the stance phase, with the fascicles operating at ca. 50 mm; and (ii) the tendon stretched by ca. 7 mm during single support, and recoiled in push-off. The behaviour of the muscle in our experiment indicates consumption of minimal metabolic energy for eliciting the contractile forces required to support and displace the body. On the other hand, the spring-like behaviour of the tendon indicates storage and release of elastic-strain energy. Either of the two mechanisms would favour locomotor economy     

Joint forces in the human pelvis-leg skeleton during walking

For the calculation of the forces in the hip, knee and ankle joints during walking the knowledge of the three-dimensional movements of the human body and of the forces between foot and ground is a prerequisite. It is shown how this information may be obtained and what accuracy is obtainable. For the calculation of the statically indeterminate system of the lower limbs, consisting of muscles, bones and joints an optimization method is applied. The optimization criterion is the minimization of the muscle forces. Measurements were taken with seventeen male and five female persons. The maximum joint forces are plotted against gait speed, body weight and body size. In addition some statistical distributions are presented.     

Three-dimensional kinematics of the human knee during walking.

Three-dimensional kinematics of the tibiofemoral joint were studied during normal walking. Target markers were fixed to tibia and femur by means of intra-cortical traction pins. Radiographs of the lower limb were obtained to compute the position of the target markers relative to internal anatomical structures. High-speed cine cameras were used to measure three-dimensional coordinates of the target markers in five subjects walking at a speed of 1.2 m s-1. Relative motion between tibia and femur was resolved according to a joint coordinate system (JCS). The measurements have identified that substantial angular and linear motions occur about and along each of the JCS axes during walking. The results do not, however, support the traditional view that the so-called 'screw home' mechanism of the knee joint operates during gait.     

Variation in the human Achilles tendon moment arm during walking

The Achilles tendon (AT) moment arm is an important determinant of ankle moment and power generation during locomotion. Load and depth-dependent variations in the AT moment arm are generally not considered, but may be relevant given the complex triceps surae architecture. We coupled motion analysis and ultrasound imaging to characterize AT moment arms during walking in 10 subjects. Muscle loading during push-off amplified the AT moment arm by 10% relative to heel strike. AT moment arms also varied by 14% over the tendon thickness. In walking, AT moment arms are not strictly dependent on kinematics, but exhibit important load and spatial dependencies.     

Dynamic margins of stability during human walking in destabilizing environments

Understanding how humans maintain stability when walking, particularly when exposed to perturbations, is key to preventing falls. Here, we quantified how imposing continuous, pseudorandom anterior-posterior (AP) and mediolateral (ML) oscillations affected the control of dynamic walking stability. Twelve subjects completed five 3-minute walking trials in the Computer Assisted Rehabilitation ENvironment (CAREN) system under each of 5 conditions: no perturbation (NOP), AP platform (APP) or visual (APV) or ML platform (MLP) or visual (MLV) oscillations. We computed AP and ML margins of stability (MOS) for each trial. Mean MOS(ml) were consistently slightly larger during all perturbation conditions than during NOP (p≤0.038). Mean MOS(ap) for the APP, MLP and MLV oscillations were significantly smaller than during NOP (p<0.0005). Variability of both MOS(ap) and MOS(ml) was significantly greater during the MLP and MLV oscillations than during NOP (p<0.0005). We also directly quantified how the MOS on any given step affected the MOS on the following step using first-return plots. There were significant changes in step-to-step MOS(ml) dynamics between experimental conditions (p<0.0005). These changes suggested that subjects may have been trying to control foot placement, and consequently stability, during the perturbation conditions. Quantifying step-to-step changes in margins of dynamic stability may be more useful than mean MOS in assessing how individuals control walking stability.     

Fibre operating lengths of human lower limb muscles during walking

Muscles actuate movement by generating forces. The forces generated by muscles are highly dependent on their fibre lengths, yet it is difficult to measure the lengths over which muscle fibres operate during movement. We combined experimental measurements of joint angles and muscle activation patterns during walking with a musculoskeletal model that captures the relationships between muscle fibre lengths, joint angles and muscle activations for muscles of the lower limb. We used this musculoskeletal model to produce a simulation of muscle-tendon dynamics during walking and calculated fibre operating lengths (i.e. the length of muscle fibres relative to their optimal fibre length) for 17 lower limb muscles. Our results indicate that when musculotendon compliance is low, the muscle fibre operating length is determined predominantly by the joint angles and muscle moment arms. If musculotendon compliance is high, muscle fibre operating length is more dependent on activation level and force-length-velocity effects. We found that muscles operate on multiple limbs of the force-length curve (i.e. ascending, plateau and descending limbs) during the gait cycle, but are active within a smaller portion of their total operating range.     

Differences between local and orbital dynamic stability during human walking

Currently there is no commonly accepted way to define, much less quantify, locomotor stability. In engineering, "orbital stability" is defined using Floquet multipliers that quantify how purely periodic systems respond to perturbations discretely from one cycle to the next. For aperiodic systems, "local stability" is defined by local divergence exponents that quantify how the system responds to very small perturbations continuously in real time. Triaxial trunk accelerations and lower extremity sagittal plane joint angles were recorded from ten young healthy subjects as they walked for 10 min over level ground and on a motorized treadmill at the same speed. Maximum Floquet multipliers (Max FM) were computed at each percent of the gait cycle (from 0% to 100%) for each time series to quantify the orbital stability of these movements. Analyses of variance comparing Max FM values between walking conditions and correlations between Max FM values and previously published local divergence exponent results were computed. All subjects exhibited orbitally stable walking kinematics (i.e., magnitudes of Max FM < 1.0), even though these same kinematics were previously found to be locally unstable. Variations in orbital stability across the gait cycle were generally small and exhibited no systematic patterns. Walking on the treadmill led to small, but statistically significant improvements in the orbital stability of mediolateral (p = 0.040) and vertical (p = 0.038) trunk accelerations and ankle joint kinematics (p = 0.002). However, these improvements were not exhibited by all subjects (p < or = 0.012 for subject x condition interaction effects). Correlations between Max FM values and previously published local divergence exponents were inconsistent and 11 of the 12 comparisons made were not statistically significant (r2 < or = 19.8%; p > or = 0.049). Thus, the variability inherent in human walking, which manifests itself as local instability, does not substantially adversely affect the orbital stability of walking. The results of this study will allow future efforts to gain a better understanding of where the boundaries lie between locally unstable movements that remain orbitally stable and those that lead to global instability (i.e., falling).     

Simultaneous recording of longitudinal displacements of both feet during human walking

1. Recordings of longitudinal displacements of both feet have been performed by linking each foot to a length-voltage transducer by means of threads. The movement transmitted to the transducer was reduced by winding the thread around a sixteen strand pulley block. 2. The aspect of the displacement curves allows a direct analysis of the walk. Some typical curves of normal and pathological gaits are presented. Graphic measurements done on several right and left cycles of a 6 m walk episode in 50 adult subjects provided data to determine the mean and the standard deviation of spatial and temporal parameters of the walk. 3. An advantage of this method is to permit the measurement of the parameters of several successive cycles of both sides and so to detect changes in the length, the duration and the velocity of successive cycles of the same foot and of alternated cycles of both feet. This being important to characterize pathological gaits.     

Pendular activity of human upper limbs during slow and normal walking

When walking at normal and fast speeds, humans swing their upper limbs in alternation, each upper limb swinging in phase with the contralateral lower limb. However, at slow and very slow speeds, the upper limbs swing forward and back in unison, at twice the stride frequency of the lower limbs. The change from “single swinging” (in alternation) to “double swinging” (in unison) occurs consistently at a certain stride frequency for agiven individual, though different individuals may change at different stride frequencies. To explain this change in the way we use our upper limbs and individual variations in the occurrence of the change, the upper limb is modelled as a compound pendulum. Based on the kinematic properties of pendulums, we hypothesize that the stride frequency at which the change from “single swinging” to “double swinging” occurs will be at or slightly below the natural pendular frequency (NPF) of the upper limbs. Twenty-seven subjects were measured and then filmed while walking at various speeds. The mathematically derived NPF of each subject's upper limbs was compared to the stride frequency at which the subject changed from “single swinging” to “double swinging.” The results of the study conform very closely to the hypothesis, even when the NPF is artificially altered by adding weights to the subjects' hands. These results indicate that the pendulum model of the upper limb will be useful in further investigations of the function of the upper limbs in human walking. © 1994 Wiley-Liss, Inc.     

The forces and couples in the human trunk during level walking

The intersegmental force and couple exchanged between upper and lower body across a transverse section passing through the fourth lumbar vertebra were estimated during level walking on a straight line at speeds ranging from 0.99 to 2.23 ms-1. This was done using 3-D kinematic information relative to the head, upper limbs, and upper torso, obtained through a stereophotogrammetric technique, and the relevant inertial parameters obtained using anthropometric measurements and estimation techniques provided in the literature. Twenty walking cycles of five normal adult male subjects were analysed. The intersegmental force and couple components are presented as referenced to both a laboratory and pelvic set of axes. Using these results some considerations are made concerning the variations which the overall trunk muscles effort undergoes because of mean walking speed changes. The muscular action on the trunk is inferred from the intersegmental couple components. The various factors that contribute to the build-up of the intersegmental force and couple are analysed and their relative importance assessed.     

Characterization of a system for studying human gait during slope walking

Sloped walking surfaces provide a unique environment for examining the biomechanics and neural control of locomotion. While sloped surfaces have been used in a variety of studies in recent years, the current literature provides little if any discussion of the integrity, i.e., validity, of the systems used to collect data. The goal of this study was to develop and characterize a testing system capable of evaluating the kinetics of human locomotion on sloped surfaces. A ramped walkway system with an embedded force plate was constructed and stabilized. Center of pressure and reaction force data from the force plate were evaluated at 6 ramp grades (0, 5, 15, 25, 35, and 39 %). Ground reaction force data at 0 % grade were effectively the same as data from the same force plate when mounted in the ground and were well within the range of intrasubject variability. Collectively, data from all tests demonstrate the fidelity of this ramp system and suggest it can be used to evaluate human locomotion over a range of slope intensities.     

Dynamic optimization of human walking

A three-dimensional, neuromusculoskeletal model of the body was combined with dynamic optimization theory to simulate normal walking on level ground. The body was modeled as a 23 degree-of-freedom mechanical linkage, actuated by 54 muscles. The dynamic optimization problem was to calculate the muscle excitation histories, muscle forces, and limb motions subject to minimum metabolic energy expenditure per unit distance traveled. Muscle metabolic energy was calculated by slimming five terms: the basal or resting heat, activation heat, maintenance heat, shortening heat, and the mechanical work done by all the muscles in the model. The gait cycle was assumed to be symmetric; that is, the muscle excitations for the right and left legs and the initial and terminal states in the model were assumed to be equal. Importantly, a tracking problem was not solved. Rather only a set of terminal constraints was placed on the states of the model to enforce repeatability of the gait cycle. Quantitative comparisons of the model predictions with patterns of body-segmental displacements, ground-reaction forces, and muscle activations obtained from experiment show that the simulation reproduces the salient features of normal gait. The simulation results suggest that minimum metabolic energy per unit distance traveled is a valid measure of walking performance.     

The human ankle during walking: implications for design of biomimetic ankle prostheses

The non-disabled human ankle joint was examined during walking in an attempt to determine overall system characteristics for use in the design of ankle prostheses. The hypothesis of the study was that the quasi-stiffness of the ankle changes when walking at different walking speeds. The hypothesis was examined using sagittal plane ankle moment versus ankle angle curves from 24 able-bodied subjects walking over a range of speeds. The slopes of the moment versus ankle angle curves (quasi-stiffness) during loading appeared to change as speed was increased and the relationship between the moment and angle during loading became increasingly non-linear. The loading and unloading portions of the moment versus angle curves showed clockwise loops (hysteresis) at self-selected slow speeds that reduced essentially to zero as the speed increased to self-selected normal speeds. Above self-selected normal speeds, the loops started to traverse a counter-clockwise path that increased in area as the speed was increased. These characteristics imply that the human ankle joint could be effectively replaced with a rotational spring and damper for slow to normal walking speeds. However, to mimic the characteristics of the human ankle during walking at fast speeds, an augmented system would be necessary. This notion is supported by the sign of the ankle power at the time of opposite heel contact, which was negative for slow speeds, was near zero at normal speeds, and was positive for fast walking speeds.     

Selective activation of human soleus and medial gastrocnemius muscles during walking in water

During walking in water (WW) the vertical component of ground reaction forces decreases, while the greater propulsive force is required to move forward against the greater resistance of water. In such reduced gravity environment, Hutchison et al. (1989) have demonstrated that the relative activation of rat medial gastrocnemius (MGAS) increased compared to that of the soleus (SOL) during swimming, suggesting different effects of peripheral information on motoneuron excitability of these muscles. It is conceivable that both buoyancy and resistance of water have different effects on the activation patterns of triceps surae muscles during WW, since the reduced weight in water might decrease the peripheral inflow relating load information while greater volitional command might be needed to propel a body forward against the water resistance. The present study was designed to assess each peripheral inflow and efferent input by adjusting the load and walking speed voluntarily during WW. The aim of this study is to investigate the dissociative activation pattern between the SOL and the MGAS during WW.     

Three-dimensional modular control of human walking

Recent studies have suggested that complex muscle activity during walking may be controlled using a reduced neural control strategy organized around the co-excitation of multiple muscles, or modules. Previous computer simulation studies have shown that five modules satisfy the sagittal-plane biomechanical sub-tasks of 2D walking. The present study shows that a sixth module, which contributes primarily to mediolateral balance control and contralateral leg swing, is needed to satisfy the additional non-sagittal plane demands of 3D walking. Body support was provided by Module 1 (hip and knee extensors, hip abductors) in early stance and Module 2 (plantarflexors) in late stance. In early stance, forward propulsion was provided by Module 4 (hamstrings), but net braking occurred due to Modules 1 and 2. Forward propulsion was provided by Module 2 in late stance. Module 1 accelerated the body medially throughout stance, dominating the lateral acceleration in early stance provided by Modules 4 and 6 (adductor magnus) and in late stance by Module 2, except near toe-off. Modules 3 (ankle dorsiflexors, rectus femoris) and 5 (hip flexors and adductors except adductor magnus) accelerated the ipsilateral leg forward in early swing whereas Module 4 decelerated the ipsilateral leg prior to heel-strike. Finally, Modules 1, 4 and 6 accelerated the contralateral leg forward prior to and during contralateral swing. Since the modules were based on experimentally measured muscle activity, these results provide further evidence that a simple neural control strategy involving muscle activation modules organized around task-specific biomechanical functions may be used to control complex human movements.     

Dynamic arm swinging in human walking

Humans tend to swing their arms when they walk, a curious behaviour since the arms play no obvious role in bipedal gait. It might be costly to use muscles to swing the arms, and it is unclear whether potential benefits elsewhere in the body would justify such costs. To examine these costs and benefits, we developed a passive dynamic walking model with free-swinging arms. Even with no torques driving the arms or legs, the model produced walking gaits with arm swinging similar to humans. Passive gaits with arm phasing opposite to normal were also found, but these induced a much greater reaction moment from the ground, which could require muscular effort in humans. We therefore hypothesized that the reduction of this moment may explain the physiological benefit of arm swinging. Experimental measurements of humans (n = 10) showed that normal arm swinging required minimal shoulder torque, while volitionally holding the arms still required 12 per cent more metabolic energy. Among measures of gait mechanics, vertical ground reaction moment was most affected by arm swinging and increased by 63 per cent without it. Walking with opposite-to-normal arm phasing required minimal shoulder effort but magnified the ground reaction moment, causing metabolic rate to increase by 26 per cent. Passive dynamics appear to make arm swinging easy, while indirect benefits from reduced vertical moments make it worthwhile overall.