Visual cue spatial context affects performance of anticipatory postural adjustments.

Past research indicates that anticipatory postural adjustment (APA) errors may be due to the incorrect selection of responses to visual stimuli. In the current study we used the Simon task as a methodological tool to challenge the response selection stage of processing by presenting visual cues with conflicting spatial context; in this case generating a step response to a left pointing arrow which appears to the participant's right side or vice versa. We expected greater mediolateral APA errors, delayed APA and step onset times, and greater lateral CoP displacement prior to stepping for visual cues with incongruent spatial contexts compared to cues with congruent. Thirteen healthy young adults completed step initiation trials (n = 40) from a force platform while whole-body kinematic motion was tracked. Participants were presented with arrows pointing to the left or right, indicating to step with the left or right limb, respectively. These arrows were presented on the same side as the desired step direction (congruent) or the opposite side (incongruent). Results revealed that incongruent trials resulted in significantly more incidences of mediolateral APA errors and greater mediolateral CoP deviations during the APA compared to congruent visual cue context trials. No effects were observed for the temporal outcomes, suggesting that young adults can maintain temporal execution of steps despite these motor control errors. This study demonstrates that the spatial context of visual information significantly impacts the success of response selection processes during step initiation, furthering our knowledge of how humans integrate visual information to initiate whole body movement.

Positional Differences in Pre-Season Scrimmage Performance of Division I Collegiate Football Players.

This study aimed to describe the physical demands of American football players using novel performance analysis techniques. Heart rate (HR) and accelerometer-based activity levels were observed across two pre-season scrimmages in 23 Division I collegiate football players (age: 19 ± 1 y, height: 1.90 ± 0.06 m, weight: 116.2 ± 19.4 kg). Data were analyzed using a MATLAB program and inter-rater reproducibility assessed using inter-class correlations (ICC). Players were analyzed by side (offense/defense) and position (skill/non-skill). Performance variables assessed in bursts of activity included burst duration, HRmean and HRmax (bpm), and mean activity (vector magnitude units [vmu]). Exercise intensity was categorized as time spent in % HRmax in 5% increments. The burst duration (8.1±3.9 min, ICC = 0.72), HRmean (157 ± 12 bpm, ICC = 0.96) and mean activity (0.30 ± 0.05 vmu, ICC = 0.86) were reproducible. HRmean (p = 0.05) and HRmax (p = 0.001) were greater on defense. Offense spent more time at 65-70% HRmax (p = 0.01), 70-75% HRmax (p = 0.02) while defense spent more time 90-95% HRmax and ≥95% HRmax (p = 0.03). HRmean (p = 0.70) and HRpeak (p = 0.80) were not different between positions across both sides. Skilled players demonstrated greater mean activity (p = 0.02). The sport-specific analysis described HR and activity level in a reproducible manner. Automated methods of assessing HR may be useful in training and game time performance but ultimately provides support to coaching decision making.

The Effect of Aspartate and Sodium Bicarbonate Supplementation on Muscle Contractile Properties Among Trained Men.

Farney, TM, MacLellan, MJ, Hearon, CM, Johannsen, NM, and Nelson, AG. The effect of aspartate and sodium bicarbonate supplementation on muscle contractile properties among trained men. J Strength Cond Res 34(3): 763-770, 2020-The focus of this investigation was to examine the effects of aspartate and NaHCO3 supplementation on muscle contractile properties within trained men. Eleven men (21.9 ± 1.5 years) ingested supplementation as 4 conditions all separated by 1 week and included the following: placebo (PLA), L-aspartate (12.5 mg) (ASP), NaHCO3 (0.3 g·kg) (SBC), or combination of ASP and SBC (CBO). For each day of testing, participants performed 1 high-intensity exercise session along with a pre- and postexercise (pre- or postex) isometric mid thigh pull test to measure peak force (PF) production and rate of force development (RFD). Blood was collected for all testing sessions before and after the high-intensity exercise to determine ammonia accumulation (AMM). Exercise sessions consisted of 4 exercises: barbell thrusters, squat jumps, lunge jumps, and forward jumps, with the total amount of work being equated for all 4 exercises across all 4 testing sessions. Participants performed the exercises in the aforementioned order, which was designated as 1 round. Each participant performed 3 rounds, with the work-to-rest ratio being 20-second work, 30-second rest. A 1-minute rest was given between the rounds. There were no treatment effects (p > 0.05) for PF, RFD, or AMM. However, there was a significant main effect for supplement consumption for the total time of work with the ASP, SBC, and CBO treatments having a lower time to completion compared with the PLA treatment. Ammonia was significantly elevated postexercise (p = 0.004), whereas there were no differences from preexercise to postexercise for PF or RFD (p > 0.05). The only significant treatment × time interaction was for RFD (p = 0.03) with CBO increasing postexercise, with the other 3 treatments all decreasing postexercise. The combination of ASP and SBC together may have the potential to reduce fatigue by mitigating the effects of metabolic by-product accumulation.

Shoulder Muscle Activity Dampens Arm Swing Motion When Altering Upper Limb Mass Characteristics During Locomotion.

Weighting the arms during locomotion results in decreased swing motion and increased shoulder muscle activity. To determine the functional relevance of this activity, participants walked on a treadmill with the arms unweighted, or weighted unilaterally or bilaterally. Similar to past work, the weighted arms decreased in swing amplitude and increased their shoulder muscle activity. A close examination of shoulder muscle activities in specific regions of the arm swing cycle suggested these muscles primarily acted eccentrically for all weighting conditions. These findings suggest that the increased shoulder muscle activities when weighting the arms act to dampen the arms when the inertial characteristics of the arms are altered, as opposed to assisting in driving swing of the heavier arms.

Visually-guided saccades attenuate postural sway under non-fatigued, fatigued, and stretched states.

Muscular fatigue, which reduces force output and position sense, often leads to increased sway and potential balance impairments. In contrast, visually-guided saccadic eye movements (saccades) can attenuate sway more than fixating gaze on an external target. The goals of this study were to determine whether the use of saccades could reduce the increased postural sway in a fatigued state and to better understand the contributions to fatigue-induced increased sway. We compared the effects of gazing at a fixation point (FP) and performing saccades (SAC) on various spatial and temporal measures of the center of pressure (CoP) while participants stood as still as possible on a force plate. Participants used either a narrow or wide base of support and performed three trials for each eye movement condition (SAC, FP) in three states (non-fatigued-NF, stretched-S, and fatigued-F). Calf raises to exhaustion induced ankle fatigue. Extreme plantar- and dorsi-flexion induced stretch. SAC significantly decreased sway and increased time-series complexity (sample entropy) compared to FP. F increased sway and decreased time-series complexity compared to NF and S states, which were similar. Reduced force production, which accompanies muscle fatigue and stretching, did not account for increased sway associated with acute bouts of ankle muscle fatigue. Increased position sense often associated with muscle stretching likely compensated for any reduced force output for S, while the decreased position sense associated with F probably explained the increased sway in this state. Performing saccadic eye movements during quiet stance can help reduce sway under various states.

Backward walking highlights gait asymmetries in children with cerebral palsy.

To investigate how early injuries to developing motor regions of the brain affect different forms of gait, we compared the spatiotemporal locomotor patterns during forward (FW) and backward (BW) walking in children with cerebral palsy (CP). Bilateral gait kinematics and EMG activity of 11 pairs of leg muscles were recorded in 14 children with CP (9 diplegic, 5 hemiplegic; 3.0-11.1 yr) and 14 typically developing (TD) children (3.3-11.8 yr). During BW, children with CP showed a significant increase of gait asymmetry in foot trajectory characteristics and limb intersegmental coordination. Furthermore, gait asymmetries, which were not evident during FW in diplegic children, became evident during BW. Factorization of the EMG signals revealed a comparable structure of the motor output during FW and BW in all groups of children, but we found differences in the basic temporal activation patterns. Overall, the results are consistent with the idea that both forms of gait share pattern generation control circuits providing similar (though reversed) kinematic patterns. However, BW requires different muscle activation timings associated with muscle modules, highlighting subtle gait asymmetries in diplegic children, and thus provides a more comprehensive assessment of gait pathology in children with CP. The findings suggest that spatiotemporal asymmetry assessments during BW might reflect an impaired state and/or descending control of the spinal locomotor circuitry and can be used for diagnostic purposes and as complementary markers of gait recovery. NEW & NOTEWORTHY Early injuries to developing motor regions of the brain affect both forward progression and other forms of gait. In particular, backward walking highlights prominent gait asymmetries in children with hemiplegia and diplegia from cerebral palsy and can give a more comprehensive assessment of gait pathology. The observed spatiotemporal asymmetry assessments may reflect both impaired supraspinal control and impaired state of the spinal circuitry.

Active and passive contributions to arm swing: Implications of the restriction of pelvis motion during human locomotion.

Current research has yet to determine how passive dynamics and active neural control contribute to upper limb swing during human locomotion. The present study aimed to investigate these contributions by restricting pelvis motion during walking, thereby altering the upward energy transfer from the swinging lower limbs. Ten healthy individuals walked freely on a treadmill (CON) and with an apparatus that reduced pelvis motion (PR) at three walking speeds (0.9, 1.3, and 1.8m/s). Spatiotemporal characteristics of limb movement and muscle activation were recorded and analyzed. When wearing the apparatus, the ranges of the sagittal and transverse rotations of the trunk and shoulders, as well as vertical trunk center of mass movement all decreased. At higher treadmill speeds, the movement amplitudes of the upper and lower limbs increased. This increase was less pronounced in the upper limbs when the apparatus reduced pelvis motion. However, this decrease in arm swing was accompanied with a preservation of upper and lower limb muscle activity amplitudes. The temporal coordination between upper and lower limbs was also conserved irrespective of the PR or CON conditions. Relating shoulder muscle activities to upper limb kinematics suggested these muscles mainly acted eccentrically, providing evidence that passive elements are a significant factor in arm swing control. However, the conserved muscle activity patterns and temporal coupling of limb movements when pelvis motion was reduced are suggestive of an underlying active maintenance of the locomotor pattern via linked upper and lower limb neural networks.

Modular organization of muscle activity patterns in the leading and trailing limbs during obstacle clearance in healthy adults.

Human locomotor patterns require precise adjustments to successfully navigate complex environments. Studies suggest that the central nervous system may control such adjustments through supraspinal signals modifying a basic locomotor pattern at the spinal level. To explore this proposed control mechanism in the leading and trailing limbs during obstructed walking, healthy young adults stepped over obstacles measuring 0.1 and 0.2 m in height. Unobstructed walking with no obstacle present was also performed as a baseline. Full body three-dimensional kinematic data were recorded and electromyography (EMG) was collected from 14 lower limb muscles on each side of the body. EMG data were analyzed using two techniques: by mapping the EMG data to the approximate location of the motor neuron pools on the lumbosacral enlargement of the spinal cord and by applying a nonnegative matrix factorization algorithm to unilateral and bilateral muscle activations separately. Results showed that obstacle clearance may be achieved not only with the addition of a new activation pattern in the leading limb, but with a temporal shift of a pattern present during unobstructed walking in both the leading and trailing limbs. An investigation of the inter-limb coordination of these patterns suggested a strong bilateral linkage between lower limbs. These results highlight the modular organization of muscle activation in the leading and trailing limbs, as well as provide a mechanism of control when implementing a locomotor adjustment when stepping over an obstacle.

Immature Spinal Locomotor Output in Children with Cerebral Palsy.

Detailed descriptions of gait impairments have been reported in cerebral palsy (CP), but it is still unclear how maturation of the spinal motoneuron output is affected. Spatiotemporal alpha-motoneuron activation during walking can be assessed by mapping the electromyographic activity profiles from several, simultaneously recorded muscles onto the anatomical rostrocaudal location of the motoneuron pools in the spinal cord, and by means of factor analysis of the muscle activity profiles. Here, we analyzed gait kinematics and EMG activity of 11 pairs of bilateral muscles with lumbosacral innervation in 35 children with CP (19 diplegic, 16 hemiplegic, 2-12 years) and 33 typically developing (TD) children (1-12 years). TD children showed a progressive reduction of EMG burst durations and a gradual reorganization of the spatiotemporal motoneuron output with increasing age. By contrast, children with CP showed very limited age-related changes of EMG durations and motoneuron output, as well as of limb intersegmental coordination and foot trajectory control (on both sides for diplegic children and the affected side for hemiplegic children). Factorization of the EMG signals revealed a comparable structure of the motor output in children with CP and TD children, but significantly wider temporal activation patterns in children with CP, resembling the patterns of much younger TD infants. A similar picture emerged when considering the spatiotemporal maps of alpha-motoneuron activation. Overall, the results are consistent with the idea that early injuries to developing motor regions of the brain substantially affect the maturation of the spinal locomotor output and consequently the future locomotor behavior.

Comparison of kinetic strategies for avoidance of an obstacle with either the paretic or non-paretic as leading limb in persons post stroke.

The task of stepping over obstacles is known to be particularly risky for persons post stroke. A kinetic analysis informing on the movement strategies used to ensure clearance of the leading limb over an obstacle is, however, lacking. We examined obstacle avoidance strategies in six community dwelling stroke survivors comparing the use of paretic and non-paretic limb as the leading limb for clearance over obstacles measuring 7.5% and 15% of their total leg length. Subjects were able to increase foot clearance height in both limbs in order to avoid the two obstacles. Obstacle clearance with the non-paretic limb leading was associated with positive knee flexor work that increased when stepping over each obstacle, thus showing a normal knee strategy that flexes both the knee and the hip for foot clearance. There was also slightly increased hip flexor contribution for non-paretic obstacle clearance that was the same for both obstacle heights. When the paretic limb led during obstacle clearance, there was also evidence of an increased knee flexor moment, suggesting a residual knee strategy, but it was less pronounced than for the non-paretic limb and was assisted by greater vertical hip elevation and additional positive hip flexor work that both gained greater importance with increased obstacle height. These findings suggest that rehabilitation should explore the ability to improve the residual, but less powerful, knee flexor strategy in the paretic limb in specific patients, with further promotion of a hip flexor and limb elevation strategy depending on patient deficits and obstacle height.

Locomotor-like leg movements evoked by rhythmic arm movements in humans.

Motion of the upper limbs is often coupled to that of the lower limbs in human bipedal locomotion. It is unclear, however, whether the functional coupling between upper and lower limbs is bi-directional, i.e. whether arm movements can affect the lumbosacral locomotor circuitry. Here we tested the effects of voluntary rhythmic arm movements on the lower limbs. Participants lay horizontally on their side with each leg suspended in an unloading exoskeleton. They moved their arms on an overhead treadmill as if they walked on their hands. Hand-walking in the antero-posterior direction resulted in significant locomotor-like movements of the legs in 58% of the participants. We further investigated quantitatively the responses in a subset of the responsive subjects. We found that the electromyographic (EMG) activity of proximal leg muscles was modulated over each cycle with a timing similar to that of normal locomotion. The frequency of kinematic and EMG oscillations in the legs typically differed from that of arm oscillations. The effect of hand-walking was direction specific since medio-lateral arm movements did not evoke appreciably leg air-stepping. Using externally imposed trunk movements and biomechanical modelling, we ruled out that the leg movements associated with hand-walking were mainly due to the mechanical transmission of trunk oscillations. EMG activity in hamstring muscles associated with hand-walking often continued when the leg movements were transiently blocked by the experimenter or following the termination of arm movements. The present results reinforce the idea that there exists a functional neural coupling between arm and legs.

Changes of gait kinematics in different simulators of reduced gravity.

Gravity reduction affects the energetics and natural speed of walking and running. But, it is less clear how segmental coordination is altered. Various devices have been developed in the past to study locomotion in simulated reduced gravity. However, most of these devices unload only the body center of mass. The authors reduced the effective gravity acting on the stance or swing leg to 0.16g using different simulators. Locomotion under these conditions was associated with a reduction in the foot velocity and significant changes in angular motion. Moreover, when simulated reduced gravity directly affected the swing limb, it resulted in significantly slower swing and longer foot excursions, suggesting an important role of the swing phase dynamics in shaping locomotor patterns.

Use of segmental coordination analysis of nonparetic and paretic limbs during obstacle clearance in community-dwelling persons after stroke.

OBJECTIVE: To use a segment coordination analysis to identify coordination differences between the paretic and nonparetic limbs for obstacle clearance in community-dwelling persons after stroke. DESIGN: Within-participant design. SETTING: Gait analysis laboratory. PARTICIPANTS: Six community-dwelling persons with a stroke (excluding cerebellar stroke). METHODS: Participants stepped over obstacles of 2 different heights (7.5% and 15% of leg length), leading alternately with their paretic and nonparetic limbs. MAIN OUTCOME MEASUREMENTS: Kinematic data were collected, and segment elevation angles (absolute segment angular position with respect to vertical) were calculated for the thigh, shank, and foot segments. Established mathematical techniques related to the planar law of intersegmental coordination (principal component analysis to quantify covariance and temporal phase relationships among elevation angles) were then applied to compare and contrast the coordination of these segment elevation angle trajectories between paretic and nonparetic limbs. RESULTS: Segment covariance in elevation angles followed the planar law of intersegmental coordination during level walking (ie, 3 elevation angles that form a plane and the variance explained by 2 principal components) for both paretic and nonparetic limbs. During obstacle clearance, however, relationships between covariance plane characteristics and phase differences for elevation angles of adjacent segments differed in the nonparetic limb, likely related to a need for greater limb elevation for obstacle clearance during paretic limb support or an altered foot trajectory, which resulted from preobstacle foot placement. CONCLUSIONS: The present coordination analysis suggests the preservation of basic control mechanisms in the paretic limb during obstacle clearance after stroke and also reveals its specific motor control compensations. However, a larger study with differing levels of stroke severity must be conducted to understand how the evaluation of intersegmental coordination during walking could guide treatment of specific locomotor control deficits in stroke rehabilitation.

Proximal lower limb muscle energetics and the adaptation of segment elevation angle phasing for obstacle avoidance.

The purpose of this study was to better understand phase differences in the previously shown reorganization of elevation angles for obstacle avoidance and to relate them to active and passive energetic contributions at proximal lower limb joints. Ten healthy young adults stepped over obstacles of different heights. The fundamental harmonics representing elevation angles of the thigh and shank segments, their relative phase relationship as well as joint and muscle mechanical power and related work at the hip and knee joints were calculated. As higher obstacles were cleared, phase shifts between the thigh and shank increased due to a greater lead by the thigh for the leading limb and a greater lag by the shank for the trailing limb. While kinematic patterns were relatively constant, mechanical work differed greatly with passive energy transfer from the shank to the thigh during mid-swing dominating in the leading limb, but passive energy transfer from the shank to the thigh segment during toe-off coupled with active hip flexor generation in the trailing limb. Shank elevation angle waveform shifts were related to active knee flexor power in both limbs. However, different power bursts appeared to be related to thigh elevation waveform shifts in the leading (shank to thigh passive transfer offset) and trailing (active hip flexor offset) limbs. These results suggest limb specific temporal organization and underlying energetic patterns to realize thigh-shank phase shifts necessary for obstacle avoidance further supporting the theory of independent bilateral control.

Changes to control of adaptive gait in individuals with long-standing reduced stereoacuity.

PURPOSE: Gait during obstacle negotiation is adapted in visually normal subjects whose vision is temporarily and unilaterally blurred or occluded. This study was conducted to examine whether gait parameters in individuals with long-standing deficient stereopsis are similarly adapted. METHODS: Twelve visually normal subjects and 16 individuals with deficient stereopsis due to amblyopia and/or its associated conditions negotiated floor-based obstacles of different heights (7-22 cm). Trials were conducted during binocular viewing and monocular occlusion. Analyses focused on foot placement before the obstacle and toe clearance over it. RESULTS: Across all viewing conditions, there were significant group-by-obstacle height interactions for toe clearance (P < 0.001), walking velocity (P = 0.003), and penultimate step length (P = 0.022). Toe clearance decreased (approximately 0.7 cm) with increasing obstacle height in visually normal subjects, but it increased (approximately 1.5 cm) with increasing obstacle height in the stereo-deficient group. Walking velocity and penultimate step length decreased with increasing obstacle height in both groups, but the reduction was more pronounced in stereo-deficient individuals. Post hoc analyses indicated group differences in toe clearance and penultimate step length when negotiating the highest obstacle (P < 0.05). CONCLUSIONS: Occlusion of either eye caused significant and similar gait changes in both groups, suggesting that in stereo-deficient individuals, as in visually normal subjects, both eyes contribute usefully to the execution of adaptive gait. Under monocular and binocular viewing, obstacle-crossing performance in stereo-deficient individuals was more cautious when compared with that of visually normal subjects, but this difference became evident only when the subjects were negotiating higher obstacles; suggesting that such individuals may be at greater risk of tripping or falling during everyday locomotion.

Segmental control for adaptive locomotor adjustments during obstacle clearance in healthy young adults.

Anticipatory locomotor adjustments (ALAs) are used during locomotion to perform tasks, such as obstacle clearance, although not much is known as to how these ALAs are implemented by the central nervous system (CNS). The current study applied the planar law of intersegmental coordination to both leading and trailing limbs in a paradigm in which obstacle height and depth were manipulated to propose how ALAs are controlled. Ten healthy young adults stepped over nine obstacle conditions. Full-body 3D kinematic data were collected and elevation angles of the foot, shank, and thigh in the sagittal plane were calculated. For each limb within each trial, a principal component analysis was applied to limb segment trajectories. As well, a Fourier harmonic series was used to represent segment elevation angle trajectories, and phase differences between adjacent segments were determined. Planarity was consistently high in both limbs for all obstacle conditions, although significant differences between obstacle heights were observed. Increases in covariance loop width and rotation of the covariance plane accompanied changes in planarity. As observed in previous studies, fundamental harmonic phase differences between adjacent segments were highly correlated to plane characteristics and these phase differences changed systematically with increases in obstacle height. From the results, it is proposed that if a given environment requires a change in locomotion, the CNS adjusts a basic locomotor pattern if needed through the manipulation of the phase differences in the fundamental harmonics of the elevation angles between adjacent segments and elevation angle amplitude (with a constraint being intersegmenal elevation angle planarity).

Visual guidance of landing behaviour when stepping down to a new level.

When stepping down from one level to another, the leading limb has to arrest downward momentum of the body and subsequently receive and safely support bodyweight before level walking can begin. Such step downs are performed over a wide range of heights and predicting when and where contact between the landing limb and the lower level will be made is likely a critical factor. To determine if visual feedback obtained after movement initiation is habitually used in guiding landing behaviour, the present study determined whether pre-landing kinematics and the mechanics of landing would be modulated according to the type of visual feedback available during the stepping down phase. Ten healthy participants (32.3 +/- 7.9 years) stepped, from a standing position, down from three different heights onto a forceplatform, either coming immediately to rest or proceeding directly to walking across the laboratory. Repeated trials were undertaken under habitual vision conditions or with vision blurred or occluded 2-3 s prior to movement initiation. Pre-landing kinematics were assessed by determining, for the instant of landing, lead-limb knee and ankle angle, stepping distance, forwards positioning of the body CM within the base of support and the forwards and downwards body CM velocity. Landing mechanics for the initial contact period were characterized using lead limb vertical loading and stiffness, and trail limb un-weighting. When vision was occluded movement time, ankle plantarflexion and knee flexion were significantly increased compared to that determined for habitual vision, whereas forwards body CM positioning and velocity, vertical loading and stiffness, and trail limb un-weighting, were significantly reduced (p < 0.05). Similar adaptations were observed under blurred conditions, although to a lesser extent. Most variables were significantly affected by stepping task and step height. Subjects likely reduced forwards CM position and velocity at instant of landing, in order to keep the CM well away from the anterior border of the base of support, presumably to ensure boundary margins of safety were high should landing occur sooner or later than expected. The accompanying increase in ankle plantarflexion at instant of landing, and increase in single limb support time, suggests that subjects tended to probe for the ground with their lead limb under modified vision conditions. They also had more bodyweight on the trail limb at the end of the initial contact period and as a consequence had a prolonged weight transfer time. These findings indicate that under blurred or occluded vision conditions subjects adopted a cautious strategy where by they 'sat back' on their trail limb and used their lead limb to probe for the ground. Hence, they did not fully commit to weight transfer until somatosensory feedback from the lead limb confirmed they had safely made contact. The effect of blurring vision was not identical to occluding vision, and led to several important differences between these conditions consistent with the use of impoverished visual information on depth. These findings indicate that online vision is customarily used to regulate landing behaviour when stepping down.

Stepping over an obstacle on a compliant travel surface reveals adaptive and maladaptive changes in locomotion patterns.

Adaptive human locomotion is dependent on safe clearance of obstacles encountered in the path of locomotion. When the terrain is uneven or compliant, stability along with safe obstacle clearance are competing demands presented to the central nervous system (CNS). To examine how the CNS deals with the two competing demands, six participants walked under four conditions: normal ground walking, normal ground walking with an obstacle in the travel path, compliant surface walking, and compliant surface walking with an obstacle in the travel path. Full body kinematics were measured and swing limb kinetics were derived from these measurements. Results showed that on a compliant surface, the CNS was able to decrease foot placement variability at foot contact when approaching an obstacle, similar to the normal ground terrain. Limb trajectory over the obstacle showed that toe elevation was maintained while clearance over the obstacle was lower in the compliant surface condition due to depression of the surface during push off. This illustrates that the CNS controls toe elevation, not toe clearance when stepping over an obstacle. Work done in the knee during elevation and hip during lowering was similar in the compliant and ground conditions even though a lower clearance over the obstacle was achieved in the complaint condition. This shows the inability of the CNS to account for compression of the surface prior to obstacle clearance and provides further evidence the CNS controls toe elevation, not clearance when stepping over an obstacle.

Adaptations of walking pattern on a compliant surface to regulate dynamic stability.

Dynamic stability can be threatened by various travel surface changes that humans encounter on a daily basis. The central nervous system (CNS) must acquire appropriate information about upcoming surface changes and provide necessary proactive and reactive changes to maintain stability. The purpose of this study was to examine stability control by characterizing adaptations in step patterns, center of mass (COM) trajectory, and lower limb muscle activity when stepping onto and walking on a compliant surface. Eight young adults walked under two conditions: baseline ground walking and while walking on a large foam mat (compliant surface). Optotrak system was used to collect 3D-full body kinematics and electromyography was collected for the rectus femoris, biceps femoris, tibialis anterior, medial gastrocnemius, and soleus bilaterally. Vertical COM decreased on the compliant surface while medio-lateral COM was not affected. This lowering of the vertical COM peak would provide a more stable posture when walking on the surface. Toe trajectory during the swing phase was elevated to avoid tripping on the deformable compliant surface. Step width and length increased on the compliant surface which would increase base of support and provide better control of COM. Increases in gastrocnemius and soleus activity during push-off accounted for increases in step length seen on the compliant surface. Dynamic stability margin in the anterior-posterior direction demonstrated a constant overcompensation and subsequent correction in COM control. These proactive and reactive changes in motor patterns show how the CNS actively coordinates all body segments while traveling on a compliant surface in order to maximize stability.