Papers by Michael MacLellan
The Cerebellum, Jan 1, 2011
Obstacle clearance can be a hazardous locomotor task if not coordinated with the utmost accuracy.... more Obstacle clearance can be a hazardous locomotor task if not coordinated with the utmost accuracy. The current study explored changes in leading limb segment coordination during obstacle clearance in a population with cerebellar ataxia using the planar law of intersegmental coordination. Eight participants with ARCA-1, caused by mutations in the SYNE-1 gene, and eight healthy adults stepped over obstacles. Healthy adults walked at natural speeds, as well as a velocity similar to the participants with cerebellar ataxia, resulting in three groups [healthy (H), matched velocity (MV) and cerebellar ataxia (CA)]. Elevation angles of the foot, shank and thigh in the sagittal plane were calculated. A principal component analysis was applied to limb segment trajectories, and a Fourier harmonic series was further used to determine temporal phase differences between adjacent segments. Although obstacle clearance was greater in the CA group, the planar nature of the 3D covariance plot of segment elevation angles, the covariance loop width and orientation did not differ between the CA, H and MV groups, suggesting that the planar patterns between elevation angles may not be heavily influenced by the cerebellum. Further analysis led to the observation of a nonlinear relationship between covariance loop width and thigh-shank fundamental harmonic phase difference, and a decrease in covariance loop width was observed when the fundamental harmonic phase difference between the thigh and shank segments is >90°. This study supports previous work that a greater safety margin is used in people with cerebellar ataxia when stepping over obstacles, but reveals a mechanism of segmental coordination to facilitate this increase in toe clearance. Further work is required to determine whether ataxia severity has an effect on the observed coordination variables.
A major function of the central nervous system (CNS) during locomotion is the ability to maintain... more A major function of the central nervous system (CNS) during locomotion is the ability to maintain dynamic stability during threats to balance. The CNS uses reactive, predictive, and anticipatory mechanisms in order to accomplish this.
Gait & Posture, Jan 1, 2012
Journal of …, Jan 1, 2012
the upper limb to decrease step frequency at lower speeds in humans may be due to weaker ascendin... more the upper limb to decrease step frequency at lower speeds in humans may be due to weaker ascending propriospinal connections and/or a larger influence of cortical control on the upper limbs which allows for an overriding of spinal CPG control.
Experimental brain research, Jan 1, 2010
Anticipatory locomotor adjustments (ALAs) are used during locomotion to perform tasks, such as ob... more 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).
Changes to control of adaptive gait in individuals with long-standing reduced stereoacuity
… ophthalmology & visual …, Jan 1, 2010
Gait during obstacle negotiation is adapted in visually normal subjects whose vision is temporari... more 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. 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. 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). 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.
Experimental brain research, Jan 1, 2006
Adaptive human locomotion is dependent on safe clearance of obstacles encountered in the path of ... more 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 oV. 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.
Experimental Brain …, Jan 1, 2008
When stepping down from one level to another, the leading limb has to arrest downward momentum of... more 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 diVerent heights onto a forceplatform, either coming immediately to rest or proceeding directly to walking across the laboratory. Exp Brain Res 123 was not identical to occluding vision, and led to several important diVerences between these conditions consistent with the use of impoverished visual information on depth. These Wndings indicate that online vision is customarily used to regulate landing behaviour when stepping down.
Experimental brain research, Jan 1, 2006
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Papers by Michael MacLellan