Stephen D. Prentice

Visual control of locomotion: strategies for changing direction and for going over obstacles

Dynamics of gait adjustments required to go over obstacles and to alter direction of locomotion when cued visually were assessed through the measurement of ground reaction forces, muscle activity, and kinematics. The time of appearance of obstacles of varying heights, their position within the step cycle, and cue lights for direction change were varied. Direction change must be planned in the previous step to reduce the acceleration of the body center of mass toward the landing foot to 0. The inability of steering within the step cycle is due to the incapacity of muscles to rotate the body and translate it along the mediolateral axes. For obstacle avoidance, Ss systematically manipulated the gait patterns as a function of obstacle height and position and the time available within the ongoing step. Greater supraspinal involvement in control of locomotion is found.

Cortical and brainstem control of locomotion

While a basic locomotor rhythm is centrally generated by spinal circuits, descending pathways are critical for ensuring appropriate anticipatory modifications of gait to accommodate uneven terrain. Neurons in the motor cortex command the changes in muscle activity required to modify limb trajectory when stepping over obstacles. Simultaneously, neurons in the brainstem reticular formation ensure that these modifications are superimposed on an appropriate base of postural support. Recent experiments suggest that the same neurons in the same structures also provide similar information during reaching movements. It is suggested that, during both locomotion and reaching movements, the final expression of descending signals is influenced by the state and excitability of the spinal circuits upon which they impinge.

The role of active forces and intersegmental dynamics in the control of limb trajectory over obstacles during locomotion in humans

The focus of this paper is to examine the contributions of active and passive forces in the control of limb trajectory over obstacles during locomotion. Kintetic analyses of the swing phase of locomotion were carried out to determine the power profiles at various joints and to parcel the joint moments into moments due to muscle action, gravitational force and motion-dependent terms. The analyses revealed that toe elevation over the obstacles was achieved primarily by flexing at the hip, knee and ankle joint. Power analyses showed that translational energy applied at the hip joint and rotational energy applied at the knee joint were modulated as functions of obstacle height. This demonstrates that increased hip and ankle joint flexion are achieved not through active muscle action but rather through passive forces induced by translational action at the hip (representing contribution by the stance limb muscles) and rotational action at the knee joint. Parcelling the joint moment terms into various components clearly shows how the nervous system exploits intersegmental dynamics to simplify control of limb elevation over obstacles and minimize energy costs.

Age-related changes in balance control system: initiation of stepping

The balance control system of a group of healthy and fit, young and elderly subjects was studied during the initiation of stepping in one of three directions: forward, sideways, and backwards in response to a light cue. The performance of these movements requires shifting support from two to one foot, moving the centre of mass outside the initial base of support and creating a new support configuration. By recording and analysing the vertical ground reaction force beneath the subjects stepping foot, we were able to examine the two phases prior to limb lift-off for stepping: reaction time and weight transfer time. Both reaction time and weight transfer time increased with age: The elderly subjects had a proportionately larger increase in weight transfer time compared to the reaction time. The peak force generated showed an age by stepping direction effect: the elderly had a significantly lower peak force for the forwards stepping compared to the younger subjects. The larger increase in weight transfer results in a slower stepping response. Since a stepping task is often recruited to avoid a fall, the increase in response execution time can have undesirable consequences.

Visual Control of Obstacle Avoidance During Locomotion: Strategies in Young Children, Young and Older Adults

Abstract The focus of this chapter is on understanding how obstacle avoidance during locomotion is affected by normal aging process and how this adaptability in locomotor system develops as children acquire independent bipedal locomotion. Obstacle avoidance paradigms offer a rich source of material for understanding the unique sensorimotor integration common to many visually guided movements. Based on studies on young healthy adults, we have proposed a jigsaw puzzle metaphor summarizing the key ingredients for successful obstacle avoidance. The nature of visual and kinesthetic input and the contribution of the effector system properties form the pieces of the puzzle. Studies on healthy older adults reveal relatively well preserved obstacle avoidance strategies, although there are some differences when compared to the healthy young adults. Deterioration in sensory input and effector system characteristics shows up as adaptive changes in feedforward control of limb trajectory over obstacles. This suggests that the puzzle is relatively robust with cracks appearing in some pieces. Preliminary studies on children provide interesting signposts for the development of stable obstacle avoidance strategies. High failure rate and poorer control of limb trajectory over obstacles characterize the gait patterns of young children in a cluttered environment. This suggests that the pieces of the puzzle have to be sculpted and merged into a coherent picture during the development process.

Artificial neural network model for the generation of muscle activation patterns for human locomotion

Skilled locomotor behaviour requires information from various levels within the central nervous system (CNS). Mathematical models have permitted researchers to simulate various mechanisms in order to understand the organization of the locomotor control system. While it is difficult to adequately characterize the numerous inputs to the locomotor control system, an alternative strategy may be to use a kinematic movement plan to represent the complex inputs to the locomotor control system based on the possibility that the CNS may plan movements at a kinematic level. We propose the use of artificial neural network (ANN) models to represent the transformation of a kinematic plan into the necessary motor patterns. Essentially, kinematic representation of the actual limb movement was used as the input to an ANN model which generated the EMG activity of 8 muscles of the lower limb and trunk. Data from a wide variety of gait conditions was necessary to develop a robust model that could accommodate various environmental conditions encountered during everyday activity. A total of 120 walking strides representing normal walking and ten conditions where the normal gait was modified in terms of cadence, stride length, stance width or required foot clearance. The final network was assessed on its ability to predict the EMG activity on individual walking trials as well as its ability to represent the general activation pattern of a particular gait condition. The predicted EMG patterns closely matched those recorded experimentally, exhibiting the appropriate magnitude and temporal phasing required for each modification. Only 2 of the 96 muscle/gait conditions had RMS errors above 0.10, only 5 muscle/gait conditions exhibited correlations below 0.80 (most were above 0.90) and only 25 muscle/gait conditions deviated outside the normal range of muscle activity for more than 25% of the gait cycle. These results indicate the ability of single network ANNs to represent the transformation between a kinematic movement plan and the necessary muscle activations for normal steady state locomotion but they were also able to generate muscle activation patterns for conditions requiring changes in walking speed, foot placement and foot clearance. The abilities of this type of network have implications towards both the fundamental understanding of the control of locomotion and practical realizations of artificial control systems for use in rehabilitation medicine.

Bilateral lower limb strategies used during a step-up task in individuals who have undergone unilateral total knee arthroplasty.

OBJECTIVE The purpose of this study was to determine how bilateral lower limb joint function is altered by the combined effects of osteoarthritis and its treatment by total knee arthroplasty. DESIGN Lower limb joint work of age-matched healthy, control participants was compared to surgical and non-surgical limb work in individuals who had undergone total knee arthroplasty. BACKGROUND Research investigating outcomes following total knee arthroplasty has focussed primarily on the surgical knee, identifying deficits in surgical knee function. The existence of additional lower limb deficits and adjustments made by unaffected joints to complement these deficits, has yet to be examined. METHODS Joint moments, power and work were calculated using bilateral lower limb force and kinematic data collected during a step-up to heights of 11.25 and 20 cm. RESULTS Fifty percent of patients were unable to step onto the 20 cm step. At both step heights, when the surgical limb led the step-up, surgical knee work was less than controls. When the non-surgical limb led, deficits in non-surgical lead knee work were observed. In both cases, lead hip work increased. CONCLUSIONS Work done by both surgical and non-surgical knees in a step-up task was lower than that done by healthy controls. This deficit was balanced by increased lead hip extensor work. RELEVANCE These findings highlight the importance of including exercises that optimize bilateral knee and hip function in rehabilitation programs used following knee replacement. Clinicians working with this population can use this information to assist in the design of evidenced based treatment programs.

Swing phase kinetics and kinematics of knee replacement patients during obstacle avoidance

Proper knee joint function is essential for safe and effective mobility within a complex environment. Following knee joint replacement, joint structure and function are altered, often requiring individuals to adjust normal movement patterns in order to adapt to these changes. Such adaptations may either improve function or lead to movement patterns that may potentially be unsafe for individuals. To investigate this issue, a group of individuals who had undergone knee replacement was examined while performing an obstacle avoidance task. Their performance was compared with that of healthy age-matched controls. Participants walked along a 10 m walkway a total 48 times. Trials were divided equally between unobstructed walking and clearing a 6 or 18 cm obstacle during both right and left limb lead. Lead limb kinetic and kinematic variables were examined and revealed that members of the surgical group exhibited decreased active surgical knee flexion and diminished surgical knee flexor work. In order to maintain toe clearance at control levels, patients were observed to increase hip hiking and hip flexor work, in addition to laterally displacing the surgical toe during swing over the obstacle. Despite allowing individuals to maintain adequate toe clearance, these compensatory strategies may lead to an increased instability and pose a threat to safety in this population. In addition, an increased demand placed on the hip joint of the surgical limb may be undesirable in this population. Research aimed at determining how to best maximize surgical knee function must continue if these potential negative sequela are to be minimized.

The influence of ankle muscle activation on postural sway during quiet stance

Although balance during quiet standing is postulated to be influenced by multiple factors, including ankle stiffness, it is unclear how different mechanisms underlying increases in stiffness affect balance control. Accordingly, this study examined the influence of muscle activation and passive ankle stiffness increases on the magnitude and frequency of postural sway. Sixteen young adults participated in six quiet stance conditions including: relaxed standing, four muscle active conditions (10%, 20%, 30% and 40% maximum voluntary contraction (MVC)), and one passive condition wearing an ankle foot orthotic (AFO). Kinetics were collected from a force plate, while whole-body kinematics were collected with a 12-sensor motion capture system. Bilateral electromyographic signals were recorded from the tibialis anterior and medial gastrocnemius muscles. Quiet stance sway amplitude (range and root mean square) and frequency (mean frequency and velocity) in the sagittal plane were calculated from time-varying centre of gravity (COG) and centre of pressure (COP) data. Compared to the relaxed standing condition, metrics of sway amplitude were significantly increased (between 37.5 and 63.2%) at muscle activation levels of 30% and 40% MVC. Similarly, frequency measures increased between 30.5 and 154.2% in the 20-40% MVC conditions. In contrast, passive ankle stiffness, induced through the AFO, significantly decreased sway amplitude (by 23-26%), decreased COG velocity by 13.8%, and increased mean COP frequency by 24.9%. These results demonstrate that active co-contraction of ankle musculature (common in Parkinsons Disease patients) may have differential effects on quiet stance balance control compared to the use of an ankle foot orthotic (common for those recovering from stroke).

Age-related changes in mediolateral dynamic stability control during volitional stepping

The control of mediolateral dynamic stability during stepping can be particularly challenging for older adults and appears to be related to falls and hip fracture. The specific mechanisms or control challenges that lead to mediolateral instability, however, are not fully understood. This work focussed on the restabilisation phase of volitional forward stepping, subsequent to foot contact, which we believe to be a principal determinant of mediolateral dynamic stability. Twenty younger (age 24±5 years; 50% women) and 20 older participants (age 71±5 years; 50% women) performed three different single-step tasks of various speed and step placement, which varied the challenge to dynamic stability. The trajectory of the total body centre of mass (COM) was quantified. Mediolateral COM incongruity, defined as the difference between the peak lateral and final COM position, and trial-to-trial variability of incongruity were calculated as indicators of dynamic stability. Older adults exhibited increased instability compared to young adults, as reflected by larger COM incongruity and trial-to-trial variability. Such increases among older adults occurred despite alterations in COM kinematics during the step initiation and swing phases, which should have led to increased stability. Task related increases in instability were observed as increased incongruity magnitude and trial-to-trial variability during the two rapid stepping conditions, relative to preferred speed stepping. Our findings suggest that increased COM incongruity and trial-to-trial variability among older adults signify a reduction in dynamic stability, which may arise from difficulty in reactive control during the restabilisation phase.