Jody L. Jensen

A dynamical systems investigation of lower extremity coordination during running over obstacles

OBJECTIVE To investigate intralimb coordination during running over a level surface and over obstacles of three different heights. DESIGN The phasing relationships between the foot and leg motions in the frontal plane, and the shank and thigh motions in the sagittal plane were used to compare patterns of coordination. BACKGROUND The coordinated actions of lower extremity segments are necessary to absorb the impact forces generated during running. The behavioral patterns of these segments can be studied under changing task demands using analysis techniques from the Dynamical Systems Theory. METHODS Ten subjects ran at their self-selected pace under four conditions: over a level surface and over obstacles of different heights (5%, 10%, 15% of their standing height). A force platform was used to record impact forces during landing after obstacle clearance, while kinematics were collected using a two-camera system. RESULTS The increases in obstacle height resulted in significant changes in impact forces (34% increase between the two extreme conditions) and more in-phase relationships between the segments during early stance. No changes were observed in the variability of the phasing relationships. CONCLUSIONS The coordination changes observed might be compensatory strategies aimed to reduce forces and potential injury. However, since the impact forces still increased significantly, it is also possible that the observed changes might be at-risk movement patterns predisposing runners to injury.

Compensatory stepping : The biomechanics of a preferred response among older adults

Abstract The purpose of this study was to evaluate age-related differences in the mechanics of the compensatory stepping response to balance threats. A moving platform was used to disturb the balance of 16 younger (21 to 35 years) and 19 older (68 to 88 years) adults. Backward platform translations consisted of 15-cm displacements with peak accelerations ranging from 9.4 to 15.2 m/s2. Older adults were more likely to use a step to recover balance and stepped at lower perturbation magnitudes than younger adults. Group differences were not found in time to step initiation or segmental momentum. The lack of group differences in momentum revealed that lower perturbation accelerations created an equivalent or greater magnitude of body motion in older adults compared to higher accelerations experienced by younger adults. Older adults also showed a reduced ability to attenuate the input acceleration and experienced significantly greater linear acceleration of the head.

Development of Lower Extremity Kinetics for Balance Control in Infants and Young Children

Abstract Developmental changes in the kinematics and kinetics underlying balance control were studied in 61 children, 9 months to 10 years of age. The children were classified according to developmental milestones as standers; new, intermediate, and advanced walkers; runners-jumpers; hoppers; gallopers; and skippers. The children experienced support-surface translations of varying size and speed. Children with greater locomotor experience withstood larger balance threats without collapsing or stepping. Analyses of scaled trials (perturbations normalized in size to foot length and center of gravity height) revealed that improvement in balance was not related to initial configuration parameters surrounding the task (degree of crouch or lean). Children with advanced locomotor skills had faster recovery times and relatively larger muscle torques than children with less experience. Relative torque-time histories of the more experienced children began to match the adult response to similar perturbations. With increased experience and changing muscle torque regulatory abilities, balance skills became more robust.

The translating platform paradigm: perturbation displacement waveform alters the postural response

The translating platform paradigm is widely used to investigate the regulation of upright standing and locomotion. This study investigated how the displacement waveform characteristics underlying the translating platform perturbation are revealed in the resulting postural response. Eight participants experienced a series of backward-directed perturbations using a hydraulically driven forceplate. Two ranges of platform displacement (5 and 15 cm) in combination with two peak velocities (40 and 60 cm/s) were achieved using three distinct waveforms for platform displacement: (a) RAMP: ramp onset and ramp offset, (b) Ramp-to-Parabola (R-P): ramp onset with parabolic offset and (c) SINE: sine-wave onset with sine wave offset. Our findings indicated that the unique and distinctive acceleration and deceleration characteristics that result from the three different platform displacement waveforms significantly altered the postural response to the perturbation.

How do non-muscular torques contribute to the kinetics of postural recovery following a support surface translation?

The common platform translation paradigm used in balance control studies employs a disturbance event that applies non-muscular forces to the body for the duration of the disturbance. Previous research has explored the process of constructing the balance recovery by considering these perturbations to be trigger events, not events with an ongoing force application timeline. The purpose of this study was to quantify the effect of muscular and non-muscular torques on post-perturbation balance with particular interest in the role of the external perturbation in balance recovery. Five young adult males experienced backward translations of the support surface at three different speeds. Integration intervals were defined for each segment and angular impulses were calculated for a period of increasing angular momentum (destabilization), and a period of decreasing angular momentum (restabilization). Destabilization of distal segments was primarily due to impulse generated by the motion of the support surface. For the trunk, however, muscle and motion-dependent sources contributed most to increasing momentum. Restabilization of distal segments was achieved by muscle and platform impulses while trunk restabilization was achieved by muscle and motion-dependent terms in opposition to gravity. Increased platform speed resulted in increased muscular contribution only in the control of the trunk, while demand on distal musculature decreased with change in platform speed as the platform contribution to restabilization increased in these segments. Therefore, impulses from non-muscular sources, including the perturbation itself, are significant modifiers of the response to balance disturbances and must be accounted for in balance research.

For Young Jumpers, Differences are in the Movement's Control, Not its Coordination

Previous research on the development of jumping has indicated that the coordination of the jump is stable from its inception, but that individual differences lie in the control of the task. The purpose of this study was to examine control in the vertical jump as performed by novice jumpers. Two groups of children who varied in their ability to regulate the takeoff angle were compared on measures of coordination and control. The childrens groups were also compared to a group of skilled adults. Both childrens groups demonstrated temporal coordination patterns comparable to the adult pattern, though the children were distinguished from the adults on selected spatial (control) measures. The results indicate that a mature pattern of coordination for the jump exists at the earliest stages of the behaviors appearance in the movement repertoire. The novice, however, lacks the ability to precisely control, or tune, the movement to task demands.

Age-related differences in Lower extremity power After support surface perturbations

OBJECTIVES: The purpose of this study was to examine a comprehensive measure relating to the ability to generate, absorb, and transfer mechanical energy introduced by a perturbation. It was hypothesized that this measure would reveal age‐related differences leading to different balance recovery responses (i.e., feet‐in‐place and compensatory step).

Intralimb coordination following obstacle clearance during running: the effect of obstacle height

The purpose of this study was to investigate the different coordination strategies used following obstacle clearance during running. Ten subjects ran over a level surface and over obstacles of six different heights (10, 12.5, 15, 17.5, 20 and 22.5% of their standing height). Analysis based upon the dynamical systems theory (DST) was used and the phasing relationships between lower extremity segments were examined. The results demonstrated that the increasing obstacle height elicited behavioral changes. The foot and the leg became more independent in their actions, while the leg and the thigh strengthened their already stable relationship. The 15% obstacle height seems to be a critical height for the observed changes.

Postural response in older adults with different levels of functional balance capacity

Background and aims: Aging is frequently accompanied by a deterioration in postural control, but it is not clear whether the primary contributor is increasing age or a progressive loss of functional balance capacity. The purpose of this study was to test the hypothesis that functional balance capacity contributes more than age to changes in postural response in the elderly. Methods: The study considered 3 groups of healthy young, and functionally-stable and functionally-unstable older adults (N=16 each group). Postural responses, including behavioral response patterns, joint angular displacement, displacement of the center of mass and center of pressure, and ground reaction forces, were induced and examined by submitting standing subjects to unexpected backward displacements in the surface supporting them. Results: Functionally-stable older adults showed similar postural response patterns to those of young adults, whereas functionally-unstable older adults differed from young adults in the control of hip, trunk, shear force, and center of pressure trajectory during balance recovery. Conclusions: These findings support the hypothesis that changes in postural control in the elderly correlate with their functional balance capacity and are not just a matter of age.

Adapting to changing task demands: variability in children's response to manipulations of resistance and cadence during pedaling.

Abstract Reduction in performance variability is characteristic of skill acquisition during childhood. Less understood is the role of variability in adaptive skill. The purpose of this study was to determine childrens capacity for adapting to changing task requirements. Children ages 4–14 years and adults rode a stationary ergometer at different levels of cadence and resistance. Younger children were less successful in meeting task requirements. When they did succeed, the younger children were more variable. However, no interactions were found. Variability did not change with resistance, and all groups showed increasing variability as cadence increased. It was concluded that in spite of a weaker synergy (more variability), children were adept to changes in task demand within tested limits.