Three-dimensional analyses of gait initiation in a healthy, young population

1. Three-dimensional analyses of gait initiation in a healthy, young population Drew Smith1 and Del P. Wong21 Motion Analysis Research Center (MARC), Samuel Merritt University, Oakland CA, USA 2 Technological & Higher Education Institute (THEi), Hong Kong SAR, PRC

2. Outline • Dynamic stability • Brief review of gait initiation • Three-dimensional analyses of gait initiation in a healthy, young population: • Introduction, Methods, Results, Discussion • Directions for future study

3. Dynamic stability • Inverted pendulum model • Most of our mass is located high above a smallbase of support • Gravity acts to de-stabilize system • Only 1 position is not unstable • Requires a dynamic stability control system • Sensory + motor + reflexes + mechanical • Failure of control system = falls • (Gait = ‘controlled falling’)

4. Dynamic stability • Definition: • Refers to the ability of the human to recover from perturbations while maintaining an upright posture over a stationary or moving base of support • Perturbations can be internal (muscle forces) or external (gravity, pushes, pulls, changes in friction) • Posture is the alignment of the body segments with respect to gravity • Balance is the maintenance of posture

5. Dynamic stability • Spectrum of inverted pendulum activities: • Sit-to-stand => quiet standing => walking => running => walking => quiet standing , and so forth • Special aspects: turns, ramps, stairs, obstacles • Each phase meets criteria of pendulum model • What about the transitions? • E.g., in what ways does quiet standing resemble walking? At what point does quiet standing become walking? Walking become running? And so forth. • What can successful transitions tell us about unsuccessful ones? • E.g., ‘freezing’ in Parkinson's disease, or falls in the elderly while negotiating turns while walking

6. Gait initiation research • Gait initiation • Complete after 1, 2, or 3 steps • Quiet standing => gait initiation • Step 1: • Soleus (-) and tibialis anterior (+) activity • COP moves backward • Paradoxically, swing limb is initially loaded then unloaded • Swing limb lifted by hip flexors

7. Gait initiation research • Differing views on when completed • Depend on which variable being considered: COM velocity, acceleration, joint angle patterns, GRF • To date, no 3D studies have been conducted • Steady-state gait and quiet standing are well-studied • Pronounced change in energy states make energy a good variable to study

8. Three-dimensional analyses of gait initiation • Purpose • Examine patterns of joint powers and patterns of energy absorption and generation during the first 3 steps, defined as gait initiation • Compare sagittal and frontal planes powers and energies • Hypotheses • Sagittal powers and energies will have little contribution before and during the initial transfer of weight in the first step (STEP1) • Most of the energy in STEP1 will come from hip joint • In successive steps (STEP2, STEP3), sagittal plane energies will dominate

9. Three-dimensional analyses of gait initiation • Methods • Subjects: • 15 undergraduate students provided informed consent to participate • Mean (± SD) mass: 74.0 (±16.7) kg, height: 1746.9 (±102.3)mm, BMI: 24.1 (±4.2)kg-m² • Protocol: • Standing, feet shoulder-width apart on two force platforms for min of 5s • Initiate walking with left foot landing on 3rd force platform. • Continue walking for a minimum of 4m • 5 trials per subject

10. Three-dimensional analyses of gait initiation • Model: • 21-markers defining 11 segments • 8-camera VICON MX system (100Hz) • 3 Bertec force platforms (1,000Hz) • Data analysis: • Joint and segment kinematics and kinetics • Inverse dynamics • Time-normalised (0-100%) of 3 successive steps using 6 gait events • Identified from averaged subject data

11. Three-dimensional analyses of gait initiation

12. Three-dimensional analyses of gait initiation • Joint powers: • Product of joint moment of force and joint angular velocity • Joint absorption and generation energies: • Calculated by integrating joint powers using a trapezoidal technique • Statistics • 3x2x2 ANOVA to examine main and interaction effects of step, plane, and joint on absorption and generation energy • Post hoc test with Bonferroni correction where appropriate • Paired-sample t-tests used where there were 2 groups

13. Three-dimensional analyses of gait initiation Sagittal Frontal

14. Three-dimensional analyses of gait initiation Sagittal Frontal

15. Three-dimensional analyses of gait initiation Sagittal Frontal

16. Three-dimensional analyses of gait initiation • Results

17. Three-dimensional analyses of gait initiation • Results #s: compare across STEPs K,H: comparebetween joints

18. Three-dimensional analyses of gait initiation • Hip has smallest correlations, ankle has largest for both legs • Right knee (STEP2) is negatively correlated

19. Three-dimensional analyses of gait initiation • Discussion: • Analysis supported hypotheses • Correlations between planes: • STEP1 and STEP3 (left leg) – increasing + correlations from proximal to distal joints • STEP2 (right leg) – similar in magnitude but knee is – correlation • Raises possibility of energy flow not just between joints in the same plane but between planes within the same joint • Increases potential of system to be even more efficient than previously believed

20. Directions for future research • Gait termination: • Reverse pattern of gait initiation? • More energy absorption is likely • ‘Freezing’ in Parkinson’s disease: • Most interventions have focused on sagittal plane • Frontal plane has significant energies in frontal plane in STEP1 • Joint stiffness: • Calculating joint stiffness as a time-series • How does this pattern change in each plane? • Can this have clinical implications?

21. Dr Shirley RietdykPurdue University ACKNOWLEDGEMENT

22. Thank You!