1. Three-dimensional analyses of gait initiation in a healthy, young population Drew Smith 1 and Del P. Wong 2 1 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 small base 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 3 rd 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.  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 SagittalFrontal

14. Three-dimensional analyses of gait initiation SagittalFrontal

15. Three-dimensional analyses of gait initiation SagittalFrontal

16. Three-dimensional analyses of gait initiation  Results

17. Three-dimensional analyses of gait initiation  Results #s: compare across STEPs K,H: compare between 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 Rietdyk Purdue University ACKNOWLEDGEMENT