1. KINETIC ANALYSIS OF GAIT INITIATION D. Gordon E. Robertson, PhD, FCSB 1 Richard Smith, PhD 2 Nick O’Dwyer, PhD 2 1 Biomechanics Laboratory, School of Human Kinetics, University of Ottawa, Ottawa, Canada 2 Biomechanics Laboratory, School of Exercise and Sport Science, University of Sydney, Sydney, Australia

2. Introduction Gait initiation usually consists of the period between quiet stance and steady-state gait. In most cases, it takes only 2 steps to reach steady-state. The leg that lifts first will be called lead-leg; the other leg will be the trail-leg. There are many studies that have documented the paths of the centres of pressure, the motion patterns and EMGs but very few studies which have computed inverse dynamics or moment powers.

3. Purpose To examine the patterns of the moments of force and their powers of the lower extremities during gait initiation. To determine the sequence of events required to initiate gait from quiet stance. To observe whether similar patterns occur with a variety of subjects.

4. Methods Seven subjects performed five trials starting with their right leg Started with each foot on a separate force plate and stepped “briskly” to two other force plates and then continued several steps onto the floor Video taped at 200 fps, forces sampled at 1000 Hz using a Motion Analysis, EVaRT system 42 markers tracked to make 13-segment, 3D model Inverse dynamics and powers computed by Visual3D

5. Initiation of Gait: Events Quiet stance First toe-off (lead-leg) First toe-off (lead-leg) Second toe-off (trail-leg) Second toe-off (trail-leg)

6. Results movements started approximately 1 to 1.5 seconds before trail-leg toe-off greatest variability and least movement occurred in first 0.5 seconds (loading phase of lead, unloading of trail) called Phase 1 after Phase 1, consistent patterns of moments and powers began Phase 2 ended at first toe-off (lead-leg TO) Phase 3 ended at second toe-off (trail-leg TO)

7. Results: Centres of Pressure step 1 step 2 lead trail both line of gravity Notice that as the combined centre of pressure moves backwards and towards the lead-leg the line of gravity proceeds forwards and towards the trail-leg. The line of gravity is outside the base of support shortly after the lead-leg lifts.

8. Results – flexion/extension ankle knee hip ang.vel’s moments powers

9. Results – flexion/extension Lead-leg (until 2 nd TO) nonzero ‘postural’ moments at ankle and knee with high variability before toe-off nonzero ‘postural’ moments at ankle and knee with high variability before toe-off 1 st toe-off hip flexors act concentrically before toe-off and early swing to flex hip and swing lead-leg hip flexors act concentrically before toe-off and early swing to flex hip and swing lead-leg - small plantar flexor power burst at push-off - knee extensors do negative work until midswing - small plantar flexor power burst at push-off - knee extensors do negative work until midswing

10. Results – flexion/extension Trail-leg (until 2 nd TO) 2 nd toe-off plantar flexors inactive until just before lead-leg FS when concentric work done for a push-off plantar flexors inactive until just before lead-leg FS when concentric work done for a push-off - knee extensors stiffen knee during midstance - knee flexors do positive work at push-off - knee extensors stiffen knee during midstance - knee flexors do positive work at push-off - hip flexors act eccentrically before toe-off - switch to concentric work to create swing of trail-leg - hip flexors act eccentrically before toe-off - switch to concentric work to create swing of trail-leg

11. Results – flexion/extension Both Legs (until 2 nd TO) 1 st toe-off moments and powers normalized to body mass moments and powers normalized to body mass trail-leg ankle plantar flexors major source of energy lead and trail-leg hip flexors used to swing legs trail-leg knee flexors do positive work at push-off

12. Results – abduction/adduction Both Legs (until 2 nd TO) 1 st toe-off Adduction Abduction lead-leg hip abductors activate to initiate lateral shift while trial-leg abductors release lead-leg hip abductors activate to initiate lateral shift while trial-leg abductors release trail-leg abductors begin just before lead-leg TO while lead-leg abductors become silent trail-leg abductors begin just before lead-leg TO while lead-leg abductors become silent

13. Results – flexion/extension First Two Steps (1 st to 3 rd TO) 2 nd toe-off after first step, patterns of moments and powers are almost the same as normal steady-state patterns

14. Summary Phase 1 (posterolateral shift of CofP): –highly variable period consisting of release of lead- and trail-leg plantar flexor moments (gastrocnemius & soleus going silent and tibialis anterior activating) –simultaneous hip abductor moment from lead-leg (probably by TFL & gluteus medius) and release of the trail-leg abductors Phase 2 (lead-leg raise to 1 st TO): –lead-leg hip flexors activate to lift thigh –slight activity by plantar flexors of lead-leg –trail-leg passive or isometric

15. Summary Phase 3 (lead-leg swing and trail-leg pushes): –trail-leg plantar flexors act to push body forward, largest burst of positive power –hip abductors of trail-leg act to shift body’s CM towards midline and landing area of lead-leg –knee flexors of trail-leg assist hip flexors to flex knee and hip Phase 4 (trail-leg swing, start normal walking): –begin normal walking pattern with slightly higher dorsiflexor and knee extensor activity in early stance of lead-leg

16. Thank You Any questions? Comments?