KINETIC ANALYSIS OF GAIT INITIATION D. Gordon E. Robertson, PhD, FCSB 1 Richard Smith, PhD 2 Nick ODwyer, 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

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. Leg that lifts first will be called lead leg other leg is trail leg. There are many studies that have documented the paths of the centres of pressure, the motion patterns and EMGs but there have been very few studies which have computed inverse dynamics or moment powers.

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

Methods Seven subjects, five trials starting with preferred leg, than five trials with other leg Started with each foot on a separate force plate and stepped to two other plates and then several steps onto floor (4 force plates) Video taped at 200 fps, forces at 1000 Hz using Motion Analysis, EVaRT system 42 markers to make 13-segment 3D model of body Inverse dynamics done by Visual3D

Initiation of Gait Events and Phases Quiet stance First (lead) toe-off End phase 2 First (lead) toe-off End phase 2 Third toe-off End phase 3 Third toe-off End phase 3

Results movements started approximately 1.5 seconds before toe-off of trail leg (start of Phase 4) greatest variability and least movement occurs in first 0.5 seconds (loading phase of lead, unloading of trail) called Phase 1 afterwards consistent patterns of moments and powers begin Phase 2 ends at first toe-off (lead-leg TO) Phase 3 ends at second toe-off (trail-leg TO)

Results Centres of Pressure step 1 step 2 lead trail both line of gravity Notice that as the combined c. 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 after the lead-leg lifts.

Results – flexion/extension ankle knee hip ang.vels moments powers

Results – flexion/extension Lead-leg (until 2 nd TO) nonzero postural moments with high variability before toe-off at ankle and knee nonzero postural moments with high variability before toe-off at ankle and knee 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

Results – flexion/extension Trail-leg (until 2 nd TO) 2 nd toe-off - plantar flexors relatively inactive until just before TO - concentric work done at push-off - plantar flexors relatively inactive until just before TO - concentric work done at push-off - knee extensors stiffen knee during midstance - knee extensors do positive work at push-off - knee extensors stiffen knee during midstance - knee extensors 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

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 - hip flexors mainly used to swing legs - trail-leg ankle plantar flexors major source of energy - hip flexors mainly used to swing legs

Results – abduction/adduction Both Legs (until 2 nd TO) 1 st toe-off Adduction Abduction lead-leg hip abductors initiate lateral shift of CM lead-leg hip abductors initiate lateral shift of CM trail-leg abductors begin as just before lead-leg TO and continue to shift CM back to midline

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

Results – abduction/adduction First Two Steps (1 st to 3 rd TO) Adduction Abduction 2 nd toe-off

Conclusions Phase 1 (posterolateral shift of CofP): –release of both plantar flexor moments (by gastrocnemius & soleus going silent and tibialis anterior activating) –simultaneous hip abductor moment from lead-leg (probably by TFL & gluteus medius) Phase 2 (lead leg raise to 1 st TO): –hip flexors activate to lift thigh and flex of lead-leg –slight activity by plantar flexors of lead-leg –trail-leg relatively inactive –falling phase

Conclusions 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 bodys CM towards midline and landing area of lead-leg –knee flexors of lead-leg assist hip flexors to flex knee and hip Phase 4 (lead-leg midstance, trail-leg swing): –begin normal walking pattern with slightly higher dorsiflexor and knee extensor activity in early stance of lead-leg

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