Power Production During Swim Starting

D. Gordon E. Robertson, Ph.D.

Vivian L. Stewart, M.Sc.

Biomechanics Laboratory,

School of Human Kinetics,

University of Ottawa, Ottawa, CANADA

Introduction

• theory holds that with explosive motions muscles are recruited sequentially from proximal to distal (CAC)

• vertical jumping research suggests simultaneous recruitment of leg moments (Robertson & Fleming, 1987)

• arm moments in running contribute little work to propulsion

• swim starters’ projectile angles are not optimized for maximum flight (< 10 deg)

Purpose

• determine determine whether changes in starting position increases projection angle

• determine recruitment ordering of moments

• power contributions of shoulder, elbow, hip, knee and ankle moments to swimming starts

Methods

• eight female varsity or club swimmers

• filmed at 50 frames/second

• force plate on starting platform synchronized to 50 Hz

• three starting positions:– declined platform with toes over edge

– declined platform with toes flush with top

– level platform with toes over edge

• flight phase distance calculated from fitting trajectory to least squares quadratic

Experimental Setup

ForceForceplatformplatform

Cine-cameraCine-camera

A/DA/Dconverterconverter

A/DA/Dconverterconverter

ChargeChargeamplifieramplifierChargeCharge

amplifieramplifier

ComputerComputer

Results

• no significant differences in horizontal flight phase distances among three starting positions (mean distance was 2.0 m)

• level start produced significantly reduced takeoff velocity (4.9 vs. 6.0 and 6.3 m/s)

• toes-back start produced greater takeoff angles (mean 4.1 vs. 1.2 and 2.7 deg)

Stick-figures of Grab Start

Results - Moments

• greatest peak moments were hip extensors then hip flexors, then knee extensors and ankle plantar flexors and lastly shoulder, elbow and knee flexors

Averaged (n=8) Moments of Force

Percentage of Thrust

0 10 20 30 40 50 60 70 80 90 100

-300

-150

0

150

-150

0

150M

omen

ts

(N.m

)Toes-over

ShoulderElbow

HipKneeAnkle

Extensor

Flexor

Extensor

Flexor

Results - Sequencing of Moments

• hip extensors recruited first

• later ankle extensors (plantar flexors) followed immediately by knee extensors

• hip flexors recruited to perform negative power synchronously with ankle/knee extensors

Variability of Moment Powers

0 10 20 30 40 50 60 70 80 90 100

Percentage of Thrust

-1000

0

1000

-1000

0

1000

0

1000

2000

3000

4000P

ower

s (w

atts

)Toes-over

Hip

Knee

Ankle

Results - Powers

• greatest variability with hip powers, least with ankle powers

• largest positive work by hip extensors, then knee extensors and then ankle plantar flexors

• hip flexors perform negative work prior to takeoff

Averaged (n=8) Moment Powers

Percentage of thrust

-2000

-1000

0

1000

0

1000P

ower

s (w

atts

)

0 10 20 30 40 50 60 70 80 90 100

Toes-over

ShoulderElbow

HipKneeAnkle

Discussion

• some evidence to support proximal to distal sequencing (i.e., hips preceded knee and ankle)

• knee and ankle extensors recruited simultaneously (like vertical jumping)

• hip moments dominated thrust

• negative work by hip flexors transfer energy from upper body to whole body (transfer of momentum)