Transcript Built for Speed: Musculoskeletal Structure and Sprinting Ability

Authors: Sabrina Lee and Stephen Piazza
I. Introduction

The fastest sprinters:
 Higher proportion of fast-twitch muscle fibers
 Larger leg muscles
 A muscle with a large moment arm about a joint
(generates large torques)

The ability to accelerate separates good
sprinters from world class sprinters.
 Accelerate the most during the first 30m of
100m sprint
 The leg muscles must perform maximum
concentric work
Morphological Research

The structure of the foot and ankle in
combination with muscle moment arms
are determinative of locomotor speed.
 Cheetahs, who can reach 70mph in 3 sec.,
have plantar flexor muscles that attach close
to the ankle joint center with a large distance
between the point of application for the
ground reaction force
 Thus the fastest runner’s have higher gear
ratios (MA of the GFR divided by the
moment arm of the muscle)
MA Ratios: Cheetah vs. Lion


Cheetahs have long
toes and very short
heel bones large
“gear ratio”
High gear ratios can
place a muscle at a
disadvantage, but is
overridden by the
high muscle forces it
permits
Fascicle Length/MA Ratio

The ratio of the fascicle length to
moment arm has been identified as a
determinant of the range of joint motion
over which a muscle may generate
force.
 The muscle moment arm and the number of
sacromeres determine the magnitude
sacromeres will shorten/lengthen for a given
rotation
Purpose
The purpose of the study was to
measure the plantar flexion moment
arms of the Achilles’ tendon, plantar
flexor muscle architecture parameters,
and foot and ankle anthropometric
characteristics in sprinters and nonsprinters.
 As far as the authors’ knowledge
extends this is the first study of plantar
flexor moment arms in sprinters.

II. Materials and Methods


Participants: 12 collegiate sprinters & 12
height matched non-athletes
Ultrasonography:
 This technique was used to determine muscle
fascicle lengths as well as pennation angles
(specifically of the lateral gastrocnemius)
 It was also used to determine tendon excursion
during ankle plantar flexion and to compute the
plantar flexion moment arm of the Achilles’ tendon.

Computer Model:
 To study the effects of muscle and joint structure on
sprint performance, a planar, forward-dynamic
simulation of the push-off phase was developed
III. Data & Results





Plantar flexion moment arms of the sprinters
were 25% smaller than non-sprinters
Sprinters had longer gastronemius fascicles
than non-sprinters
The mean ratio of fascicle length to plantar
flexor lever arm was almost 50% higher in
sprinters than non-sprinters
The average toe length was 9 mm longer in
sprinters than non-sprinters
The length of the fibular head to lateral
malleolus was 30 mm shorter in sprinters.
Table 1. Anthropometric characteristics and
ages of the sprinter and non-sprinter subjects
Spinters
Non-sprinters
P-value
Stature (cm)
181.4 (+/-) 8.0
180.9(+/-)7.6
0.874
Body Mass (kg)
77.0 (+/-) 6.5
76.8(+/-)9.5
0.954
Age (yrs)
19.3(+/-) 1.2
25.4(+/-)2.8
<.001
Fibular head to lateral malleolus (cm)
41.1 (+/-) 2.6
44.1(+/-)3.5
0.026
Heel to toe (cm)
27.4 (+/-) 1.1
26.9(+/-)2
0.473
Heel to 1st metatarsal head (cm)
19.2 (+/-) .9
19.5(+/-)1.6
0.485
Heel to lateral malleolus (cm)
5.5 (+/-)0.7
5.6(+/-)0.3
0.594
Lateral malleolus to 1st metatarsal head* (cm)
13.7 (+/-) 0.7
13.9(+/-)1.4
0.589
Lateral malleolus to toe* (cm)
21.9(+/-)0.9
21.3(+/-)1.8
0.305
First metatarsal head to toe* (cm)
*These values were not measured directly but
were derived by subtraction from measured
quantities
8.2(+/-)1.0
7.3(+/-)0.9
0.032
Table 2. Musculoskeletal architecture
parameters for the lateral gastrocnemius
measured from ultrasound images
Sprinters
Non-sprinters P-value
LG thickness (mm)
15.5(+/-)1.9
14.7(+/-)2.0
0.33
LG pennation angle (deg.)
12.8(+/-)1.2
13.7(+/-)2.1
0.212
LG fascicle length (mm)
Achilles' tendon moment arm
(mm)
69.9(+/-)6.2
62.7(+/-)8.3
0.024
31.0(+/-)3.7
41.6(+/-)5.5
<.001
Fascicle length:moment arm
2.28(+/-).32
1.53(+/-).27
<.001
LG, Lateral gastrocnemius
Forward Impulse TL Varied and Held
Constant at 35mm
Forward Impulse PF MA varied
and TL Held Constant at 75mm
IV. Applications

Evolution- Early hominids had 40%
longer toes than modern humans,
suggesting that shorter toes evolved
because they require less muscle effort
and are more economical over
endurance distances.
 Longer toes allow more propulsive force,
permitting greater acceleration

Specially designed track shoes
V. Conclusions
Sprint performance depends upon
muscle structure as well as composition.
 Sprinters achieve rapid acceleration with
short plantar flexor moment arms in
combination with long fiber lengths.
 Longer toes increase contact time,
increasing time for acceleration.

Questions?