Conferences

The effect of shock absorbing sports
surfaces in jumping
Juan V. Durá
Instituto de Biomecánica de
Valencia
Abstract

A jumping movement is performed over
five different sports surfaces within a wide range of shock
absorbency. The sports surfaces tested have an influence in
performance and in the accelerations measured on people. The
subjects tested changed their movements and adaptation to the
surface has been observed. The surfaces with high shock absorbency
(more than 70% shock absorbency) do not improve protection
significantly and reduce performance in the jumping
movement.

Keywords

Sports Surfaces, Shock Absorption,
Jumping

Introduction

The shock absorbing property and the
compliance of sports surfaces have been related with injuries and
performance in sports. It seems that there is an opposite relation
between the necessity to damp impacts to avoid injuries and
performance in sports. But some authors have shown the possibility
of finding an optimal point for compliance in sports surfaces
(MacMahon 1979, Bosco 1985).

There are different techniques to
measure the sports surface behaviour under impacts. One of the
easiest methods is to drop a weight with an attached accelerometer.
This technique may not be appropriate since it does not simulate the
impact that a sportsman could produce in jumping or running.
Additionally, the results depend on the weight dropped and the
contact surface (Nigg 1987, Francis 1988, Maki
1990).

Nowadays, one of the commonest standard
test methods for multipurpose indoor surfaces is DIN18032-2 (DIN
1991). This standard uses the "Artificial Berlin Athlete" machine
(Figure 1) to simulate the impacts that a person weighing 80-90 kg
would produce in a vertical jump. The Artificial Berlin Athlete has
been accepted by the IAAF and other international sports
organisations to be the best practical solution for measuring the
shock absorbing property of the sports
surfaces.

Figure 1: Artificial Berlin
Athlete

The DIN method consists of measuring the
maximum impact force over the sports surface and calculating the
force reduction percentage in comparison to a very rigid surface,
such as concrete.

For simulating the behaviour of people,
the "Berlin Artificial Athlete" has a calibrated spring. The purpose
of the spring is to produce an impact similar to the impact that a
person would cause when jumping and falling on his or her feet, by
increasing the time of impact with the hip, knee and ankle flexion
movements.

The problem is to know how good is the
simulation of a person’s behaviour and to decide what the limits for
a good or bad surface are regarding shock
absorbency.

The DIN18032-2 standard requirements are
51% of force reduction for ‘pointelastic’ surfaces and 53% for
‘areaelastic’ surfaces in multipurpose indoor surfaces. Some elite
sportsmen think that this shock absorption is excessive since
performance is highly reduced. FIBA (International Basketball
Federation), for example, recommends 40% for basketball
courts.

Although footwear could be as important
as the surface in shock absorption, the general opinion is that a
minimum of shock absorbency is necessary to assure comfort and
reduce overuse injuries in sports. The aim of this study is to find
the relation between the shock absorbing property of surfaces,
measured with the DIN method, the performance and the impacts in
jumping movements.

Methods

Five different surfaces were selected
and tested according to DIN18032-2. The surfaces are within a wide
shock absorbency range, from 0 to 75% (Table
1).

Table 1: Surfaces
tested

SURFACE

DIN FORCE REDUCTION
PERCENTAGE

DESCRIPTION

A

0.0%

Rigid terrazzo
pavement

B

19.0%

Synthetic surface (PVC). 4 mm
thick

C

44.0%

Synthetic surface (Synthetic
rubber). 16 mm thick

D

62.3%

Wood surface, beech parquet
with crossbeams and elastics
pads

E

75.1%

Synthetic surface. PVC top layer
4mm and a PU base layer 70mm
thick

Five healthy young persons, non-elite
sportsmen, were selected with the following
characteristics:

Age:

from 17 to 24
years

Weight:

from 64 to 69.5
kg

Height:

from 1.75 to 1.82
m

Shoe size:

42 (French
scale)

The movement (Figure 2) consists in
falling from a 42 cm high bench onto the surface. They were asked to
jump as high as possible after the first contact with the surface
and to keep their arms crossed on their chest. With the arms in this
position the variability of movement performance is reduced, since
arm movement could help to obtain more or less height by movement
synchronisation.

Figure 2: Movement and accelerometer
position.

Before starting the measurements the
subjects performed several jumps in order to adapt their movements
to the surface. The movement was then repeated 5 times on each
surface. There were 25 repetitions (5 persons by 5 times) on each
sports surface.

The test subjects wore the same sports
shoes, since the objective was to find the effect of the surface. If
the worn shoes had been different, variability would have increased
and perhaps the effect of surface would have been
hidden.

Two extensometric accelerometers were
attached to the subjects: one to the lower limb and another to the
forehead. The lower limb placement was chosen to be the proximal
anterior part of the tibia, 3-4 cm under the tibial tuberosity in
the internal part.

The accelerometer specifications
are:

Forehead: Range 20 g, resonance
frequency 1200 Hz, sensitivity 2.1 mV/g, weight 0.3
grams.

Lower limb: Range 50 g, resonance
frequency 1200 Hz, sensitivity 1.0 mV/g, weight 0.3
grams.

The accelerometers were attached to the
skin by a double sided adhesive tape and an aluminium support. The
weight of the system was less than 2.5 grams. An elastic bandage
wrapped tightly around the shank and forehead was used to fasten the
accelerometer and to preload the skin (Figure
2).

A contact sensor was placed into the
shoe sole for measuring the time that the subject was on the surface
and in the air. These times were used as a measurement of
performance.

The signals from the accelerometers and
shoe sensor were acquired with a personal computer with an A/D
board. The sample frequency was 6000Hz (2000Hz per
channel).

Eight reflective markers were attached
to define four segments (two for each segment) and the movements
were recorded in a KINESCAN-IBV video system at 50 Hz with one
camera. A 2D movement analysis was done, and the flexo-extension
angles of the hip, knee and ankle were calculated. The decision to
do only a 2D analysis was based on the opinion that in the jumping
movement the flexo-extension movement is the most relevant and the
angles in other planes are not so important (Sussman
1988).

The subjects were recorded in standing
position and the angles of the joints calculated. The joint angles
in standing position were used as the origin of the measure (0
degrees).

Temporal and kinematic parameters were
obtained, and with each of these parameters a multifactor analysis
of variance of repeated measures was performed. Subject and surface
were considered as factors. A multiple range test of Least Squares
Differences (LSD) at 95% was used for post hoc analysis to determine
on which surfaces the differences were
significant.

The temporal parameters studied
were:

tfloor

time that the person is in contact
with the floor between the first and the second
impact.

tair

time that the person is in the air
between the first and the second
impact.

The kinematic parameters studied
were:

maximum forehead acceleration in
the first impact.

maximum shank acceleration in the
first impact.

maximum ankle
flexion.

maximum knee
flexion.

maximum hip
flexion.

Results

The results obtained for the different
parameters with regard to the surface factor are shown in Table 2.
The P value that appears in the table is for the surface factor. The
sports surface factor was always significative.

Table 2: Results of multifactor
variance analysis
* The interaction surface-subject has
been significant

SPORTS
SURFACE

PARAMETER

P value

ERROR

A

B

C

D

E

tfloor
(s)

0.000

± 0.006

0.387

0.427

0.390

0.413

0.434

tair
(s)

0.000

± 0.003

0.566

0.566

0.562

0.574

0.545

(g)

0.000

± 0.07

3.39

3.02

3.24

2.96

2.87

(g)

0.040

± 1.43

15.99

13.07

18.55

15.35

12.97

*
(º)

0.000

± 0.34

29.71

29.25

29.01

34.84

26.50

*
(º)

0.000

± 0.90

92.61

92.22

89.03

84.23

94.06

*
(º)

0.000

± 1.02

90.47

94.28

89.18

98.40

93.47

The results of multiple range tests of
Least Squares Differences (LSD) at 95% are shown in Table
3.

Table 3 Homogenous groups. LSD method
at 95%

A

B

C

D

E

tfloor
(s)

1

2,3

1

2

3

(s)

12

1,2

1

3

1

(g)

2

2

2

1

1

(g)

1,2

1

2

1,2

1

*
(º)

2

2

2

3

1

*
(º)

2

2

2

1

3

*
(º)

1

2

1

3

2

The multiple range tests classify the
surfaces in homogeneous groups in function of the parameter value.
The numbers indicate the groups. Two numbers appear when a surface
could be classified in two different groups. For example, surface A
belongs to group 1 in the case of  parameter, and surface B can belong to group 2 or group 3.
Number 1 groups the surfaces with the minimum parameter value. Group
3 includes the maximum parameter values. Figures 4, 5, 6 and 7 show
these results.

The homogeneous groups for accelerations
are shown in Figure 3. In the case of forehead acceleration (), surfaces A and C showed the greatest values, and
surfaces B, and E the lowest. The highest value of shank
acceleration () appeared on C surface. B and E showed the lowest values.
A and D showed intermediate values.

Figure 3: Means and 95.0 LSD intervals
for accelerations

Although the surfaces are very different
according to the DIN 18032 test method, the differences in
accelerations do not permit to classify surfaces in more than two
groups. The low and high absorbing surfaces are mixed in
the  groups. The  separates the surfaces in two groups: low (A, B, C) and
high (E, D) absorbing.

The homogeneous groups for times are
shown in Figure 4. Time differences between surfaces are bigger than
those in accelerations. In this case it is possible to classify
surfaces in three groups. But the groups do not classify the
surfaces in function of the shock absorbing property measured by the
DIN18032 test method.

Figure 4: Means and 95.0 LSD intervals
for times

The time that the subjects remain on the
floor is higher when shock absorption increases, except in the case
of surface B (Figure 4 left).

Figure 5: Means and 95.0 LSD
intervals for Knee flexion angle

Figure 6: Means and 95.0 LSD
intervals for ankle flexion angle

Figure 7: Means and 95.0 LSD
intervals for hip flexion angle

The time that the subjects remain in the
air is more or less alike for surfaces A, B and C. The three
surfaces belong to the same group. It is slightly longer for surface
D and clearly shorter for surface E (Figure 4
right).

The homogeneous groups for angles are
shown in Figures 5, 6 and 7. Angle differences between surfaces are
bigger than acceleration differences and it is possible to classify
surfaces in three groups. But the groups do not classify the
surfaces in function of the shock absorbing property measured by the
DIN18032 test method.

Surface D shows the highest ankle and
hip flexion, and the lowest knee flexion. Although surface E is the
most shock absorbent, it shows the highest knee
flexion.

Discussion

The maximum acceleration could be
interpreted as a measurement of the protection against possible
overuse injuries and the time values as a measurement of jump
performance. If contact time with the floor is low, the sportsmen
need less time to perform repeated jumps. If time in air is high,
the jump is higher. These times could be important in sports like
basketball or volleyball.

The results show that surface E (with
shock absorption of 75%) does not increase the protection in
comparison with other surfaces with less shock absorbency and
reduces jump performance. For surface E
tfloor  is high
and  is low.

The maximum jump performance,  high, appears for surface D (shock absorption of 65%) and
it is coincident with the minimum knee flexion angle, the maximum
hip flexion angle and the maximum ankle flexion angle. This surface
also shows a low  tfloor.

When tfloor is low, accelerations are higher. The impact energy
absorbed by the athlete body causes more acceleration if the energy
is dissipated in less time.

Although the interaction subject-surface
is significant for the angle parameters, the tendency observed in
the five subjects is similar. Anyway in future research more
subjects are necessary because interaction could have influenced in
the results obtained.

The subjects perform more knee flexion
on the surfaces with low shock absorption and in the surface with
very high shock absorption (E surface). In the case of low shock
absorption the reason could be the need of using the knee joint for
reducing impact levels. In the case of the E surface it is necessary
to find another explanation. One reason could be that the subjects
remain more time over the E surface because the surface deformation
is longer along time. Then the elastic energy stored in the muscles
cannot be recovered in an elastic fashionand they need to flex more
the knee. More studies are necessary to confirm this
hypothesis.

Discrepancies have been found between
surface B and C. Surface C shows higher impacts on the leg than
surface B, although it is more shock absorbent. But shock absorbency
has been measured using the force reduction percentage parameter
used in DIN18032-2, and this parameter does not consider other
energy aspects that could explain this different
behaviour.

Conclusions

Different shock absorbency levels create
changes in joint flexion to maintain impacts (accelerations) in
acceptable levels. The athlete tries to adapt his or her movements
to maintain protection (impact levels) and performance (jump height)
in surfaces with different shock absorbing
properties.

This adaptation is visible in the
changes of flexion angles for the knee, hip and ankle. When the
surface is more rigid, knee flexion is higher to maintain impacts
(accelerations) in acceptable levels. As the objective of the
movement studied was to jump as high as possible, the changes in
knee angles caused changes in the other joints (ankle and hip) to
maximise the jump height. But this tendency changes when shock
absorbency is very high (more than 70%). Very shock absorbing
surfaces suffer higher deformations for more time. Then the elastic
energy stored in the muscles cannot be recovered in an elastic
fashion and they need to flex the knee more. More studies are
necessary to confirm this hypothesis.

The results seem to show the need of
fixing a shock absorbency limit for force reduction parameter at
about 70%, although it is necessary to validate it with studies with
other type of test subjects. The results with heavier people, such
as elite basketball players, could be different. Besides, other
parameters, like absorbed energy, should be considered in measuring
deformation and force at the same time.

Acknowledgements

This work was supported by the Spanish
Interministry Commission for Science and Technology (Reference
Number SAF94-0518) and JUNCKERS INDUSTRIER A/S.

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