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).
 

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

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.

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 t_floor  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  t_floor.

When t_floor 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.

References