Methods of impact absorption when
landing from a jump*
A LEES
Department of Sport and Recreation Studies, Liverpool Polytechnic
The vertical reaction force was measured during impact landings following a vertical jump. It was observed
that variations between subjects occurred in the magnitude of the peak force over the first 150-200 ms after
impact. This period of the landing is termed the impact absorption phase. Two extreme cases of impact
landings are taken for analysis. These are referred to as ’hard’ and ‘soft’ landings, describing the magnitude
of the vertical force peak during impact absorption. These differences are produced as a result of the
interactions which occur between the individual segments of the human body. The segmental contribution to
the total force curve was derived by film analysis techniques.The specific influence of each segment on the
total force curve was isolated and made comparable with other segments by using the relative acceleration of
the segment mass centre to the lower joint about which it rotates. By using this variable to compare hard and
soft landings, it was established that a soft landing was produced by a phased and controlled deceleration of
body segments
Introduction
One aspect of sports biomechanics is to analyse human
sporting actions in terms of how segments of the body
interact with each other to produce an optimum end
result.
One particular sporting action which has received
much attention is the jump for height (Adamson and
Whitney, 1971; Smith, 1972; Miller, 1976). This is an important element in many sporting activities, for example
gymnastics, volleyball and basketball, in addition to the
well known athletics event of high jumping. The natural
consequence of a jump is the following impact landing.
This has received much less attention despite the fact that
it is more likely to result in injury (both immediate and
long term) as a consequence of the large impact forces.
Impact absorption at landing may be performed in a
variety of ways, depending on the circumstances of the
landing. These circumstances relate to the expected force
of impact and can be observed by viewing landings on
both hard and soft surfaces. For example when landing
on a soft surface such as a trampoline, trampette or
springboard, the body is typically straight and there is
little flexion in the major joints of the body during the
impact. However, on a hard surface, there is a marked
flexion of the joints and an appreciable amount of ‘give’
in the body. In these cases the body is adapting itself to
the harder surface so as to reduce the force of impact.
Although these are extreme cases, they illustrate very
well the differences in landing techniques which are used
by people in general when landing on a fairly hard surface. In this situation landings are produced which can be
described as ‘hard’ or ‘soft’ depending on the relative
magnitudes of the vertical component of the initial force
peak.
Impact landing can be expressed in mechanical terms.
The downward momentum of the body must be reduced
to zero, and the change in momentum is related to
Newton’s second law:
Amu=
.c
F(t).dt
This paper was originally presented at a conference on ‘The Biomechanics of
Sports Medicine and Physical Education’ held at the University of Leeds in
January 1981.
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1981 VoI. 10. No. 4
If the body is kept fairly rigid, all the body segments
decelerate together. The time for total body deceleration
is small and so the force level is high. The body behaves
like a pole being dropped on its end. However if the body
adopts a more flexible state, it behaves like a damped
spring. The time for deceleration increases and so the
force level becomes smaller.
These results are not only a consequence of Newton’s
second law, but are observed experimentally, in for
example, the vertical jump and the corresponding landing
as measured by a force platform (Fig. 1).
From a sports point of view, if large forces are applied
when the body is slightly off balance then these can serve
to cause the performer to lose balance more easily. From
a medical point of view, the high force levels are undesirable as they can lead to immediate or long-term
injury. From a mechanical point of view, the body is a
multi-linked system and the reduction of force levels,
desirable both sportingly and medically, is the result of a
complex interaction between the links or segments of the
system.
This study is concerned with how the segments of the
body interact to reduce the force levels acting on it during
an impact landing.
Characteristics of the landing action
The activity used for investigation in this study is the
landing from a vertical jump. This activity is a skill which
can be performed by both young children and mature
adults.
Although the landing (i.e. the duration from the
moment of impact to the establishment of the body in a
stationary balanced position) takes place over nearly one
second, the feature which is described as ‘impact absorption’ lasts for only 150-200 ms. After this time the downward momentum of the body has been substantially
reduced and the landing, from the point of view of
‘momentum reduction, is nearly over (Lees, 1977). The
rest of the landing action is concerned with the maintenance of balance, and can be illustrated by the variation in
force levels, F , ,and point of application of force, A , ,over
eight landings as measured by a force platform (Fig. 2).
Therefore in order to investigate the techniques of impact
absorption an analysis must be made of body segment
interaction over the first 200 ms after landing commences.
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207
I\
HARD
4
-
-+flight
Je---jump
landing 1*-
SOFT
weight
time
Fig. 1. Comparison of the vertical reaction force of a ‘hard’ and ‘soft’ landing
produced from a vertical jump
Segmental contributions to the total
force curve
The differences shown between subjects in the vertical
force curve over the impact absorption phase (first 200
ms) is a consequence of segmental interactions in the
multi-linked system. These interactions are predominantly influenced by the levels of muscular excitation.
They are a result of human skilled performance, and as
such cannot be explicitly defined.
For any particular landing force curve, the contributions of each segment, i, to the vertical force component, F,(t), may be obtained from film analysis
methods (Smith, 1972). Considering the limbs of the
human body as rigid segments whose centre of mass
location, T i , is known, the total body centre of mass
location, R, is given from the principle of moments as:
MR =
mili
L
I
I
0.2
0.4
0.6
0.8
1.0
1.2
1.4
b
balance
absorption
Fig. 2. The landingfrom a vertical jump can be divided into the ‘impact absorption’ phase, demonstrated by highly consistent movement patterns, and the
‘balance’ phase, demonstrated by variability in muvement patterns
particularly the A, (point of application offorce) variable
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1981
where the total body mass M , and segmental masses mi
are known. Taking the vertical, z, component of this
equation and differentiating twice with respect to time,
gives the total body acceleration, A, in terms of the
segmental accelerations a, :
MA, =
1miaZi
i
The observed acceleration of the total body centre of
mass may be related to the net force acting on the total
body system, according to Newton’s second law. The
forces acting on the system (in the vertical direction) are
the ground reaction force, F,(t), and the gravitational
force M g . Thus:
F,(t) - Mg
= MA, =
miuzi
i
and hence the segmental accelerations, aZi,can be related
to the measured ground reaction force. The vertical component of the acceleration ofeach segment is calculated in
practice directly from the segmental displacement, I ,
using appropriate numerical methods to produce acceptable acceleration values, sensitive to changes over the
time period of interest (Lees, 1980).
If this is done for each major segment of the body
(lower legs, upper legs, head and trunk, and arms), the
proportional contribution to the total vertical force curve
can be obtained, and is illustrated in Fig. 3.
0.4
0
Q)
L
U
0
From this segmental analysis, it is noticed that each
segment contributes in different ways to the vertical force
curve. However, the magnitude of the contribution is
closely related to the segmental mass. For a direct comparison to be made of the segmental contribution to the
total vertical force curve, the segmental acceleration, aZi,
rather than force, miuzi,should be used. This variable is
independent of segmental mass, and can be obtained
directly from film analysis as described above.
Components of segmental acceleration
While the segmental acceleration enables comparisons to
be made between subject performances,this variable does
not isolate the segment from the influences of the rest of
the body. For example, it is not known whether the
measured acceleration is due to that imparted by an
adjacent segment, or due solely to rotation about that
segment, or due to a combination of them both.
For a segment undergoing both linear and rotational
motion the acceleration of its mass centre in a particular
direction is given by the component of acceleration of the
joint to which it is attached, plus a component of
acceleration due to the rotation of the segment about
that joint.
This can be expressed as:
a,(total) = a,(ioint) -I-u,(relative)
where the total segment acceleration in the vertical, z,
direction is given by the sum of the components due to
the acceleration of the joint and the acceleration of the
segment mass centre relative to that joint.
The u,(relative) component of acceleration isolates the
behaviour of the individual segment, resulting from the
activity of the muscles which surround the joints of the
segment. This variable, which is easily obtained experimentally from film analysis, can be used to interpret the
contribution of individual segments to the total acceleration, and hence force, curves. For the case of landing
from a jump, it is practical to take the lower or distal
joint as a reference point, for determining a,(relative).
Before impact each segment is approximately vertical
and during impact, flexion at the joint occurs followed by
extension.At the end of the landing all segments are again
approximately vertical. This knowledge concerning the
general sequence of events is useful as it enables an interpretation of the details of u,(relative) to be made without
detailed reference to angular kinematic data. From
standard kinematic analysis, the magnitude of a,(relative)
will be related to the angle of orientation of the segment,
as well as its angular velocity and angular acceleration.
Magnitudes of a,(relative) thus obtained can be related
to the motion of the segment. However, of greater importance for the interpretation of a,(relative) are the
timing of sequential acceleration components and their
relation to the total force curve.
Application to impact landing
0.6
0
0.1
0.2
0.3
0.4
0.5
Time, s
Fig. 3. Segmentalcontributions tothe landingfrom a verticaljump
The above analysis can be applied to two contrasting
landings from a vertical jump performed by two adult
male subjects.One demonstrates a high force peak during
the impact absorption phase and is referred to as a ‘hard’
landing. The other demonstrates a low force peak and is
referred to as a ‘soft’ landing (see Fig. 1). The relative
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3.0
HARD
g
SOFT
2.0
L
O
d
U
L
0
LL
1.o
body wtight
I
0.1
0.2-
I
0.2
1
0.3
I
I
0.4 0.5
I
0.6
1
0.7
.-
Time, s
lower legs
lower legs
I
I
1-1
-0.2-1
-0.2’
N
0.2-
v)
\
upper legs
N
E
c
1
,-
upper legs
-1
z
E -0.2 J
v)
E
rn
i
‘
C
a,
.o
-m -0.2
A
E
u
L
L
0)
0.21
trunk
-0.2
-0.2
-0.2J
trunk
1 arms
o.21
o2’1=-
-0.2
-0.2
Fig. 4 . ‘Hard’ landing. Relative acceleration of segment mass
centres about the lower (distal)joint centres of rotation
accelerations of the major segments for each subject are
shown in Figs. 4 and 5. Several points can be made concerning each landing as follows.
Hard landing
1. Immediately after impact the two leg segments show a
negative acceleration. This is an expected feature as the
impact force will tend to cause joint flexion to take
place.
2. The magnitude of the negative acceleration is indicative of the stabilizing effect of the muscles surrounding
the joint about which the segment rotates. The larger
this negative acceleration, the less stabilized is the
joint.
3. The sequential nature of the negative accelerations is
obvious: first the lower and upper legs, and then the
head and trunk segments.
4. After the initial negative acceleration each segment
shows a positive acceleration. This can be attributed to
the reaction or stabilization of the system (but not due
to conscious reaction as this usually takes longer than
about 150 ms).
5. The reverse in acceleration occurs very rapidly, and
produces large acceleration and hence force peaks. Of
particular importance is the lower leg. This shows an
early peak and indicates that the musculature controlling rotation of the leg around the ankle joint generates
tension quickly and strongly.
Fig. 5 . ‘Soft’ landing. Relative acceleration of segment mass
centres about the lower (distal) joint centres of rotution
6. At the time when the lower leg shows its maximum
positive acceleration, the thigh and head and trunk
segments have a very small relative acceleration. This
means that the joints about which these segments
rotate are fairly well stabilized and so acceleration of
the lower leg is transferred to the other segments. This
is the reason why the peak in the lower leg curve
corresponds so closely to the total force curve.
7. From this data it may be concluded that the major
peak occurring during impact absorption is due to,
firstly, a quick and strong response to leg flexion about
the ankle joint, and secondly, joint stabilization at the
hip and knee joints. The rapid reduction in force is due
to the subsequent deceleration of the head and trunk
segment.
Soft landing
1. There is a similar negative acceleration of segments on
impact, starting firstly at the lower leg, and then progressing to the head and trunk, but these are more
obviously phased than in the hard landing.
2. However the magnitude of the negative accelerations
for the lower leg is small. In addition it changes only
slowly to a small positive acceleration. Thus one of the
causes of the high force peak is absent in this subject. It
is assumed that the musculature around the legs is
correctly pretensioned, and its reaction to impact
occurs over a longer period of time-extending into
the individual’s reaction time.
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Similarly with other segments, the acceleration values
are in general lower and are extended over a larger
period of time. This implies that the musculature controlling joint actions is better prepared and better
controlled.
The initial force peak is reduced in magnitude, but it is
noticed that a second peak is present when the lower
legs and thighs show a positive acceleration. However,
the effect of this is reduced due to a negative acceleration of the trunk.
It may be concluded from this data that a soft landing
is producedby aphaseddeceleration of segments, and a
pattern of muscular activity which anticipates the
demands made upon it. An additional comment may
be made on the use of the arms in each subject. In the
hard landing, there is a positive relative acceleration at
the start and this increases the total vertical force. In
the soft landing, the opposite is observed, and the effect
is to reduce the total vertical force.
Is prevention better than cure?
An analysis of two extreme forms of impact landing
indicate that differences do occur which can be related to
the musculature controlling segmental interaction. However, the important features of impact landing occur in a
time period which is shorter than human reaction time. If
any attempt at trying to reduce force levels during the
impact absorption phase is to be successful then attention
must be paid to altering the structure of what is known as
the ‘motor programme’ (i.e. the sequence of muscular
commands which are issued to cope with an event, but
which are not influenced by consciously mediated feedback, see for example, Stelmach, 1976).
Such changes in motor behaviour are produced as a
result of a training programme. The details of such a
programme designed to reduce force levels in impact
situations are not the concern of this paper, but they
would require the details concerning sequential interaction presented here, translated into communicable
teaching or coaching points.
Reduction of impact force levels as noted here is a
feature of skilled performance. In a related study (Lees,
1977)subjects producing soft landings also had particular
experience in gymnastic skills although they were not
competitive gymnasts. In a survey of school boys
( N , = N z = 20, age R, = 11.4, X, = 8.9 years) there
were no subjects who could produce what is described
here as a soft landing. The typical peak deceleration
demonstrated by these subjects was between 4 0 m/s2
but for adults only 2&30 m/s2.
These data suggest that the skill of landing from a jump
in order to reduce the magnitude of impact forces, is very
much under-developed at the school ages considered. The
general skill of absorption of impact landings is not
specificallytaught in schools as a movement skill, and so
it is hardly surprising that many adults have failed to
master it. Indeed many people are not aware that landing
from a jump is a skill until injury results-but at this point
it is too late !
References
Adamson, G. T. and Whitney, R. J. (1971) Critical appraisal of jumping
as a measure of human power. In Medicine and sport, Vol. 6 .
Biomechanics I I , pp 208-21 1.
Lees, A. (1977) A biomechanical analysis of the movement patterns
associated with selected static and dynamic balance activities. Doctoral
thesis, University of Leeds.
Lees, A. (1980) An optimised film analysis method based on finite
difference techniques. J. of Human Movement Studies, 6, 165-180.
Miller, D. I. (1976)A biomechanical analysis of the contribution of the
trunk to standing vertical jump take-offs. In Physical education, sports
and the sciences, edited by J. Broekhoff.
Smith, A. J. (1972) A study of forces on the body in athietic actions
with particular reference to jumping. Doctoral thesis, University of
Leeds.
Stelmach, G. (1976) Motor control: issues and trends, Academic Press.
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