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Every sporting
movement involves numerous different forces, which are subject to
certain laws. A rough grasp of, and taking due note of, these laws will
ensure that not only your strength training is easy on your joints but
your endurance training is energy saving. It will also help you to
recognize to some extent unusual strains exerted on the passive motor
system during certain lift techniques and thereby avoid incorrect
movements.
The diagram
below shows how expenditure of energy is determined by the proximity of
the weight to be lifted from the initial position. The forces are shown
as arrows, which also indicate the size and direction of the force. A
weight of roughly 20 kgs. is thus subject to a gravity of about 100
newtons. The Newton unit is a measure of force, and when directed
vertically downwards the force expressed in newtons is always ten times
the relevant weight. If a force operates on a point from a given
distance, torque is exercised on it. The torque is thus the product of
the effective force and the length of the lift. If two forces from
opposite directions operate on a point, they stand in balance if the
product of the length of the lifting arm and the force is equal.
Fig. 1 :
the balance of forces of two different loads. Being at different
distances from the pivot point, they develop different torque. The
product (force x distance from pivot) is equal, e.g. 100 newtons x 3 ft.
= 50 newtons x 6 ft.
Fig. 2:
the energy used by the back muscles for establishing balance at the
center of gravity: if the center of gravity of the upper body as pivot
is thrice as far from the vertebral bodies as the distance of the back
muscles behind the pivotal point, the tractive power of the back muscles
must be thrice as large as the weight of the upper body.
As the torque
with an unchanged load and longer lift gets greater and greater, the
load on the structure concerned also increases. The consequence for both
strength and endurance training is that weights moved or parts of the
body moved should be as close to the body as possible, partly in order
not to overstrain the kinesthetic system and partly to save energy.
In an erect,
immobile body there are a multiplicity of levers and forces which must
ultimately be in balance so that the body does not move. In every
immobile body there is a point where it can be hung up so that, however
it is rotated from this point, it will always be in balance. This is the
center of gravity. The upper bodys center of gravity lies in its
dynamic effect line in front of the spinal column, which, depending on
the type of physique, is more or less far in front of the hip joint.
This means that, to stand erect, the back extensor muscles and gluteal
muscles are needed to generate an immobile balance (Fig. 2).
If the
center of gravity moves forward when the upper body is bent towards at
the hip joint, the forces required to maintain balance rise to a
multiple of the original value. |
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Fig. 3: the load on the
lumbar vertebral column when holding a weight at various angles of the
spinal column.
To be able to judge which
muscles are greatly or less greatly involved, biomechanical awareness is
necessary as well as an understanding of the insertion and origin of
musculature. Particularly central to the former are the concepts of
torque and center of gravity presented here, together with the laws of
lifting derived from them. If for example one lifts a weight by bending
the forearm at the elbow, the strength with which individual muscles are
involved in the movement varies according to the technique used to lift
the weight. If the elbow moves backwards while lifting the load (fig. 4
a) the load arm between the dynamic effect line of the weight (passing
directly under the shoulder joint) and the pivotal point in the elbow is
shortened. In this lifting technique, the arm-bending muscles are less
strained by the relatively shorter lift as in the diagram beside it (Fig
4 b) In the latter case, not only is the lift clearly longer between the
elbow and the weight, the strain on the shoulder joint is also clearly
higher because a longer load arm lies between the center of gravity in
the weight and the pivot in the shoulder, whereby greater torque is
effected in the shoulder joint. In the first picture on the other hand,
the center of gravity of the weight lies just beneath the pivot of the
shoulder, so that the torque, and therefore also its load, is virtually
zero. If on the other hand the upper arm rests on a inclined cushion
(Scott-curl), the arm flexor muscles are indeed maximally loaded, but at
the same time the shoulder muscles and joint are relieved by locking the
body and body support (fig 4 c). As this example shows, a biomechanical
understanding can help the trainer, when comparing other exercises as
well, choose the exercise best suited for the given training task. |
