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.3
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.
Fig. 3:
the load on the lumbar vertebral column when
holding a weight (of 110 lbs) at various angles
of the spinal column.
Weight based fitness
equipment
Our Hydraulic fitness
equipment
180 deg. – 200 lbs
150 deg. – 795 lbs
120 deg. – 1390 lbs
90 deg. – 1590 lbs
180 deg. – 200 lbs
150 deg. – 200 lbs
120 deg. – 200 lbs
90 deg. – 200 lbs
In case of
the weight based equipment, the load of 110 lbs
at 180 degree is equivalent to 200 lbs, at 150
degree is equivalent to 795 lbs, at 120 degree
is equivalent to 1390 lbs and the same load is
equivalent to 1590 lbs at 90 degree.
In case of our
hydraulic fitness equipment, the load of 110 lbs
at 180 degree is
equivalent
to 200 lbs and it is the same throughout the
range of motion i.e. at each angle it is 200 lbs
as these equipment adjust automatically and
continuously to the
strength, power, speed output and the need of
the person using it or with the capacity of the
user. The
user can never apply more force than his
capacity.
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).
This is
the center of gravity. The upper body's 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.
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.
Fig. 4,
4a, 4b, 4c
Fig. 4:
Sundry joint loadings on the shoulder and
elbow with various lift techniques following a
shift in the center of gravity of the weight.
