Kinetics is a branch of classical mechanics that is
focused on the movements of various bodies and the forces that can act on both
bodies in motion and bodies at rest. Some people confuse the term with
“kinematics,” because the two words sound similar, and they both have to do
with the science of motion. In fact, “kinetics” itself is an out-dated term.
Scientists prefer to say “analytical dynamics” or simply “dynamics.” The
science of motion in the form of kinematics relies on some very clearly-defined
laws, such as the concept that an object in motion tends to remain in motion.
Kinetics expands upon these laws, adding some additional principles that help
to explain what happens when external forces act on an object. This branch of
classical mechanics recognizes the fact that many things can influence the
outcome of a series of events, ranging from whether an object is dropped or
hurled, to the obstacles the object encounters on its way to a destination. Many people are unconsciously aware of the laws of kinetics,
because they use them on a daily basis, whether they are tossing a dirty fork
into the sink from across the kitchen or designing bridges. Kinetics application is relevance in Occupational Therapy
in various ways. It is basically apply to prevent injury and improve
rehabilitation in terms of technique analysis and exercise given to the client.
(Smith 2013)
One specific performance areas in occupation that I
chose are leisure. Leisure is a non-obligatory activity that is intrinsically
motivated and engaged in during discretionary time, that is, time not committed
to obligatory occupations such as work, self-care, or sleep. (American Journal
of Occupational Therapy, 2008). Example of leisure activities is playing
basketball which involves jumping movement of human body. Jumping is
acceleration of body parts upward to increase the mutual force between us and
the earth above the force of body weight. Sir Isaac Newton penned three laws of
motion that capture the essence of movement of bodies, human as well as
inanimate. These laws have stood the test of time in our state of being close
to the earth and travelling at modest speeds. One in particular, the third law,
states that "action and reaction are equal and opposite." If you
accelerate your arms upward, the muscular force required to do this has an
equal and opposite reaction pushing the remainder of your body against the
ground at your feet. The reaction to this force is that of the earth pushing up
on you. When being try on your bathroom scales, you will see your weight
apparently increase and decrease. The reason is that the bathroom scale is a
force transducer that measures the ground reaction force. The subsequent
apparent decrease in force is due to upward deceleration of the arms, which
requires a downward force on the arms and an equal upward force on the
remainder of the body. If our initial upward arm motion is sufficiently
vigorous, the ground reaction force will go very high and subsequently drop to
zero as we jump upward off our bathroom scale. This is jumping. We experience
jumping as going upward because the earth is our reference for all things
stable.
Kinetics knowledge can be applied in clinical
practice for gait analysis of the patient. The integration of posture and
movement utilizes anticipatory and reactive postural control mechanisms. The
postural orientation of the individual relative to the base of support and
gravity determines the movement strategies that will be accessible and
effective. The alignment of body segments both at the initiation of movement
and throughout the evolvement of movements plays a critical role in the
postural control strategies utilized. The alignment of body segments in
relation to each other and the base of support and the expression of postural
control in relation to gravity and the environment are the key areas of focus
in stroke rehabilitation and the treatment of other neurological conditions.
(Graham et al. 2009)
One clinical case study for physical dysfunction is
Pyia, who is a 75-year old woman who has treated for breast cancer 8 years
earlier. She developed metastases, with an onset of acute, bilateral lower
extremity weakness and loss of sensation. For 2 days she felt “unsteady” when
she was walking and had one fall. By the time she was admitted to the hospital,
she was unable to walk. After few weeks later, Pyia was asked to identify what
areas of occupational performance were still problematic for her, what she
could do well, and what her goals were. She replied that she was happy to be
walking better but felt endurance was still a problem. She used the walker
independently in the home but still needed assistance to get up and down
stairs. (Heidi McHugh Pendleton and Winifred Schultz-Krohn. 2007) The kinetics
application here can be explained by the third of Newton’s laws of motion which
states that every applied force is accompanied by a reaction force. For every
action, there is an equal and opposite reaction. During gait, every contact of
foot with the floor or ground generates an upward reaction force. In the case
of Pyia, she used walkers instead of her own foot due to the physical
dysfunction. The upward reaction force is generated to the Pyia’s walkers while
she walks instead of her foot. The
weight of Pyia is distributed over the walker so that she can also have good
postural control while walking and maintain a stable gait with respect to the
force from the ground.
One of biomechanics instrumentation for kinetics
application is the use of dynamometer.
The client should be seated with the shoulder adducted and neutrally
rotated, the elbow flexed at 90 degrees, forearm in neutral position, and wrist
between 0 and 30 degrees extension between 0 and 15 degrees of ulnar deviation.
It is important for the client to have an appropriate position while the
therapist is taking the joint measurement of the patient. The force against
gravity to the weight of the client can affect the reading of the grip strength
of the dynamometer. Dynamometer is a device for measuring mechanical force, or
power, transmitted by a rotating shaft. Since power is the product of torque
(turning force) and angular speed, all power-measuring dynamometers are
essentially torque-measuring devices; the shaft speed is measured separately.
Among force-measuring devices are a flexible metallic ring that bends when a
force is applied in such a manner as to tend to collapse it—the amount of
bending being a measure of the applied force—and a hydraulic “load cell” that
measures compressive loads in terms of fluid pressure. (Encyclopaedia
Britannica 2013) A dynamometer has been designed for measuring isometric forces
in human body limb segments.
Isokinetic contraction is the muscular contraction
that accompanies constant velocity limb movements around a joint. The velocity
of movement is maintained constant by a special dynamometer. The resistance of
the dynamometer is equal to the muscular forces applied throughout the range of
movement. This method allows the measurement of the muscular forces in dynamic
conditions and provides optimal loading of the muscles . However, during
movements in the vertical plane, the torque registered by the dynamometer is
the resultant torque produced by the muscular and gravitational forces. The error
depends on the angular position and the torque potential of the tested muscle
group. Several methods have been developed for the correction of gravitational
errors in isokinetic data. The torque output also contains artefacts that are
associated with the inertial forces during acceleration and deceleration
periods before the development of the constant pre-set angular velocity. For an
accurate assessment of muscle function, only constant velocity data should be
analysed.
The most frequently used isokinetic parameters are
the maximum torque and the angular position where it was recorded, the torque
output at different angular velocities of movement, the torque ratio of
reciprocal muscle groups and the torque output during repeated contractions. The
unique features of isokinetic dynamometry are optimal loading of the muscles in
dynamic conditions and constant preselected velocity of movement. These
features provide safety in the rehabilitation ofpatients with muscular and
ligamentous injuries. Isokinetic dynamometry has also been used for the
training of various muscle groups in order to improve the muscular performance
in dynamic conditions. The movement velocity of different activities can be
simulated during training in order to improve the training effect. Data
acquisition and analysis have been improved by using computer systems
interfaced to isokinetic dynamometers. Recently developed computer systems
provide correction for gravitational and inertial errors, accurate computation
of isokinetic parameters and real-time display of the torque output .
In a nut shell, knowledge of kinetics is important
in Occupational Therapy practice. It is basically apply to prevent injury and
improve rehabilitation in terms of technique analysis and exercise given to the
client. Therapist observes the client’s movement and relates it to the ground
or gravitational force with respect to the mass or weight of the client. Any
postural impairments or physical dysfunction can affect the result of the
client’s areas of occupation and performance areas respectively. The conditions
face by the client should also being include in other to identify the best
technique will be given to the client.
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