During your time
training at Brunel Kobu-Jitsu Karate Dojo you will
hear terms such as PNF, Kinesiology and Biomechanics,
this is an important part of our understanding with
regards to training safely. We teach biomechanics in
the form of lectures along with other topics which
you will be able to attend as part of your rank requirement
seminars. It is a requirement of the YKKF that all
instructors are qualified to teach with a detailed
and full understanding of how forces effect the human
body. The YKKF in the late 1970’s was one of
the first International federations to have bio-mechanically
correct martial art techniques taught in their dojo’s
worldwide. This section of the website is devoted to
the explanation of what biomechanics is all about,
in addition to an explanation of the terms and names
that are generally used when discussing biomechanics.
So what exactly is biomechanics?
Biomechanics is the research and analysis of the mechanics
of living organisms. It is the study of the effects
of internal and external forces on the human body in
movement and rest. Some simple examples of biomechanics
research include the investigation of the forces that
act on limbs, the aerodynamics of bird flight, the
hydrodynamics of swimming. The biomechanics of human
beings is a core part of kinesiology and is very useful
when looking at was to maximize movement without causing
damage to the body.
Before we look into this topic we must first have
an appreciation for the following:
(Biomechanics), The Basic Forces, Inertia & Mass,
Center of gravity (mass), Basic Anatomy of arm and
Kinematics is the description of motion, including:
considerations of space and time, patterns and speeds
of movement sequencing, the forces causing the motion
are not considered in kinematics.
Kinetics (Biomechanics) is the study of the relationship
between the forces acting on a system and the motion
of the system.
A concept relating to the difficulty with which an
object’s motion is altered
• the quantity of matter composing an object
• the measure of inertia for linear motion
• the property giving rise to gravitational
• Units: – SI: kilogram (kg)
Centre of Gravity
• Geometric point about which every particle
of a body's mass is equally distributed
• Position of the Centre of Mass changes with
changes in body configuration.
• Motion of the Centre of Mass represents the “average” motion
of the body as a whole
Center of gravity (Zenkutsu dachi stance)
• A mechanical interaction between an object
and its surroundings
• The “push”, “pull” or
rotation of one object on another (compressive, tensile,
• Force is a vector.
– a magnitude
– a direction
– a point of application
Force direction and
point of application
• Resultant force derived from the composition
of two or more forces
• Reflects the net effect of all of the forces
Direction of Forces and summation
• Forces cause acceleration or deformation (a
change in shape)
– To keep the description as simple as possible,
we will assume that the forces acting on a body cause
– The relationship between force (F), mass (m)
and acceleration (a) can be given by Newtons second
F = m a
A force that is applied at a single point (as seen
in the figure above)
• A force that is applied over a distributed
• Can be approximated by a concentrated force
that has the same net effect
• The force due to gravity (i.e. the pull of
Weight has magnitude:
FW = m g
m = mass
g = acceleration due to
gravity (9.81 m/s2 ) Direction of Force due to gravity
• Weight always acts at the centre of gravity
and points towards the centre of the Earth
Jit Kundalia has a mass of 58 kg.
– What is his weight in Newtons (N)?
– Does his weight depend on the position of
In the International Space Station, orbiting 400 km
above the Earth, the acceleration due to gravity (g)
is about 8.7 m/s²
– What would Jit’s mass be on the space
– What would his weight be on the space station?
should be able to calculate this after attending
the biomechanics rank requirement seminar)
In simple terms there are 3 forces of interest to
us when looking at biomechanics, these are:
1. Torsion Forces
2. Tensile Forces
3. Compressive Forces
Torque or torsion
• A measure of the extent to which a force will
cause an object to rotate about a specific axis
• A net force applied through the centre of
gravity produces translation (linear motion)
• A net force applied at any other point produces
both translation and rotation
Torque Force (rotation)
Compression, Tension, & Shear
• Compression : pressing or squeezing force
directed normal (perpendicular) to a surface
• Tension : pulling or stretching force directed
normal to a surface
• Shear : sliding or tearing force directed
Compressive, Shear Forces.
Deformation under Tensile, Compressive, Shear loading.
Bending and shear.
• Load producing a twisting of a body
• Creates shear stresses
• Shear stresses are greatest at the surface
• The motion of a body depends not only on the
force, but also on the duration that the force is applied
• Impulse : a measure related to the net effect
of applying of force (F) for a time (t):
Impulse = F t
• Impulse increases with:
– Increased force magnitude
– Increased duration of application
• Equal impulses result in equal changes in
• Materials behave elastically at small loads
• Loads above the yield point create permanent
• Rupture or fracture occurs at the ultimate
Stress vs Strain graph.
Repetitive vs. Acute Loading
• The size of the loading required to cause
a bone to fail (i.e. fracture or rupture) decreases
as the number of loading cycles increases.
Stress Causing Failure vs. # of Loading Cycles
The elbow joint (courtesy MMG)
There are several important Ligaments in the elbow.
Ligaments are soft tissue structures that connect bones
to bones. The ligaments around a joint usually combine
together to form a joint capsule. A joint capsule is
a watertight sac that surrounds a joint and contains
lubricating fluid called synovial fluid.
The Ulna Collateral Ligament (courtesy MMG)
In the elbow,
two of the most important ligaments are the medial
collateral ligament and the lateral
collateral ligament. The medial collateral is on
the inside of the elbow, and the lateral is on the
Together these two ligaments connect the humerus
to the ulna and keep it tightly in place as it slides
through the groove at the end of the humerus. These
ligaments are the main source of stability for the
In such dynamic art as karate it is extremely important
to understand the physical rules on the one hand and
their use in connection with the performance of the
techniques. When executing a karate technique it is
not only important to move the hand or the foot in
the correct way but in each technique the insertion
of the whole body has to be visible. e.g Zenkutsu Dachi
or Sanchin Dachi (especially in Go-Ju Ryu karate the
change between tension and relaxation appears very
well.) At the point of a strike all the muscles have
to be in tension. This is necessary to make sure, that
for this moment the force can be transmitted in the
best possible way from the target point at the opponent
through the body into the floor. The transmission of
the force is made sure by correct techniques, well
trained muscles of the skeleton, the muscles of the
stomach and the persons own stability in the standing
position (the stance must not include that of the extreme
that there is not enough mobility existent).
• The presupposition for a high effective technique
are declared based on the following physical rules
(simplified). E.g. for a punch (figure 14.4)
and Fist = Mass, Speed of punch = Velocity.
The Kinetic energy, which is, the ability of moved
bodies to do work, increases with the exponent of 2
of the velocity. Given that the arm size cannot be
changed during the technique, the only variable is
the velocity (v). Which should be as fast as possible
to achieve maximum effect.
It is not the only purpose
to get a great Ke which can be changed into deformation
energy by performing
a technique, but there is also another physical value
of importance. This force is know as impulse.
more information on biomechanics of punching and
kicking you can purchase Nat Peat’s Warrior Syllabus
and Handbook 2004 from the dojo, or order online.
Click here for common definitions
Acceleration--the rate of change of velocity
Accelerometer--a device that measures
Accommodation--the decrease in biological
response to an unchanging stimulus.
rate of change of angular velocity (vector).
Angular displacement--the change
in angular position (vector).
Angular momentum--the quantity of angular
motion, calculated as the product of the moment of
inertia times the angular
Angular velocity--the rate of change
of angular displacement (vector).
Angular impulse--the angular effect of
a torque acting over time, the product of the torque
and the time it
Anisotrophic--a body with different
mechanical properties for loads in different directions.
study of the physical properties of the human body.
Ballistic--fast, momentum assisted
principle--the pressure a fluid can exert decreases
as the velocity of the fluid increases.
of mass/gravity--the point that represent the total
weight/mass distribution of a body. The mass
centriod is the point where the mass of the
object is balanced in all directions.
Center of pressure--the
location of the vertical ground reaction force vector.
The center of pressure measured
by a force platform represents the net forces in
support and the COP may reside in regions of low local
Common mode rejection--a measure of the
quality of a differential amplifier in rejecting common
Compression--a squeezing mechanical loading
created by forces in opposite directions acting along
Compliance--the ratio of change in length
to change in applied force, or the inverse of stiffness(see
A material that is easily deformed has high compliance.
Concentric muscle action-- the condition
were an activated muscle(s) creates a torque greater
than the resistance
Creep--the increase in length (strain)
over time as a material is constantly loaded.
Degrees of freedom--the
number of independent movements an object may make,
and consequently the number of
measurements necessary to document the kinematics
of the object.
Direct dynamics--biomechanical simulation
technique where kinematics of a biomechanical model
calculated from muscle activation (kinetic) inputs.
Direct linear transformation (DLT)--a
short-range photogrammetry technique to create 3D coordinates
(x, y, z) from the
2D coordinates (x, y) of two or more camera views.
Displacement--change in position in a
particular direction (vector).
Double differential amplification--EMG
technique to eliminate cross-talk.
Dynamics--the branch of mechanics
studying the motion of bodies under acceleration.
Deccentric muscle action--the
condition were an activated muscle(s) creates a torque
less than the resistance
Elastic--the resistance of a body
to deformation (see stiffness).
Elastic (strain) energy--the potential
mechanical work that can be recovered from the restitution
of an body
that has been deformed by a force (see hysteresis).
Electromyography (EMG)--the amplification
and recording of the electrical signal of active muscle.
ability to do mechanical work (potential, strain,
or kinetic energy are all scalar
Excursion--the change in the
length of a muscle as the joints are moved through
their full range of motion.
External work--work done on a body by
an external force.
Finite element model--advanced biomechanical model
to study how forces act within a deformable body.
rate--the number of times a motor unit is activated
Force--an instantaneous push, pull or
tendency to distort between two to bodies.
muscle mechanical property that shows how muscle
force potential depends
on muscle length.
Force platform--a complex force transducer
that measures all three orthogonal forces and moments
Force-time relationship--(see elecromechanical
muscle mechanical property that shows how muscle force
on muscle velocity.
Fourier series--a mathematical
technique of summing sin and cos terms that can be
used to represent the
frequency content of a signal.
Frame (video)-- a complete
Free body diagram--a
technique for studying mechanics by creating a diagram
that isolates the forces acting
on a body.
Frequency domain/content--time varying
signals can be modeled as sums of weighted (see Fourier
Frequency response--the range of frequencies
faithfully reproduced by an instrument.
Global reference frame--measuring
kinematics relative to an unmoving point on the earth.
to measuring angular position.
High pass filter--a signal processing
technique that removes low frequency components of
muscle model--a common three component model of muscle
force consisting of a contractile component,
series elastic component, and parallel elastic component.
energy loss in the elastic recoil of a material.
Impulse--the mechanical effect of a force
acting over time (vector). J = F?t
relationship--principle that the change in momentum
of an object is equal to the net
applied. Original language of Newton's second law
and is equivalent to the instantaneous version:
F = ma.
Integrated EMG (IEMG)--the area under
a rectified EMG signal. Correctly the time integral
reported in units
amplitude?time (mV?s). Unfortunately, old EMG equipment
and some report IEMG that is not really integrated,
but filtered or smoothed EMG values (mV) that is
essentially a liner envelope detector.
Internal work--work done
on body segments by internal forces (muscles, ligaments,
for "in place" or structures
isolated by dissection.
In vitro--Latin for "in
glass" or tissues
removed from the body but preserved.
for "in the living" or during
Isometric muscle action--the condition
were an activated muscle(s) create a torque equal
to the resistance torque.
Jerk--the third derivative of displacement
with respect to time.
Joint center--an approximation of the
instantaneous center of rotation of a joint.
Joule--the unit of mechanical
energy and work.
measurement of the motion of an object relative to
some frame of reference.
linkage of rigid bodies. An engineering term used
to simplify the degrees of freedom needed
to document the mechanical behavior of the system.
a force or moment applied to a material.
cell--a force measuring device.
curve--the mechanical behavior of a material can
be documented by the instantaneous measurement
of the deformation and load applied to a material.
Local reference frame-- measuring kinematics
relative to an moving point on nearby rigid body (joint,
or center of mass).
Low pass filter--a signal processing
technique that removes high frequency components
of a signal.
reflective materials attached to subjects to facilitate
the location of segments
or joint centers for digitizing.
Moment (moment of
force, torque)--the rotary effect of a force.
Moment arm--the leverage of a force for
creating a moment. The perpendicular distance from
the axis of
rotation to the line of action of the force.
of inertia--the resistance to rotation (angular acceleration)
of a body.
Momentum--the quantity of
motion of an object calculated by the produce of
mass and velocity (vector).
unit--a motor-neuron and the muscle fibres it innervates.
Newton--the SI unit of force. One Newton
is equal to 0.22 pounds.
Pascal--the SI unit of pressure or stress
(force per unit area).
Pennation--the angle of the muscle fibre
bundles relative to the tendon.
Potentiometer--a device for measuring
(mechanical)--the rate of doing mechanical work.
Peak mechanical power represents the greatest mechanical
effect, the ideal combination of force and velocity.
can be calculated as W/t or F?V.
Radius of gyration--a
convenient way to summarize an object's moment of
inertia, defined as the distance
from the axis of rotation half the object's mass
would have to be to equal the object's moment of inertia.
Recruitment--the activation of motor
units of muscles by the central nervous system.
of vibration that matches the physical properties
of the body so the amplitudes
of the vibration grow rather than decay over time.
Scaling--converting image measurements
to actual size.
Scalar--simple quantity completely defined
by a single number (magnitude).
Shear--mechanical loading in opposite
directions and at right angles to the surface of
speed--limiting the time that a photographic/video
image is made (e.g. 1/1000 of a second) to prevent
blurring of moving objects.
Simulation--use of a biomechanical
model to predict motion given input conditions in
order to study the
factors that affect motion (see direct dynamics).
parameter--a index of the amount of smoothing allowed
in splines. The larger the smoothing parameter
the more smoothing (allowable deviation between the
raw and fitted curve).
Snap--the fourth derivative
of displacement with respect to time.
Statics--the branch of mechanics studying
bodies at rest or uniform motion.
Strain--the amount of deformation
of a material by an applied force, usually expressed
as a percentage
change in dimensions.
Strain gage--a small array that
is bonded to materials and senses the small changes
in size (strain) as the
material is loaded. Usually used to measure force
Strength (muscular)--the maximum force
or torque produced by a muscle group in an isometric
action at a specific
joint angle. Research has found several domains of
strength expression depending on the time, velocity,
and resistance involved.
total work or peak force required to break a material.
of a material, measured as the slope of the stress/strain
curve in the elastic region (Young's modulus of elasticity)
of a loaded material.
Stress (mechanical)--The force
per unit area in a material.
Stress-relaxation--the decrease in stress
in a material with time when subjected to a constant
cycle (SSC)--a common coordination strategy where
agonists for a movement are eccentrically
loaded in a countermovement, immediately before the
concentric action and motion in the intended direction.
SSC result in larger initial force and greater concentric
work than purely concentric actions (reversible muscle
Telemetry--a technique to send biomechanical
signals to recording devices without wires, using
a FM radio
transmitter and receiver.
Tension-- a pulling apart
(making longer) mechanical loading created by forces
in opposite directions acting
along a longitudinal axis of a material.
an averaging/smoothing value in EMG processing, the
larger the time constant the
larger the time interval averaged over, meaning more
Vector--a complex quantity requiring
the description of size and direction.
due to gravitational force.
Work (mechanical)--work is done when
a force moves an object in the direction of the force
and is calculated
by the product of force and displacement.
relationship--principle of physics that the work
done on a body is equal to the net change
Yield point--point on the load-deformation
curve where a material continues to deform without
Peat, N. L. (2004) – The Warrior Syllabus and
Handbook (2nd ed).
Fung, Y.C. (2003) - "Biomechanics: Mechanical
Properties of Living Tissue" (2nd ed.). New York
Vogel, Steven. (2003). Comparative Biomechanics: Life's
Physical World. Princeton: Princeton University Press.
Medical Multimedia Group 2004.