Biomechanics - the application of mechanical laws to __living___
structures, specifically to the ___locomotor____ system of the human body.
I. Uses of Biomechanical Analyses
• Improvement of sports skill ___techniques__
• Design of sports ____equipment_____
• Prevention of ____injuries__
• Clinical analysis of movement ___pathologies__
• Design of ___prostheses___
• Design of ____rehabilitation______ devices
Qualitative analysis - a __non-numerical_____ description of a movement
based on direct ___observation____. Conducted primarily by teachers and
Quantitative analysis - a movement is analyzed __numerically__ based on
___measurements_____ from data collected during the performance of the
movement. Conducted by researchers.
II. Levers of the Human Body
Refer to Unit 13 in Lab Manual.
Lever –a rigid bar that turns about an axis – in the body, the bones
represent the bars and the joints are the axes.
Force point – the exact point where the effort is applied
Resistance point –the exact point on which the resistance acts
Fulcrum – the axis of motion 2
Force arm – the perpendicular distance from the fulcrum to the line of
action of the force acting on the force point
Resistance arm – the perpendicular distance from the fulcrum to the line
of action of the resistance acting on the resistance point
First-class – its fulcrum at some location between the force point and the
resistance point [ex. Is a teeter-totter]
Second-class – have their resistance point at some location between the
force point and the fulcrum [ex. Wheel barrow, swinging door]
Third-class levers – has its force point at some location between the
resistance point and the fulcrum [Most common in the body since it
permits the muscle to be inserted near the joint and to produce distance
and speed of movement although at a sacrifice of force. Ex. Shoveling,
swinging a golf club]
Mechanical advantage -
Mass – the quantity of matter contained in an object. Units = __kilograms
Force – __Mass__ X acceleration. Units = Newtons (N)
1 N = (1 kg) (1 m/s )2
Weight – the amount of ___gravitational___ force exerted on a body
Weight = mass X ___acceleration of gr2vity___ = ma g
Acceleration of gravity = 9.81 m/s
Units of weight – _Newtons (N)____
If a person has a mass of 80 kg, his weight = (80 kg) (9.81 m/s ) = 785 N
Torque – the product of force and the perpendicular __distance__ from the
force’s line of action to the axis of rotation. It may be thought of as rotary
Torque = Force (N) x Moment arm (m)
Units = Newton-meters (N∙m)
Moment arm – the perpendicular distance between the force’s line of action
and the axis of __rotation____.
The Principle of Levers
-This principle states that a lever of any class will balance when the produce
of the force times the distance from the point of application of the force to
the fulcrum is equal to the resistance times the distance from the point of
application of the resistance to the fulcrum or axis.
Force x force arm = Resistance x Resistance Arm F x FA = R x RA
Elbow flexed at 90 degrees
10-kg mass held in the hand
Angle of muscle pull is 90 degrees [not true]
FA = 5cm [i.e biceps muscle inserts 5cm from the joint axis]
Question: How much force does the biceps muscle have to exert to hold the
Solution: [TIP – Remember to convert the units of measure!!!!]
F x FA = R x RA [You can use 10 instead of 9.81]
F x 5cm = (10kg x 9.81) x 0.35m
F = 70 x 9.81N
F = 687 N
Levers of the human body
Conversely, when the resistance arm is longer than the force arm, the lever
gfavours speed and range of motion at the sacrifice of force nad is called a
speed lever this is the most common lever in the human body 4
Mechanical advantage = force arm/resistance arm
-Is the mechanical advantage of a first class lever greater than, less than or
equal to one?
IT COULD BE ANY!! – This is because of the axis or rotation. RA and FA
could be exactly the same or it could be closer to either the FA or RA.
-For a second class lever The 2 class lever is always MA > 1 because
it’s always a force lever
3 class MA<1 and is ALWAYS a speed lever because the mechanical
advantage is always less than one.
Mechanical advantage of a lever – the ratio of force arm length to
__reisistance arm___ _________ length
Volume – the amount of __space__ that a body occupies
Pressure – force distributed over a given __area__. Units = N/cm 2
Pressure = F/A
Compression – pressing or squeezing force directly _axially__ through a
Tension – pulling or stretching __force__ directly axially through a body
Shear – force directed parallel to a _surface_
Mechanical stress = F/A Similar to __pressure____.
Is it better to be stepped on by a woman wearing a spike heel or a woman 5
wearing a smooth-soled athletic shoe?
-Woman’s mass = 50kg
-the spike heel area (As) =5cm^2
-the athletic shoe area (Aa)=100cm^2
-Weight (fg) =mag (assume ag=10m/s^2)
-Pressure (P) =Force/Area
Woman weight = 50kg x 10m/s^2 = 500N
The spile heel pressure = Fg/As = 500N/5CM^2 = 100N/cm^2
The athletic shoe pressure = Fg/Aa = 500M/100cm^2 = 5N/cm^2
Comparison = 20 times more pressure with high heel
Lifting A Heavy Object From the Floor
1. If the object is very heavy, get someone to _help__ you. Don't be a
"hero". Use techniques that __minimize___ the actual weight of the load
2. Stand facing the object with your feet _flat_ on the floor, at shoulder
_width__, and pointing straight _ahead_. Ensure that you have a stable
base of support so that you don’t __slip__ as you are lifting the load.
3. Face the object in the __direction__ which you intend to move with it, so
that you don't have to turn while holding the object. Avoid twisting and
the simultaneous generation of high twisting torques.
4. Keep the object as _close__ to your body as is convenient to minimize
the reaction torque on the low back.
When 40cm away, Tobject = 0.4m X 100N = 40NM [Newton meters]
When 20cm away, Tobject = 0.2 X 100N =10NM
5. Get a good _grip_ on the object so that you don’t lose control of it as you
are lifting it. 6
6. Bend at the __knees__ and __hips__, and keep your back as __straight_
as possible. Avoid a fully flexed or bent spine.
7. Lift the object using the knee and hip extensor muscles, not by pulling
upwards with the __arms__ and ___back___.
8. Carry the object close to your __center___ of gravity.
The flat back lifting posture has been found to be better overall than a
___rounded__ back in minimizing _L5/S1___ disc