Download HEALTH SCIENCES 365

HEALTH SCIENCES 365
Chapter 3 – Biomechanical Factors & Concepts
Biomechanics: the study of mechanics as it relates to functional and anatomical analysis of
biological systems.
Mechanics: the study of physical actions of forces. Divided into:
 Statics: the study of systems that are in a constant state of motion, whether at rest
with no motion or moving at a constant velocity without acceleration. (Forces acting
on the body being in balance)
 Dynamics: the study of systems in motion with acceleration. (Unequal forces acting
on the body causing it to be unbalanced)
Machines of the Body: Used to increase or multiply the applied force (muscular contraction) in
performing a task (to cause, control, prevent  push, pull, hold) to provide a mechanical
advantage.
 Mechanical advantage enables us to apply a relatively small force to move a much
greater resistance
 Mechanical advantage determined by dividing load by effort:
Load
Effort
Machines function to:
1. Balance multiple forces
2. Enhance force
3. Enhance range of motion and speed of movement
4. Alter the resulting direction of an applied force
Musculoskeletal System Machines: Levers, Wheel/Axles, Pulleys
Levers
Humans move through a system of levers – that cannot be changed but used more efficiently.
Lever: A rigid bar that turns around an axis of rotation, or fulcrum.
 Three Points of a Lever:
1. Axis (A): the point of rotation (joint) about which the lever moves
2. Force (F): the point of force application (usually muscle insertion)
3. Resistance (R): the point of resistance application
 Center of gravity of lever, or
 Location of an external resistance
NOTE: The arrangement of these three points determines the type of lever & which kind
of motion it is best suited.



First Class Lever: Axis (A) is between Force (F) and Resistance (R)
If axis is midway between force and resistance = balanced movement (seesaw)
If axis is close to the force = speed and range of motion (scissors)
If axis is close to resistance = force (crowbar)
Head on Neck
Axis: _____________________________
Force: ____________________________
Resistance: ________________________
Triceps Press
Axis: _____________________________
Force: ____________________________
Resistance: ________________________


Second Class Lever: Resistance (R) is between Axis (A) and Force (F)
Large resistance can be moved with relatively small force (wheelbarrow), (nutcracker)
Designed to produce force movements
Heel Raises
Axis: _____________________________
Force: ____________________________
Resistance: ________________________



Push ups
Axis: _____________________________
Force: ____________________________
Resistance: ________________________
Third Class Lever: Force (F) is between Axis (A) and Resistance (R)
Requires large force to move a relatively small resistance (catapult, paddle rowing,
Designed for speed and range of motion
Most levers in human body are this type
Biceps Curl
Axis: _____________________________
Force: ____________________________
Resistance: ________________________
Knee Flexion
Axis: _____________________________
Force: ____________________________
Resistance: ________________________
Mechanical advantage of levers may be determined using the following equations:
Mechanical Advantage = resistance
Mechanical Advantage = length of force arm
force
Length of resistance arm
Torque & Length of Lever Arms
Torque (moment of force) – the turning effect of an eccentric force
 Eccentric force – force applied in a direction not in line with the center or rotation of an
object with a fixed axis
 For rotation to occur , an eccentric force must be applied
 In the human body, the contracting muscle applies an eccentric force (not to be confused
with an eccentric contraction) to the bone on which it attaches and causes the bone to
rotate about an axis at the joint.
 The amount of torque can be determined by multiplying the amount of force (force
magnitude) by force arm
Force arm – the distance between the location of force and the axis.
 Distance from joint to where the muscle attaches
 Also known as moment arm or torque arm
 The greater the distance of force arm, the more torque produced by the force
Resistance Arm – the distance between the axis and the point of resistance application
 Distance from joint to center of gravity of distal segment
Inverse relationship between length of the two lever arms
 The longer the force arm, the less force required to move the lever if the resistance and
resistance arm remain constant
 Shortening the resistance arm allows a greater resistance to be moved if force and force
arm remain constant
Proportional relationship between force components and resistance components
 If either of the resistance components increase, there must be an increase in one or both
of force components
 Greater resistance or resistance arm requires greater force or longer force arm
 Greater force or force arm allows a greater amount of resistance to be moved or a longer
resistance arm to be used
 Even slight variations in the location of the force and resistance are important in
determining the mechanical advantage and effective force of the muscle
Lever Equation: For use in force calculations. Evaluation of torque with modifications in force
arms, resistance arms, and resistance.
F
x
(force)
FA
(force arm)
=
R
x
(resistance)
RA
(resistance arm)
The Human Leverage System
 Built for speed and range of motion at the expense of force
 Short force arms and long resistance arms require great muscular strength to produce
movement
 The longer the lever (or summation of several levers), the more effective it is in
imparting velocity (produces more linear force)
- A longer lever (Z’) travels faster than a shorter lever (S’) in traveling the same
number of degrees.
Torque, Length of Lever Arms, Angle of Resistance & Angle of Muscle Pull
EMC: Effective Muscular Component
ERC: Effective Resistive Component
Equations:
EMC = Muscular Force (MF) x Muscular Force Arm (MFA) x  Angle of
Muscle Pull (AMP)
ERC = Resistive Force (RF) x Resistive Force Arm (RFA) x  Angle of
Resistance (AR)
NOTE: When working these problems determine: a) type of muscle contraction, and 2) relative
speed of movement




Wheels & Axles
Used primarily to enhance speed and range of motion in the musculoskeletal system
Center of the wheel and the axle both correspond to the fulcrum
Both the radius of the wheel and the radius of the axle correspond to the force arms
If a wheel radius is 3 times greater than the radius of the axle, due to the longer force
arm, the wheel has a mechanical advantage over the axle
a. the outside of the wheel will turn at a speed 3 times that of the axle
b. the distance that the outside of the wheel turns will be 3 times that of the outside of
the axle
c. Mechanical advantage calculated by:
radius of the wheel
radius of the axle
Anatomical Example
1. Joint: Shoulder (glenohumeral)
2. Axle: ___________________________________________
3. Outside of Wheel: _________________________________
4. Force: __________________________________________
Pulleys
1. Single pulleys have a fixed axle and function to change the effective direction of force
application and have a mechanical advantage of 1.
2. Every additional rope connecting to moveable pulleys increases the mechanical advantage by
1.
Anatomical Example
1. Joint: Ankle and subtalar
2. Pulley: _________________________________________
3. Force: __________________________________________
4. Force Application: ________________________________
5. Movement: ______________________________________