PTHY 6401 Kinesiology I
Basic Biomechanics
Read Neumann Ch 1, pgs 13-25 AND Ch 4, pgs 92-98
Kinematics is not Kinetics. Body Planes & Axes, Osteokinematics & Degrees of Freedom, Closed & Open
Kinematic Chains, Arthrokinematics & Convex/Concave Rules
Kinetics - analysis of forces (i.e. gravity, center of gravity, line of gravity, vector forces, levers, torque, moment
arms, mechanical advantage, etc..) and their effect on the body
Biomechanics - the application of principles of physics to the body
TYPES OF MOTION
1. Translatory – object moving in a straight line (same direction, same speed). Translatory motion is
measured and expressed in units such as meters, mm, cm, etc..
2. Rotary (angular) - object rotating around an axis (common for joint osteokinematics). Rotary motion is
typically measured and expressed in degrees (  ).
3. Curvilinear – combined rotary AND translatory motion. Gait is also an excellent example b/c rotary
movement at the major LE joints results in translatory movement of the whole body.
FORCE & TORQUE
1. A force is a physical quantity that can accelerate and/or deform a body. We deal with tensile,
compressive and shear forces. The source of such forces are 1 or more of these:
Gravity - In biomechanics, it is the attraction of body, or body part, toward the center of the earth. It is the
most common source of force acting on human body. Includes concepts of Center of Gravity (COG) & Line of
Gravity (LOG). Gravity produces (exerts) an external torque on the body.
Muscle - Produces force on the body by contraction through its point of attachment.
Friction - Force which resists the sliding of one object past another
2. Force implies both magnitude and direction, and is a vector quantity.
Characteristics of a vector arrow:
 Base of arrow – represents point of application of force
 Orientation & Tip of arrow – represents line of application & direction of force
 Length of arrow – represents magnitude of force
3. Torque - the ability of a force to cause rotation of the lever about the
axis.
Rotation of the segment is dependent upon:
 magnitude of force exerted by the effort and resistance
 distance from the axis of movement that the force is applied
Torque= force x lever arm distance (“torque” is aka. “moment”)


Common units for torque are “ft-lbs” or “N-m”
lever arm distance = shortest distance between the line of applied force and
the axis of lever; aka: length of a line drawn perpendicular to line of force to
the axis. ALSO CALLED “MOMENT ARM”
PTHY 6401 Kinesiology I
CLASSIFICATION & PRINCIPLES OF LEVERS IN PARALLEL FORCE SYSTEMS
Lever - a rigid apparatus that rotates around an axis; the rotation is caused by force(s), which produce torque.
Components of a Lever System: Axis (A), Effort Force (EF), Resistance/weight (R)
Effort arm (EA): distance from EF to Axis “internal moment arm”
Resistance arm (RA): distance from R to Axis “external moment arm”
In the musculoskeletal system, the rigid parts of the lever are bones, the Axis (A) is commonly a joint,
the Effort Force (EF) is ms. contraction (location of EF is ms. attachment site on the bone that is
moving), Resistance (R) is weight is limb + any objects held or attached (location of R is the
(combined Center of Gravity).
MATH & FORMULAS (for lever systems in stasis):
Law of levers Equilibrium Equation: EF X EA = R X RA [aka. internal torque = external torque]
Mechanical advantage (MA):
MA = Effort Arm distance (EA) / Resistance Arm distance (RA)
General concept: as MA increases, the amount of angular
movement decreases; as MA decreases, the amount of angular
movement increases.
Essential concepts of 1st, 2nd, and 3rd class lever systems
1st Class lever (seesaw)

MA can be 1, greater than 1, or less than 1

Examples:
- Occipito-Atlantal joint (MA typically >1)
- Triceps – elbow extension (MA < 1).
PTHY 6401 Kinesiology I
2nd Class lever (wheelbarrow):

MA is ALWAYS greater than 1.

Example:
- Gastroc/Soleus – plantarflexion
- Brachioradialis - elbow flexion (depends on location of COG)

What is the functional up-side to having a 2nd class lever in the body?
3rd Class lever:

MA is ALWAYS less than 1.

Examples: Supraspinatus, Deltoid, Brachialis, Biceps, Wrist Flex/Ext, Gluteus Max, Quads, Hams, Ant.
Tibialis, many more . . .

What is the functional up-side to having a 3rd class lever in the body?
3rd class lever is most common in the body and serves us best because it allows for maximal
___________________________________________ if enough muscle force can be produced. The external torques
we encounter in our functional activities are relatively small, such as weight of limb and small objects we carry.
Questions:
[Sit-ups/crunches] Why is trunk flexion (in supine) and extension (in prone)
more difficult to perform as the arms are moved from a low position to a high
position?
[Extermity joints]
What happens to the external moment arm (and thus, the external torque) during joint AROM?
What happens to the internal moment arm (and thus, the internal torque) during joint AROM?
What happens to muscle force production (and thus, the internal torque), especially long, 2 joint muscles during joint
AROM?
Variation among people: Some individuals may be endowed with muscle insertions that are farther from their joints than
in the average person. Only a small difference is necessary to give considerable increase in mechanical advantage. Muscle
insertions further down the bone increase the internal moment arm length and thus increase internal torque. However,
don’t lose sight of the fact that a short internal moment arm is better for maximizing the speed of movement desired of
the limb.