Biomechanics
The objective of this next set of lectures is to give a
basic introduction to biomechanics: the study of force
relationships of different systems of the human body.
Biomechanics is used:
 To help understand how certain physiological
systems function (e.g. cardiovascular system)
 To model these systems to aid in the design of
biomedical devices such as prostheses
B. Amsden
CHEE 340
Mechanical Forces
Forces:
 The physical influence that produces a change in a physical quantity


(i.e. force = mass x acceleration).
Forces are vectors and thus have both magnitude and direction.
Forces can be resolved along different directions and are usually
resolved along the axes of the Cartesian coordinate system (using
Pythagoras’ theorem)
Vx = V cos q
Vy = V sin q
Note: q is measured positive counterclockwise in these equations
B. Amsden
CHEE 340
Moments
Moment:
 The moment of a force about a point or axis is a measure of its


tendency to cause rotation.
A moment is the cross-product of the force and its distance vector
about the point of rotation (i.e. M = F x r)
The scalar definition of a moment is the product of the force
magnitude and its perpendicular distance from the point of rotation
to the line of action of the force
Mo = F d
where, d = r sin q
B. Amsden
CHEE 340
Static Equilibrium
Static equilibrium. Any body (or any part of it) which is currently
stationary will remain stationary if the resultant forces and moments
acting on the body are zero.
 This leads to three equations (in 2D Cartesian space):
S Fx = 0
S Fy = 0
SM=0
B. Amsden
CHEE 340
Static Equilibrium Example 1
Russell Traction Rig:
a)
b)
What weight (W) must be attached to the free end of the cable to
maintain the leg in static equilibrium?
What is the average tensile force applied to the femur under these
conditions?
B. Amsden
CHEE 340
Anthropometrics
Anthropometric data provides physical information (weight, length,
etc.) of body segments of a typical individual (expressed in terms of
individual body weight and height).
B. Amsden
CHEE 340
Static Equilibrium Example 2
A 160 lb person is holding a 10 lb
weight in the palm of their hand
with their elbow at a 90º angle.
a)What force must be generated
by the biceps to hold the forearm
in this position?
b)What force(s) does the forearm
exert on the humerus?
B. Amsden
CHEE 340