Human Strength Evaluation - Fundamentals and Assessment Tools for Occupational Ergonomics - page 228

100

α = 15°

β = 160°

90

80

α = 0°
β = 160°

70

60

r = 13 cm

50

10 20 30 40 cm

0

R : Height of backrest above seat (SRP)

FIGURE 10.15 Effects of backrest height R — see Figure 10.11 — on pedal push force. (With permission by the
publisher from Kroemer et al., Ergonomics: How to Design for Ease and Efficiency, Prentice Hall, Upper Saddle
River, NJ, 2001. All rights reserved.)

In isometric contraction, muscle length remains constant, which establishes a static condition for the
body segments affected by the muscle. In an isotonic effort, the muscle tension remains constant, which
usually coincides with a static (isometric) effort.
Dynamic activities result from changes in muscle length, which bring about motion of body segments.
In an isokinematic effort, speed remains unchanged. In an isoinertial test, the mass properties remain
constant.
Human body (segment) strength is measured routinely as the force (or torque) exerted to an instru-
ment external to the body. This is information of great importance to the ergonomic designer engineer.

Design of equipment and work tasks to match human body strength capabilities considers these
aspects:

/

. Determine whether the exertion is static or dynamic
. Establish the part of the body on which the force or torque is exerted
.

Select the segment strength percentile (minimum and maximum) that is critical for the operation

Follow the chain of strength vectors through the involved body segments to find the “weak link”
and to improve and rearrange the conditions if possible

.

Foot

Ankle

Knee

Hip

Reaction Force
provided by
proper seat interface

Sitting:
Seat design

Standing
Not Recommended

FIGURE 10.16 Determining the “critical body segment strength” for foot operation.