Human Strength Evaluation - Fundamentals and Assessment Tools for Occupational Ergonomics - page 216

Besides the simple convenience of dealing with statics, measurement of isometric strength yields
for most cases of practical design interest a reasonable estimate of the maximally possible exertion.
That estimate also applies to slow body segment movements, especially if they are eccentric. However,
the data do not estimate fast exertions well, especially if they are concentric and of the ballistic-
impulse type, such as throwing or hammering.
10.6.2 Dynamic Strength
Dynamic muscular efforts are more difficult to describe and control than static contractions. In
dynamic activities, muscle length changes, and therefore the involved body segments move. This
results in displacement. The amount of travel is relatively small at the muscle but usually amplified
along the links of the internal transmission path to the point of application to the outside, for
example, at the hand or foot.
The time derivatives of displacement (velocity, acceleration, and jerk) are of importance for both the
muscular effort (as discussed earlier) and the external effect, for example, change in velocity determines
impact and force, as per Newton's Second Law.
Definition and experimental control of dynamic muscle exertions are much more complex tasks
than static testing. Various new classification schemes for independant and dependant experimental
variables can be developed (Kroemer, 1999; Kroemer et al., 1989, 1997; Kumar, 2004, Marras et al.,
1993). Table 10.2 shows one such a system that includes the traditional isometric and isoinertial
approaches.
Table 10.2 shows that, indeed, dynamic strength tests require more effort to describe and control
than static (isometric) measurements. This complexity explains why, in the past, dynamic measure-
ments (other than isokinematic and isoinertial testing) have been rare. In the most practical “free
dynamic” test, common in sports, the experimenter can exert very little control (if any, then usually
over mass and repetition). The independant variables, force and displacement (and their time
derivatives), are the free choice of the subject. The dependant output is likely to be some measure
of performance, such as the distance a discus is thrown.
10.7 Designing for Body Strength
The engineer or designer wanting to consider operator strength has to make a series of decisions. These
include:
1. Is the exertion mostly static or dynamic? If static, we can use information about isometric capa-
bilities, listed later. If dynamic, other considerations apply in addition, concerning, for example,
physical (circulatory, respiratory, metabolic) endurance capabilities of the operator, or prevailing
environmental conditions. Physiologic and ergonomic texts (e.g., Astrand and Rodahl, 1986;
Kroemer et al., 1997, 2001; Winter, 1990) provide such information.
2. Is the exertion by hand, by foot, or with other body segment? For each, specific design information
is available. If we can chose, we follow physiologic and ergonomic considerations to achieve the
safest, least strenuous, and most efficient performance. For example, foot movements in compari-
son to hand movements over the same distance, consume more energy, are less accurate and
slower — but they are stronger.
3. Is a maximal or a minimal strength exertion the critical design factor?
. Maximal user strength usually relates to the structural strength of the object, so that even the
strongest operator cannot break a handle or pedal. Accordingly, we set the design value with a
safety margin above the highest perceivable strength application.
. Minimal user strength is that expected from the weakest operator, which still yields the desired
result, so that a door handle or brake pedal can be operated successfully or a heavy object be
moved.