Ergonomics - Atlas Copco

Maximal capacity MVC for
of individual fi ngers trigger
Fingers Male Female operation
Thumb 100 65
Index 80 50 = sum of
Middle 130 85 the maximal
Ring 100 65 capacity of the
Little 70 45 fi ngers involved
Table 3.5 MVC for trigger operation in
Newton (N). Practical values based on scientific
results and experience.
on, for example, chipping hammers, where
force is also applied in the same direction as
the feed force.
Allowable load limits
Load level consideration
It should be noted that the above strength
limit values are average maximum voluntary
contraction (MVC) of normal operators. There
are considerable physical variations between
operators and ideally the MVC for the operator
in question should be used. In industrial work
there is a risk of musculoskeletal disorders
when the load is higher than 30% MVC of the
operator and at a load level of 40% MVC, blood
circulation can be affected. For occasional tasks,
the load level should be limited to 50% of the
maximal force capacity of the operator and must
not exceed 70% MVC. For highly repetitive
work the load should be as low as 10-15% of MVC.
Reduced MVC
To determine the maximum allowable force
limit for a specific action, one should refer to
the maximal human force generating capac-
ity data listed in Table 3.2 and 3.3. This
capacity should then be reduced due to the
speed of movement of the tool, frequency of
operation and the total use (duration) of the
tool per day (safety factors a 1 , a 2 , a 3 ).
Safety factor a 1
Speed of movement when
performing the operation
The maximal force generating capacity
of muscles decreases when they contract
rapidly, due to the viscous friction caused
by the fluid viscosity of the muscle. The
maximum force allowed should therefore be
reduced in operations involving obvious fast
movements. A safety factor of 0.8 may be
used in such cases. In most cases assembly
operations should be regarded as involving
fast movements. Material removal opera-
tions on the other hand normally do not
involve fast movements.
Type of operation Safety factor a 1
No fast movements 1
Fast movements 0.8
Table 3.6 Safety factor a 1 to reduce MVC due to
fast movements in the operation.
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