Work Tool Design
Grip Strength and Endurance



Grip strength is the maximum momentary squeeze force exerted on a hand
dynamometer (Bazar, 1978)
Endurance is the length of time a person can exert a specified force (Bazar,
1978)
A hand dynamometer is a device which measures the force an individual
exerts through the hand grip
The following factors affecting grip strength and endurance should be
considered in hand tool design:
 Size of grip span. The tool handle should fit the hand of the user
 Type of grasp/grip. Power grip strength is about four times greater than
the maximum pinch strength
 Age. Maximal muscle strength is reached at the age of about 20 for men
and a few years earlier for women (Rodahl, 1989)
 Gender. The grip strength of men is greater than that of women (Falkel,
1985)
 Handedness. The grip strength and endurance of the dominant hand are
higher than those of the non-dominant hand
 Vibration. The grip strength requirements for grasping and controlling
vibrating tools are higher than those required by non-vibrating tools
 Wrist posture. Grip strength is reduced with the deviation of the wrist
from its neutral posture
 Gloves. Gloves increase grip strength requirement
 Surgery. Grip strength is reduced after wrist surgery to relieve cts
Development in Tool Handle Design

A slightly bent handle better fits the natural contour of the hand and
minimizes the need for a tight grip to maintain the tool.

John Bennet introduced a handle for push brooms with a 19 degree bend at
its stem (Emanuel et al, 1980)

This angle as the basis for design to keep the user’s wrist straight
Bennet has patented the 19 ± 5 degree angles on virtually all tool handles
and named them BIOCURVE Tool

Ergonomics Principles for Hand-Tool Design and
Evaluation

Avoid rigid, form-fitting handles with grooves for each finger. Such handles
do not improve the grip strength and function unless they are sized to a
particular user’s hand

Avoid hand tools which require awkward movement, or cannot be operated
effectively with neutral wrist posture and low force
Tool handles should
be designed so that
the user can maintain
the hand in line with
forearm
Ergonomics Principles for Hand-Tool Design
and Evaluation-cont’

Avoid tool handles with sharp corners, edge, or pinch
point. Tool handles should be either round or oval. All
pinch points should be eliminated

The tool-handle surface should be compressible, nonconductive, and smooth. Handles should have enough
coefficient of friction to minimize hand-gripping force
required for tool control

Ergonomics Principles for Hand-Tool Design and
Evaluation-cont’

Avoid hand tools that impose concentration pressure over the soft tissues
of the hand which impair circulation and the nerve function.
More comfortable

Tool handles should fit the hand, include the proper thickness, shape, and
length so that the stress-bearing area of the hand is as large as practical. The
optimal grip with fingers, palm, and the hand is achieved by a span size
(diameter) : 6.25 cm and 9.00 cm
Ergonomics Principles for Hand-Tool Design and
Evaluation-cont’

Choose or design tools that can be used by either hand. The
handles should be designed so that the stabilizing handle can
be adjustable to either side of the tool to accommodate both
right-handed and left handed individuals.

Choose or design tools that can be used effectively by both
man and women
Ergonomics Principles for Hand-Tool Design and
Evaluation-cont’

Choose or design special-purpose tools for specific task.

Consider the angles of the grip, forearm, and tool to minimize deviation

Consider provisions for tool safety

Avoid tool handles that place concentrated pressures on the pressuresensitive areas overlying the blood vessels and nerves in the base of the
palm of the hand
The handles should be modified to
transmit the force a tougher area of
the hand-the area between the thumb
and index finger
More comfortable

Provide vibration dampening for vibrating hand tools
Use anti vibrating glove

Choose ergonomically designed tools to maintain a more natural position of
the wrist and to insure better distribution of grip forces during task
performance





Use long-handles screwdrivers.
Avoid hand tools with fluted handles surface. They concentrate
stress over a small surface in the hand
Provide workers with tools and machine control which can be
operated most effectively, with neutral body part postures and
low force
Use power grips for power, and use precision grips for
precision
Substitute power tools for hand tools that require high force
levels. Power tool should be properly designed, have adequate
grip size, not develop excessive vibration, be light enough to
handle easily, not require excessive trigger action and be easy
to use.
Design guidelines for hand tool design:
For power grip
Grip with entire hand
Grip thickness 50-60 mm
Grip length minimum 125 mm
Grip force maximum 100 N
Grip shape no cylindrical, preferably triangular
with 110 mm periphery
Tool weight maximum 2.3 kg, preferably about
1.2 kg
Trigger activated by thumb with locking
mechanism
For precision grip
Grip between thumb and finger
Grip thickness 8-13 mm
Grip length minimum 100 mm
Tool weight maximum 1.75 kg
Trigger activated by distal phalanges of
finger(s) with fast release
Biomechanical Principles for Tool Design

The force required for operating a tool should be sufficient to
provide proper sensory feedback to the musculoskeletal
system, and particularly to the tactile surface of the hand.
Therefore, the required force must be optimized

The tool should provide a precise and optimal amount of stress
concentrated at a specific location on the work-piece.

The tool should provide a contact surface area between its
handle and the user’s hand large enough to avoid concentration
of a high compressive stress
Guidelines - Summary
Description
Tool shape
Direction of force is
in-line with forearm
and wrist (typically
horizontal)
Direction of force is
perpendicular to
forearm and wrist
(typically vertical)
Separation distance
between handles (for
crushing, gripping or
clipping tools such as
pliers or tongs)
Handle length
Guideline
Slightly contoured
Bent handle
Reason
Easy grip
Minimal wrist
deviation
Straight handle
Minimal wrist
deviation
50 - 65 mm
Fits both men and
(separation distance) women with
maximum grip
strength
> 100 mm (ideally 115 Keep contact out of
- 120 mm)
palm
30 - 45 mm
Greater force and
stability
50 - 60 mm
Greater torque
Handle diameter
(power grip)
Handle diameter
(greater torque for
screwdriving tasks)
Handle diameter
5 - 12 mm
Greater control
(precision task)
Material and texture Non-slip nonFor comfort and
of handles
conductive materials reduces effort
required to use tool
Tool weight (< 1 kg) One handle
1 hand use
Tool weight (> 1 kg)
Two handles or tool
balancer
2 hand use