Using a tool which does not fit the person or task can seriously affect a worker's health. Thus, selecting the proper tool for the job and fitting it to the individual has become very important for worker health and subsequently productivity
Recall that the extrinsic hand muscles are able to generate maximum force for the power grip when the wrist is held in a neutral position. More specifically, the functional position of the wrist is approximately 20-35 degrees of extension and 10-15 degrees of ulnar deviation. This position allows for the greatest amount of flexion of the fingers and thus the most efficient power grip.
The functional position of the wrist, approximately the "handshake" position.
Image Source: Victoria Squissato
A non-neutral wrist position places stress on the tendons as they pass through the carpal tunnel preventing the extrinsic muscles from working efficiently and less power is generated with more force. Unfortunately, hand tools are often used where space is limited and access is difficult, necessitating the adoption of awkward wrist postures. Consequently, the extrinsic muscles become fatigued and susceptible to injury.
A comfortable grip with a neutral wrist. A non-neutral wrist.
Image Source: Victoria Squissato
Recall that the extrinsic hand muscles are able to generate maximum force for the power grip when the wrist is held in a neutral position. More specifically, the functional position of the wrist is approximately 20-35 degrees of extension and 10-15 degrees of ulnar deviation. This position allows for the greatest amount of flexion of the fingers and thus the most efficient power grip.
The functional position of the wrist, approximately the "handshake" position.
Image Source: Victoria Squissato
A non-neutral wrist position places stress on the tendons as they pass through the carpal tunnel preventing the extrinsic muscles from working efficiently and less power is generated with more force. Unfortunately, hand tools are often used where space is limited and access is difficult, necessitating the adoption of awkward wrist postures. Consequently, the extrinsic muscles become fatigued and susceptible to injury.
A comfortable grip with a neutral wrist. A non-neutral wrist.
Image Source: Victoria Squissato
If the arm is uncomfortable the rest of the body is likely to be uncomfortable as well. The body attempts to adapt to discomfort via repositioning of the extremities. For example, bending the back, rounding the shoulders, or tilting the neck. This exacerbates the non-neutral positioning and risk of injury.
The majority of an aesthetician's tasks are hand intensive and require small tool use. Here, the worker holds her right wrist in extension to manipulate the clippers. To accomodate the wrist extension the worker hikes her right shoulder and leans to the left (lateral trunk flexion) while supporting her body weight on her left elbow.
Image Source: Victoria Squissato
The majority of an aesthetician's tasks are hand intensive and require small tool use. Here, the worker holds her right wrist in extension to manipulate the clippers. To accomodate the wrist extension the worker hikes her right shoulder and leans to the left (lateral trunk flexion) while supporting her body weight on her left elbow.
Image Source: Victoria Squissato
To be ergonomically effective, a tool must decrease the physical demands placed on the worker using the tool. Accordingly, tools should be chosen or designed with the following considerations,
Weight
The worker should be able to operate a tool with one hand therefore the weight should not exceed 1 kg, especially if the tool is to be used repetitively. Another consideration is the centre of gravity. The centre of gravity should be aligned with the centre of the gripping hand, i.e. the tool should feel easy to hold in the position in which it will be used (usually pointing down).
A power drill's functional position is usually pointing down, therefore the center of gravity is such that the tool is comfortable to hold when pointing down.
Image Source: Victoria Squissato
A power drill's functional position is usually pointing down, therefore the center of gravity is such that the tool is comfortable to hold when pointing down.
Image Source: Victoria Squissato
Power
Whenever possible power tools should replace hand tools to minimize forecful and/or repetitive forces. The greater the force exerted with a hand tool and the more the hand has to twist to generate the force, then the greater the risk for WMSD.
Handles
Ideally, handles should be designed for a power grip with the following considerations,
Shape
A fundamental principle of ergonomic design is to bend the tool, not the wrist. Thus angled handles can be useful in the prevention of WMSD, but need to be applied appropriately given the direction of forces being applied by the worker. Angled handles are useful when the force exerted is parallel to the straightened forearm and wrist. Straight handles are useful when the force exerted is perpendicular to the straightened forearm and wrist.
Bent or angled handles or pistol-grips are best for work where the exerted force is parallel to the straightened forearm and wrist. As demonstrated above the worker maintains a straight forearm with neutral wrist while operating the power drill.
Image Source: Victoria Squissato
Tools with straight handles are best for tasks where the exerted force is perpendicular to the straightened forearm and wrist. The worker above maintains a neutral wrist while mainpulating a hammer.
Image Source: Victoria Squissato
A fundamental principle of ergonomic design is to bend the tool, not the wrist. Thus angled handles can be useful in the prevention of WMSD, but need to be applied appropriately given the direction of forces being applied by the worker. Angled handles are useful when the force exerted is parallel to the straightened forearm and wrist. Straight handles are useful when the force exerted is perpendicular to the straightened forearm and wrist.
Bent or angled handles or pistol-grips are best for work where the exerted force is parallel to the straightened forearm and wrist. As demonstrated above the worker maintains a straight forearm with neutral wrist while operating the power drill.
Image Source: Victoria Squissato
Tools with straight handles are best for tasks where the exerted force is perpendicular to the straightened forearm and wrist. The worker above maintains a neutral wrist while mainpulating a hammer.
Image Source: Victoria Squissato
Diameter
In cross section the handle should be cylindrical or oval, with a diameter of 30-45 mm (approximately the circle made by your thumb and ring finger).
On the left, the diameter of the hole made by opposition of the thumb and ring finger. On the right, grasping a manual screwdriver handle.
Image Source: Victoria Squissato
Length
To prevent unnecessary compression of the mid palm, the length of the handle should be such that it extends across the breadth of the palm. The average ideal length is 115-120 mm. A rounded end adds further comfort.
The above handle is slightly less than the optimal length, but the rounded end helps to compensate for this and minimizes the compression of the palm.
Image Source: Victoria Squissato
Separation between handles
Gripping or cutting tools such as pilers have two handles. When in the resting position, the two handles should be between 50-65 mm apart, a distance that exploits the power grip strength for both males and females.
With the pilers closed the handles of this tool are 50 mm apart, the minimum optimal distance.
Image Source: Victoria Squissato
A sheet metal worker uses tin snips to trim a piece of flashing.
Image Source: Victoria Squissato
Power tool triggers
Power tools require the depression of a button or trigger for operation. Frequent movements of the index finger to accomplish this depression represent a risk for “trigger finger”. A longer trigger (minimum 50 mm) allows the use of 2-3 fingers to activate the trigger and helps to distribute the work load, thus minimizing the risk of injury.
The trigger of the power tool above is too short for operation by 2-3 fingers. Ideally the trigger should be a minimum of 50 mm.
Image Source: Victoria Squissato
Materials and textures
Sufficient friction must exist between the hand and the handle in order to minimize the grip force needed to manipulate the tool. Thus, handles should be made of a non-slip, non-conductive, compressible material. Textured rubber handles provide a good grip and reduce the effort needed to use the tool effectively and prevent the tool from slipping out of the hand. Electrical and heat insulation properties of the handles are important for power tools.
Vibration
Vibrations generated by power tools in the high frequency range cause damage to the small blood vessels and nerves in the hands and occasionally the feet. Anti-vibration gloves contain that air or gel pockets can effectively absorb these damaging high frequency vibrations. However, some tools emit low frequency vibrations, which are not dampened by anti-vibration gloves and may contribute to other WMSDs particularly those that affect the neck. Ultimately, the most effective way to reduce vibration in power tools is at the design stage.
This sheet metal worker wears anti-vibration gloves while operating an impact screwdriver.
Image Source: Victoria Squissato
This sheet metal worker wears anti-vibration gloves while operating an impact screwdriver.
Image Source: Victoria Squissato
Condition/maintenance
Blunt or dull tools increase the effort required to use them. For example, cutters, strippers, saws, and screwdriver tips all require frequent maintenance for efficient use. Proper maintenance of tools minimizes the effort needed to use them.
Role of the Occupational Therapist
A major role of occupational therapists is to introduce modified equipment and adaptive aids. Modifying equipment and tools can correct positioning of the hand and reduce the hand force required to complete an action. For example, specialized spring-loaded pliers that reduce the force required to cut wire for electronic assembly purposes. Other ways that adaptive aids can help include, distributing the force over a larger surface area of the hand, or correcting the positioning of the hand through specially shaped handles.
Furthermore, adaptive aids can be useful in enabling individuals with CTS to participate in their chosen activities. One such adaptation is increasing the diameter of handles so that less grip strength is needed to grasp an object. Occupational therapists can easily adapt any existing handle in this way or purchase already adapted tools for a client.
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Role of the Occupational Therapist
A major role of occupational therapists is to introduce modified equipment and adaptive aids. Modifying equipment and tools can correct positioning of the hand and reduce the hand force required to complete an action. For example, specialized spring-loaded pliers that reduce the force required to cut wire for electronic assembly purposes. Other ways that adaptive aids can help include, distributing the force over a larger surface area of the hand, or correcting the positioning of the hand through specially shaped handles.
Furthermore, adaptive aids can be useful in enabling individuals with CTS to participate in their chosen activities. One such adaptation is increasing the diameter of handles so that less grip strength is needed to grasp an object. Occupational therapists can easily adapt any existing handle in this way or purchase already adapted tools for a client.
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