Cornell University Ergonomics Web
DEA 3250/6510 CLASS NOTES
Manual Materials Handling
Grasps (Grip) - For many activities the interface between a person's hand and an object to be moved is a critical factor. The design of this interface frequently influences the type of grasp that can be used.
Classifications - are made on the basis of the muscles used:
1. Pinch (Precision) Grip - Characterized by opposition of the thumb and distal (away from the body) joints of the fingers. Allows us a great deal of dexterity and precision adjustment, but, since the proximal finger surfaces (nearest to palm) and the palm are not used, the pinch grip is only about 25% of the maximum grip strength possible (the power grip). Pinch grip strength decreases rapidly at spans less than 1" or greater than 3". Examples include: holding pen or pencil, cartridges in and out of holder, and carrying a tray.
|Functional Hand Grasp (Pinch Grip)|
|5th %ile||50th %ile||95 %ile|
|True||2.1cm. (.8")||4.3cm. (1.7")||7.9cm. (3.1")|
|Max.||10.8cm. (4.2")||12.5cm. (4.9")||15.0cm. (5.9")|
- Internal Precision Grip - a pinch grip where the grip is supported by the little finger or the side of the hand, and the handle of the tool is "internal" to the hand, e.g. holding a knife (Konz, 1990).
- External Precision Grip - a pinch grip where the grip is supported by the side of the second finger or the base of the thumb, and the shaft of the tool is "external" to the hand, e.g. holding a pen (Konz, 1990).
2. Power Grip (Cylindrical) - the maximum force that can be developed by the hand. Influenced by wrist orientation and grip span. The thumb may or may not be used. Examples include: carrying pipe, forcing a screwdriver in a disassembly unit, and using hand tools.
- Wrist Orientation - The more the hand goes toward ulnar deviation, the less grip force needed. The hand is less flexible in radial direction where you can only get about 80% of grip strength.
- Grip Span - Optimum cylinder size for power grip is 1 - 2" in diameter.
3. Oblique grasp - a variant of the power grasp characterized by gripping across the surface of an object, e.g. carrying a tray with handles -lift up and power grip ends. Oblique grasp is around 65% grip strength of power grasp. Grasp strength is strongly affected by hand span. A grip of around 2 to 2 1/2 " is the strongest. The thumb is very important in this grasp.
|Functional Hand Grasp (Oblique Grip)|
|5th %ile||50th %ile||95 %ile|
|True||3.6cm. (1.4")||4.6cm. (1.8")||5.8cm. (2.3")|
4. Hook grasp - characterized by a flat hand, curled fingers, and thumb used passively to stabilize the load, e.g. auto steering wheels. Load is supported by fingers. This grip is most effective when the arms are down at the side of the body. For objects with a diameter of 2" the hook grip strength can achieve the strength of a power grip. Very narrow handles decreases hook grip strength by pressing deeply into hand and fingers. Rigid handles should generally be avoided.
5. Palm-up/ Palm down grasp -
- Palm down grasps - are primarily used in precision activities, e.g. small parts assembly, lifting cans, lifting jars, etc. When palm is turned down the stronger arm muscles are not optimally positioned to exert force and if an object weighs more than 1 lb (0.5 kg) it should not be handled repeatedly with this grasp. When opening a jar, turn it sideways to get more strength.
- Palm-up grasps - used in low lifting. Strength depends on vertical position of the arms. Maximum strength is achieved with arms around elbow level. Hands have to be re-oriented on objects at about this point so there is a potential for losing control of the load. The heavier the load, the more difficult it is to change grasp. Above elbow level, strength decreases.
Pushing and Pulling -
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