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.

• 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.

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.

Grasping -

• Rails - Power grip is the best grip to design for handles on doorways, for carrying, or for rails. For example, a door handle should be about 3/4" - 1" in diameter if the door is over 9 lbs.
• Handles - Providing a handle or handles is a good way of improving grasp possibilities. Handles should be located at or above the line passing through the center of gravity of the load and designed for bare hand and gloved hand operation. Handles should be textured to reduce finger slippage but not contoured. Objects weighing over 10 lbs should have handles. Bulky objects should have 2 handles.

Pushing and Pulling -

1. Horizontal pushing and pulling - Factors which influence this include: body weight, height of force application, distance of force application from body (amount of trunk flexion/extension), frictional coefficient of floor, frictional coefficient of shoes, duration of force applied or distance moved, availability of structure against which feet or back can push or prevent slippage (e.g. starting blocks).
2. Vertical pushing and pulling - Factors which influence this include: grip strength and height of force application. The height determines which muscles will be used. Pulls from above head level allow for the greatest force because body weight can be used. Pulls from more than 10 inches above the floor also allow the greatest force because strong leg and trunk muscles can be used. Only 85% of maximum standing vertical forces can be generated for downward pulls when seated.
3. Transverse (lateral) forces applied horizontally - Pushing across the front of the body involves weaker shoulder muscles so full arm extension leads to a marked decrease in maximum force. For example, maximum transverse forces are only 50-70% of maximum horizontal pushes and pulls for lever operation straight ahead of body at same elbow angle.

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