Levers are activated either by the whole hand or just by the fingers. In general, where fine control is needed, only the fingers should be used.
The following recommendations apply to levers:
1. The maximum resistance (force) for push-pull movements with one hand, with the control placed centrally in front of the body, is between 12 and 22 kg, depending on how far from the body the control is positioned.
2. The maximum resistance for push-pull movements for two hands is double that for one hand.
3. The maximum resistance for one hand moving in the left-right direction is about 9 kg, and is considerably lower in the opposite direction.
4. The maximum resistance for two-handed movements in the left-right direction is about 13 kg.
5. The lever movement should never be greater than the arm’s reach without moving the body.
6. Where precision is required, a supporting surface should be provided for the part of the body used: an elbow rest for large hand movements and a hand rest for finger movements.
7. When levers are used for stepwise control (e. g., gear levers), the distance between positions should be one-third the length of the lever.
8. Where the lever also acts as a visual indicator, the distance between positions can be reduced. The critical distance is then the operator’s ability to see the markings.
9. The surface of the lever handle should have a high friction coefficient so that the hand does not slip.