A model of human and object physical interaction (HOPI) has been developed to provide a more detailed understanding of how we control or environment and how to optimise the interface.
There are a number of explanations of aspects of hand and object interaction. The explanations document different levels of the interaction, from the macro of task analysis by Guo, Genaidy, Christiansen and Huntington (1996) to micro interaction at an biomechnical level by Gielo-Perczak (1994). Structures of interaction documented by Sperling, Dahlman, Wikstrom, Kilbom, Kadefors (1993) and Drury (1985), Hsia and Drury CG (1986) provide understanding of hand and object interaction based on mainly observation and biomechanics respectively.
Hand and object interaction is a description of the primarily static connection (grip) between a hand and an object formed for the purpose of the performance of a task. Due to the complexity of hand and object interaction a description of the interaction is based upon a specific moment during a task performance.
Static grip is critical to the user’s perception of comfort and ease of use. If an object slips within a user’s hand they will increase their grip until slippage stops. Conversely, a user will reduce their applied grip force until the object reaches a point just before slippage occurs (Edin, Westlin and Johansson, 1995). A product that enables a static grip to be produced that requires a minimum of grip strength to maintain it will be perceived as easy to use.
There are three levels of interaction:
- Gross interaction, where a grip pattern made by the user to resist the forces acting through the object handle. A grip pattern relies heavily on the ability of the user to be able to move their fingers and thumb into the required positions. Mobility of joints and muscle strength are required to achieve this objective.
- Intermediate interaction, where the soft tissues of the hand interlock with the coarse surface features of the object.
- Micro interaction, where the skin surface interacts with the fine surface features of the object. The interaction is primarily a form of adhesion. The adhesive is a combination of the sweat (sebum) and the dead skin layer (epidermis).
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