Motor Functions

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response orientation, rate control, arm-hand steadiness, manual dexterity, finger dexterity, wrist-finger speed, static strength, explosive strength, dynamic strength, extent flexibility, dynamic flexibility, gross body coordination and equilibrium, and stamina among others (Crepeau et al., 2003; Jacko & Vitense, 2001).

Table 5-3 Motor Skills (Crepeau et al., 2003)

Motor Skills Description Example Actions

1. Posture Relates to the stabilising and aligning of one’s body while moving in relation to task objects with which one must deal.

Stabilises, aligns, positions

2. Mobility Relates to moving the entire body or a body part in space as necessary when interacting with task objects.

Walks, reaches, bends

3. Coordination Relates to using more than one body part to interact with task objects in a manner that supports task performance.

Coordinates, manipulates, flows 4. Strength and

Effort

Pertains to skills that require generation of muscle force appropriate for effective interaction with task objects.

Moves, transports, lifts, calibrates, grips 5. Energy Refers to sustained effort over the course of task

performance.

Endures, paces

The following sections highlight pertinent issues with motor skills and its measurement for characterising motor capability loss.

5.5.1 Hand and Arm Functions

Hand function is a combination of motion control, grasping and force exertion. Most products require the use of the hands and arms to operate controls and manipulate various product features. Product interaction may demand reaching and grasping, and the exertion of linear and rotational forces with each hand separately or in combination.

Static and functional anthropometry: Anthropometry (dimensions and ranges of motion) capture the ranges of motion of the hands and arms required (distances and angles) to access a product. The capability to access and grasp product controls depends on the maximum

vertical and horizontal distances that can be reached with each arm. Reaching capabilities are more important for fixed products such as washing machines and ATMs. The maximum reach distances can be measured by determining the reach envelopes in both the vertical and

horizontal directions.

Grasping and force exertion: Forces can be exerted on product controls without any gripping required using non-prehensile movements (Napier, 1956). Linear forces can be

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exerted with fingers or the palm of the hand, for example in pushing a button. Controls and handles can also be grasped and held between fingers or within the compass of the hand using prehensile movements (Napier, 1956). Gripping actions can be sub-divided into precision gripping and power gripping. Precision grips use opposing forces of the fingers and thumb digits of the hand for fine linear or rotational movements. Power grips use the palm of the hand, in addition to the fingers, to exert larger grip forces on the product chassis and handles (Napier, 1956). Both grip types are endpoints of a continuum of grasps, and represent the movement from large force exertion with gross motions of the hand and arm to smaller force exertion with fine finger motions. Different functional grips are catalogued in taxonomies of functional grasps available in the literature (Cutkosky & Wright, 1986; Edwards, Buckland, &

McCoy-Powlen, 2002; Kroemer, 1986; Napier, 1956).

Forces exerted can be divided into linear forces and rotational forces. Linear forces can be characterised in a coordinate system of three principal directions using the body as a reference point: vertically (up-down), horizontally (left-right) and ventrally (forward-back). Rotational forces can be described in both the clockwise and anti-clockwise directions. These forces need to be applied for different durations depending on the task at hand. Forces also need to be considered along with the extent of available motion within the full range of articulation. A button push requires a linear finger force exertion for a short period of time as opposed to lifting a kettle to pour which requires lifting a load for a longer time period. Measurement of the maximum linear, rotational and grip forces that can be exerted with different grasps are required to determine the maximum performance capabilities for each hand.

Dexterity: Manual dexterity is the ability to make skilful coordinated movements of one hand, a hand together with its arm, or two hands to grasp, place, move or assemble objects (Crepeau et al., 2003). Finger dexterity is the ability to make skilful, coordinated movements of the fingers of one or both hands to grasp, place or move small objects (Crepeau et al., 2003). Dexterity can be measured with tests such as the Purdue Pegboard Test (Crepeau et al., 2003), and it can be used to assess the ability to perform actions in activities of daily living (for example buttoning a shirt) or making fine manipulations with products (Tsai & Lee, 2009).

Two handed actions and coordination: Some motor actions with products are two handed operations requiring the use of both hands with different grips. Combinations of power and precision grips with force exertions are required to accomplish tasks such as opening a jar and dialling a land-line phone while holding the receiver. Two handed actions require

coordination and adequate function in both hands and arms in order to successfully complete

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the task. The issue of handedness is also an important consideration. A product that requires the use of only the right hand would exclude or raise difficulty for left handed users.

In essence, the demanded hand actions are determined by the design of the product. There may also be multiple ways of performing an action by configuring the hands and motions.

Coping strategies are important in motor actions as more freedom and ways to manipulate a product make it easier for users to adopt different hand positions to exert forces in different directions (Kanis, 1993; Yoxall et al., 2010c). Data is also available on the range of motions, forces and grips that older and disabled users can apply (Kanis, 1993; Smith et al., 2000;

Steenbekkers & VanBeijsterveldt, 1998; Yoxall et al., 2006; Yoxall, Kamat, Langley, &

Rowson, 2010a; Yoxall et al., 2010b)

5.5.2 Gross Body Functions

Mobility functions are necessary for consumer products such as vacuum cleaners where a user is required to move while exerting forces on the product. This includes maximum bending ranges that indicate the extents of upper body flexion (Crepeau et al., 2003). For products such as washing machines that require reaching into the drum, reaching and bending

capabilities are also linked. Locomotion and balance capabilities are also required for walking and moving around. Performance measures such as maximum walking speed and distance can give an indication of locomotion ability. In impaired populations, various mobility aids compensate for the loss of locomotion capability. Measures of locomotion capability should include various aids if they are used on a regular basis. The type of aid used could also be captured. Wheelchairs affect the reaching and bending envelopes of users, and measures for this sub-population would also be necessary.

5.5.3 Speech Functions

The ability to produce speech depends on motor control of the vocal systems. Certain

products require input in the form of voice commands, making the ability to produce coherent and clear speech a requirement (Crepeau et al., 2003).

5.5.4 Common Conditions Causing Loss of Motor Function

Conditions such as arthritis, stroke, multiple sclerosis, cerebral palsy and missing or damaged limbs can cause reductions in grasp forces, ranges of motion and fatigue thresholds in

addition to the decline in these capabilities with ageing (Jacko & Vitense, 2001; Sears &

Young, 2003). Design guidelines for reduced motor ability have included minimising the forces required to operate controls; not requiring simultaneous manipulations; and allowing

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freedom and flexibility for control manipulation, such as different grips where possible (Kanis, 1993). All products require physical interaction of some form, therefore the range of movements and exertable forces with different body effectors constitute the motor demand on users with reduced motor capability.