A Study on the Perception of Forces of the Thumb
The study of the perception of forces has a long tradition in experimental psychology. Many studies have shown that forces are perceived through sensations generated by the motor commands producing the effort or motion and not from peripheral information coming from the muscles, the joints, and the skin. Generally, force experiments consist of pairing a weight held contralaterally by an equivalent weight. In such experiments, the perception of forces is evaluated in a single direction (against gravity) and only the amplitude is measured. In the present experiment, the perception of forces was studied in terms of both amplitude and direction. More specifically, the perception of the amplitude and the direction of a force exerted by the thumb was studied in eight different directions. The hypothesis is that the behavior of the perception of forces (amplitude and direction) is not identical for the eight directions considered. Some directions give identical behavior while others offer a different behavior.
Twenty-two female subjects (mean age: 23.7 years, standard deviation: 2.0 years; mean weight: 58.4 kg, standard deviation: 10.0 kg; mean height: 160.0 cm, standard deviation: 10.0 cm) were evaluated in this experiment. This convenience sample consisted of college students in occupational and physical therapy with no history of neuromuscular or orthopedic problems. Five subjects were left-handed and seventeen were right-handed.
In this experiment, the hand of the subject was placed in the anatomical position (natural position of the hand at rest) and the proximal phalanx was secured within a ring attached to a transducer from which it was possible to quantify both the vertical and horizontal components of the bending forces exerted by the thumb. These components are represented in a reference plane corresponding to the anatomical plane of flexion/extension (vertical axis) and abduction/adduction (horizontal axis) of the thumb. The experimental protocol consisted of studying two conditions, eight directions, and the force was evaluated twice.
Under the first condition, the subject was asked to exert a controlled force in a given direction with the thumb of the dominant hand, and then to reproduce the same force in the same direction with the thumb of the nondominant hand. For the second condition, the subject was asked to exert a controlled force in a given direction with the thumb of the nondominant hand, and then to reproduce the same force in the same direction with the thumb of the dominant hand.
The eight directions considered corresponded to the flexion of the thumb downwards (270deg.), the extension of the thumb upwards (90deg.), the abduction of the thumb towards the exterior of the palm of the hand (180deg.), the adduction of the thumb towards the interior of the palm (0deg.) and a combination of flexion-abduction (225deg.), flexion-adduction (315deg.), extension-abduction (135deg.), and extension-adduction (45deg.). The degrees indicated apply to the right hand while for the left hand, the data were transformed by symmetry with respect to the vertical axis in order to have directly comparable data for both hands.
For each of the two conditions and the eight directions, the evaluation of the force exerted by the thumb was done twice.
Fatigue was minimized by alternating between opposite directions for successive measurements and by allowing thirty seconds of rest between trials. In order to have comparable results for each subject, a control on the force was experimentally done by asking each subject to first exert their maximal force, and then to give 10% of that maximal force. Furthermore, a pilot trial was performed for each combination of condition and direction to make sure that the experimental protocol was entirely understood by the subjects. The results of this pilot trial are not considered in the present study.
The variable representing the force of the thumb is defined by the coordinates (x,y) of the components of the force exerted by the thumb, each of the coordinates being divided by the norm of the vector whose coordinates correspond to the maximal force exerted by the thumb of the same hand. This variable is called the normalized force of the thumb.
In the data file, we have the coordinates (x,y) representing the components of the normalized force of the thumb for each level of the experimental protocol. For each of 22 subjects, we have 16 rows for which the first 8 are for the first evaluation while the following 8 are for the second evaluation. Each of the 8 rows corresponds to a specific direction in the following order: flexion (270deg.), extension (90deg.), flexion-adduction (315deg.), extension-abduction (135deg.), adduction (0deg.), abduction (180deg.), extension-adduction (45deg.), and flexion-abduction (225deg.). On each row, we find four pairs of coordinates: (x1,y1), (x2,y2), (x3,y3), and (x4,y4) where (x2,y2) represents the normalized force of the thumb of the nondominant hand which reproduces the normalized force (x1,y1) of the thumb of the dominant hand (condition 1), while (x3,y3) is the normalized force of the thumb of the dominant hand reproducing the normalized force (x4,y4) of the thumb of the nondominant hand (condition 2).
The general objective of the project is to analyze the behavior of the normalized forces by taking into account the directions. More precisely, we need to study the reproduction process of the forces between the dominant and nondominant hands. To answer these questions, it is natural to transform the data into polar coordinates. The differences of amplitude (radius) and of direction (angle) of the normalized force between the dominant and nondominant hands could be studied separately. For the direction, it is possibly preferable to subtract the aimed direction and thus define a variable defining the bias. Also, since we want to globally describe the difference of the reproduced normalized forces, the area of the triangle determined by the coordinates (x,y) of the points representing the normalized forces of the dominant and nondominant hands and the origin (0,0) of the plane could also be used to study the behavior of the normalized forces. The analysis must be consistent with the fact that the hypothesis of independence between all the forces measured on a subject may not necessarily hold.
We are indebted to Doctor Daniel Bourbonnais, researcher at the Centre de recherche de l'Institut de réadaptation de Montréal and professor at the École de réadaptation de l'Université de Montréal, for kindly permitting us to use the data of his project on the perception of forces on the thumb.