The Aimed Movement Task has been designed to evaluate Fitts’ Law (Fitts, 1954) and consequently implemented using the PEBL (Psychology Experiment Building Language) system. Fitts’ Law (Fitts, 1954), developed by psychologist Paul Fitts, examines human psychomotor behavior in human-computer interaction. The model states that the time it takes to move or point to a target is dependent on the distance and the size of the target. This predictive model also explains the speed-accuracy trade-off characteristics of the human muscle movements, i.e., faster movements and smaller targets result in higher error rates. Fitt’s Law revolutionized the understanding of electronic systems, focusing on a human channel and the information metrics bits.
Note that the Aimed Movement Task is a computerized, refined, and modified version of the classic Reciprocal Tapping Task, which required the subjects to touch two metal plates with a stylus. In the original task, the size of the metal strips and the distance were altered to test motor capacity.
The Aimed Movement Task has evolved over the years and is now performed on a computer with the help of a mouse. The mouse cursor acts as a pointer moved across the screen towards the target. Now, the test is widely applied in user interface and user experience design. The Aimed Movement Task is easily modifiable for patients with brain disorders and cerebral palsy. For instance, a head-pointer instead of a hand mouse may be used to track the eye movements of patients with reduced mobility.
The Aimed Movement Task is usually performed on a computer with the aid of a mouse. Participants are shown a fixation point (a cross or a plus sign) on the middle-left-hand side of a screen. Note that subjects can move the cursor around for a few seconds to get accustomed to its sensitivity. The top middle of the screen displays the current trial number.
Two common versions of the Aimed Movement Task exist:
Each trial starts when subjects click on the screen. As explained above, in the first version of the Aimed Movement Task, the target (a red triangle with a black outline) appears the moment the participant clicks to start the trial. However, in the second version, the target appears once the trial begins and after participants start moving the cursor. In each trial, the distance (from the fixation point) and the size of the target variable.
The Aimed Movement Task to Study Motor Behavior in Patients with Cerebral Palsy
Gump et al. (2002) evaluated motor behaviors of patients with cerebral palsy by using the Aimed Movement Task. Gump and colleagues tested whether the Fitts’ Law principles could be applied to patients with cerebral palsy. A sample of eight patients was selected, and the Garand target board (which uses LEDs as targets) was employed. The size and the distance of the target were different in each trial. Three patients used a pointer which was affixed to their hands, while four used a head-pointer, and one used their right arm and index finger due to mobility issues. The analysis showed that Fitts’ Law and its speed-accuracy trade-off principles do not apply to the aimed movements of patients with cerebral palsy.
The Aimed Movement Task Used in a Field Study to Validate Fitts’ Law
Chapuis et al. (2007) conducted an innovative field study of aimed movements in graphical user interfaces. The study used a variation of the Aimed Movement Tasks to explore how Fitts’ Law applies to real-life electronic settings. A sample of 24 right-handed users, 20 men, and 4 women were selected, and data were collected over several months. Participants used their own computers while probing software was employed to collect the data. Results indicated that Fitts’ Law is a robust model which is applicable to computer motor behavior in the wild.
Use of the Aimed Movement Task to Assess the Role of Trial History on Fitts’ Law
Tang et al. (2018) evaluated the effect of trial history on Fitts’ Law by using variations of the Aimed Movement Task. A sample of 42 students was selected and randomized into two study groups: 1) 20 students who used their index finger for aimed movements on a touch screen; 2) 22 students who used a mouse cursor. A repeat of the trial took place two days later. Results indicated that, in terms of target width, trial history had an effect on performance on each subsequent trial. However, the target distance did not affect participants’ performance.
Use of the Aimed Movement Task to Evaluate Change in Movement Times after Aerobic Exercise
Smith & Clayton (2018) evaluated the effect of a bout of aerobic exercise on movement times during the Aimed Movement Task. The sample consisted of 19 volunteers (12 men and 7 women) between the ages of 19 and 28 years. A 30-minute bout of aerobic exercise was performed before attempting the task. Participants performed the Aimed Movement Task before and after rest. Consequently, movement times were recorded for both conditions, before and after rest trials. Results indicated that movement times decreased significantly after the 30-minute exercise session as compared to the movement times following the 30-minute rest session.
The Aimed Movement Task to Assess Motor-Skills Recovery Following Stroke
Turton & Fraser (1987) used a simplified version of the Aimed Movement Task to measure stroke recovery. The aim of the study was to examine the recovery of movement control and coordination in stroke patients. The sample consisted of three groups: participants from a healthy population, stroke patients, and a control group of non-stroke patients. The Reciprocal Tapping version of the Aimed Movement Task was used. Results indicated that movement control in stroke patients was lower compared to the control group. However, depending on the stages of recovery, stroke patients performed better as they regained control of their limbs.
While the reciprocal protocol raises some concerns about its ecological validity in real-life settings, the Aimed Movement Task has numerous applications in user interface design and electronic interactions.
Interestingly, in the classic reciprocal task, trial history and training sessions can affect performance. Tang et al. (2018) found a strong effect of the previous size of the target on the 43-in ELO touch screen (F (1,19) = 72.926, P<0.001, η2 = 0.793).
Evidence shows that the reliability of the Aimed Movement Task can be improved by instructing participants to re-click on the target after missing it. Additionally, the use of more precise cursors can improve reliability. Note that hand gestures (pitch-yaw and pitch-roll) can also imitate a mouse, with pitch-yaw movements revealing higher perceived comfortability. Widodo et al. (2019) found that, in terms of pitch-yaw movements, subjects experience less fatigue in the arm, shoulder, and neck; respectively, U = 89.5, p = 0.006; U = 107.5, p = 0.029; and U = 109.5, p = 0.035.
Strengths
The Aimed Movement Task is a simple and easy-to-use test. It has several applications in today’s user experience and user interface design world. With the help of PEBL, the Aimed Movement Task can be easily modified to change the conditions and parameters of the experiment. Moreover, different input devices can be used to perform the Aimed Movement Task, such as touch screens, mouse pointers, and head pointers.
Limitations
As mentioned above, the Aimed Movement Task raises concerns about its ecological validity as results are not always applicable to pointing and movement times in real life. Yet, the Aimed Movement Task has a wide range of benefits and applications.
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