Our sample size, hypothesis, study design and analyses was preregistered (available at https://osf.io/qx9me). The raw data and statistical analysis are also publicly available (https://osf.io/yvu97/).
Participants
We used the G*Power (version: 3.1.9.4) software to a priori determine sample size. Based on previous similar (suppression) studies 15,22, effect size was set to large (partial eta square=0.14). To achieve an alpha of 0.05 and a power of 0.95, the required minimum sample size is 31 for a repeated measures ANOVA with 5 levels. Based on this, our a priori decision was to stop when N = 35 is reached. Overall, 35 undergraduate students of the Eötvös Loránd University completed the measurements (25 women, 33 right-handed). Participants were at least 18 years old (mean age was 21.2±3.05), without severe injury or disability of the elbow joint. In average, participants spend 2.9±2.5 hours/week with sporting activity (e.g. running, calisthenics). They received partial course credit for the participation. The experiment was approved by the Research Ethics Committee of the Faculty of Education and Psychology of the Eötvös Loránd University (approval number: 2019/302-2). Every participant signed the informed consent before the experiment. All tasks were performed in accordance with the relevant guidelines and regulations.
Capacity measurement
To measure proprioceptive memory span, we adapted and modified the task developed by Horváth and colleagues 18 for assessing the ability to memorize and reproduce sequences of elbow joint positions. For the assessment, a custom-made motor-driven device (proprioceptor) was used, which enabled us to accurately set (±0.5 degree) and measure (±0.1 degree) the angle of the elbow joint. Quasi-random sequences of different lengths were composed from nine possible target positions (30, 45, 60, 75, 90, 105, 120, 135, 150 degrees, where the higher values refer to the bigger extension of the elbow joint). Every target position was presented only once in a sequence (until the length reached 10 positions). The starting position of the trials was always the same, i.e. an almost fully extended elbow (160 degree). From there, the device started to move the elbow joint of the participant, then stopped the movement and kept the arm for 4 seconds in every target position. Target positions were presented directly after each other without returning to the starting position. After the presentation of an entire sequence, the proprioceptor moved back the elbow joint to the starting position; form this position, participants were asked to replicate the whole sequence by actively moving their arm and pressing a button at every target position. The measurement started with three 2-position practice sequences; than the assessment started with 3-position sequences. If one correctly reproduced two sequences of a given length out of a maximum of three attempts, the number of presented positions increased by one in the next sequence. However, if sequences of the given length were reproduced incorrectly twice, the task ended. The capacity score was determined by the number of elements in the longest, at least two times correctly reproduced sequence. The given sequence was considered correct if: (1) the movement pattern was correct (no more or fewer positions were reproduced, and no movement were performed to the opposite direction), and (2) the difference between the target and the reproduced position was less than 30° in each case.
Procedure
Every participant performed the proprioceptive memory capacity measurement in five different conditions: (a) no suppression (control), (b) motor suppression, (c) spatial suppression, (d) visual suppression, and (e) verbal suppression (see Figure 1). The competing tasks were administered during the presentation phase of the proprioceptive measurement only. The no suppression (control) condition was administered first, followed by the remaining four conditions in a randomized order.
Control
During the control measurement, participants had no competing task, so they could fully concentrate on the proprioceptive memory task. Thus, this task measured the memory span of participants.
Motor suppression
Motor suppression task was adapted from Smyth and colleagues 15. Participants had to repeatedly touch their body parts with their dominant hand in the following order: left shoulder, right shoulder, left hip, right hip. This was presented by the experimenter at a speed of approximately 4 touch/second, and participants were instructed to keep that speed.
Spatial suppression
This task was adapted from Smyth and colleagues 15. Participants had to repeatedly touch spatial positions with eyes closed, represented by rectangular boxes (width: 3.5 cm, length: 5 cm, height: 1.5 cm), aligned in a square layout, with 2.5 cm space between them. Participants had to touch the top of the boxes. This was presented by the experimenter with approximately a 4 box/second speed, and participants were asked to try to keep that rhythm.
Visual suppression
The visual suppression task was adapted from Della Sala and colleagues 17. Participants had to watch abstract pictures on a laptop screen (e.g. pictures of Wassily Kandinsky or Jackson Pollock). The sight of the tested arm was blocked by a specific eye-mask. Each picture was presented for 5 seconds.
Verbal suppression
The verbal suppression task was adapted from Baddeley and colleagues 23. Participants had to repeatedly count from one to four aloud. The task was presented by the experimenter with approximately a four digit per second speed, and they were asked to keep that speed.
Statistical analysis
Statistical analysis was conducted in the JAMOVI (version: 1.6) software 24. The assumptions of repeated ANOVA were not met, as the Shapiro-Wilk test of normality was significant in every condition (p<0.05). To compare the experimental conditions, we used repeated measures Friedman test with 5 levels. Durbin-Conover test was used for the post hoc analysis.