Materials and methods

Main Experiment

Participants

An a priori power analysis was conducted using G_Power version 3.1.9.7 (Faul et al., 2007) to determine the minimum sample required to test the study hypothesis. The effect size was set at 0.25 considered to be medium using Cohen’s criteria (Cohen, 1992). A F-test assessing the interaction between the condition (n = 2) and the measurements (weights; n = 6) was applied with a significance criterion of a = 0.05 and power = 0.95. The minimum sample size needed was N = 28 to detect differences in discrimination sensitivity between conditions and among measurements. Thirty-five volunteers (male/females: 11/24; mean age ± SE=24.7±0.8 years) participated in the experiment. Written informed consent was obtained from all participants before data collection. The study was approved by the ethical committee of the University of Genoa (Comitato Etico per la Ricerca di Ateneo, n° 2021/42) and was conducted in accordance with the Declaration of Helsinki.

Experimental procedure

The experiment consisted of a single session where participants performed a weight discrimination video task. The task was built using jsPsych 6.3.0 library and performed off-line (de Leeuw, 2015). The task was preceded by the instructions and a questionnaire collecting personal data (i.e., gender, age, weight, height) and physical activity level (i.e., activities performed, year of experience, weekly hours of training) data. The protocol is represented in Figure 1A.
Video stimuli
The stimuli consisted of videos showing an actor performing a typical everyday activity; namely, the actor (either a woman or a man, depending on the gender of the participant, Figure 1B) moving a box (weight of the empty box 390 g) from the chest to a shelf over the head (Concentric condition, given that the main muscle involved in the movement, i.e., the anterior deltoids – based on preliminary EMG acquisitions, shortens in this kind of movement) or bring it down from the shelf to the chest (Eccentric condition, given that the deltoid anterior lengthens). In both conditions, the box was filled with varying amounts of sheets of paper in such a way as to assume 7 different additional weights (0, 2.5, 5, 7.5, 10, 12.5, 15 kg). Actors were informed about the weight of the box. Videos, whose durations range from 1.3s to 3.7s, were acquired on the same day with a video-camera positioned to record the execution of the lifting movement laterally. The face of the actor was blurred to cover facial expressions, as these reflected the level of exertion during a physical task (de Morree & Marcora, 2010).
Task
Participants (hereafter ‘observers’) sat in front of a laptop with a 16-inch LCD screen positioned on a table, at a distance of approximately 60 cm. They were required to perform a two-interval forced-choice (2IFC) task (Duarte et al., 2018). Each trial consisted of a sequence of two videos, a reference, and a comparison stimulus/video. After observing both, the observer had to indicate in which video the box was heavier. In particular, she/he had to press the left arrow key to answer “the First” and the right arrow key to answer “the Second”. The 7.5kg-video was shown in every trial as it represented the reference stimulus. The 0kg-, 2.5kg-, 5kg-, 7.5kg-, 10kg-, 12.5kg-, and 15kg-video were the comparison stimuli (the 7.5kg-video was used both as a reference and as comparison stimuli). In each trial, the order of appearance of the reference and the comparison stimuli was random. Each comparison video was displayed 12 times in random order for each condition, for a total number of trials corresponding to 168 (7 box weights, 12 repetitions, 2 conditions). The total duration of the experiment was about 30 minutes. — Figure 1 here —

Data analysis

The discrimination sensitivity ( ) was evaluated at each comparison stimulus (except 7.5 kg) using signal-detection theory as described in Norman et al. (Norman et al., 2009). The d’ was calculated according to one-dimensional classification experiments, following the procedure adopted by Norman et al. (Norman et al., 2009). A “hit” occurred when the first weight was heavier and the observers correctly responded, “first was heavier.” A “false alarm” occurred when the first weight was lighter, but the observers incorrectly responded, “first was heavier.” The hit rate was computed by dividing the number of hits by the total number of trials in which the first weight was heavier; the false-alarm rate was obtained by dividing the number of false alarms by the total number of trials in which the first weight was lighter (Norman et al., 2009). The log-linear adjustment method was used to adjust for extreme values of hits and false alarms (Macmillan & Creelman, 2004). The higher the values, the better the ability to discriminate between the object’s weight. Then data were classified as Light (0kg, 2.5kg, 5kg) and Heavy (10kg, 12.5, 15kg), andLHmean was obtained in the two conditions by averaging the answers to Light and Heavy weights. Finally, the mean was calculated considering all the weights. The ratio of responses in which the comparison stimulus was judged “Heavier” than the reference stimulus at each box weight was computed for each observer in the two conditions (Concentric and Eccentric) to build a Psychometric function. The observers’ psychometric curves were obtained by finding the best-fitting logistic functions usingpsyphy and quickpsy R package (Linares & López-Moliner, 2016; Yssaad-Fesselier & Knoblauch, 2006). The lower and upper asymptotes, threshold, and just noticeable difference (JND), were estimated for each psychometric function (Knoblauch & Maloney, 2012). Lower asymptote (ALOW) and upper asymptote (AUP) were computed according to Oh et al. (Oh et al., 2016). The lower/higher ALOW/AUP, the better the ability to discriminate low/high weights. The threshold corresponds to the curve point crosses 0.5 on the y-axis and indicates the point of subject equality (Kopec & Brody, 2010). JND is considered the smallest weight that produces changing in perception and is calculated as the half difference between the weights at which the psychometric function equals 0.75 and 0.25, respectively (von Sobbe et al., 2021). A lower JND indicated a better ability to discriminate the stimuli.

Statistical analysis

Sensitivity ( ) at each comparison stimulus (Norman et al., 2009), mean sensitivity (mean , obtained by averaging at the different comparison stimuli except 7.5-kg) (Maguinness et al., 2013), ALOW, AUP, threshold, and JND, were considered as outcome parameters. Shapiro-Wilk test was applied to evaluate data distribution and Levene’s test was used to evaluate the equality of variances. LHmean dˈ and mean were normally distributed, whilst , ALOW, AUP, threshold, JND, and heavier probability at each comparison stimulus (including 7.5 kg) were not. Statistical analyses were chosen based on data distribution. Concerning the sensitivity analysis, Wilcoxon tests were applied to compare values at each comparison stimulus between Concentric and Eccentric conditions. Within each condition, Friedman tests, followed by post hoc, were used to assess differences among at each comparison stimulus (0kg, 2.5kg, 5kg, 10kg, 12.5kg, and 15kg). An ANOVA was applied on LHmean with Amount-of-weight (2 levels, Light and Heavy) and Condition (2 levels, Concentric and Eccentric) as within subject factors. Bonferroni post hoc tests were applied in case of significant interaction. Then, a t-test was performed to statistically compare mean between Concentric and Eccentric conditions. Concerning the psychometric function, all parameters were statistically evaluated using the Wilcoxon test to assess differences between the two conditions. Normally distributed data are reported as mean value ± standard error (SE), while not-normally distributed data are given as median [interquartile range, IQR]. The significance level was set at 0.05, except for , where Bonferroni correction was applied due to multiple comparisons. Statistical analyses were performed with SPSS Statistics 26 software.

Control experiment

A Control Experiment was conducted to assess the role of the actor’s perceived effort in the ability to discriminate the weights of moving objects.

Participants

Thirteen volunteer observers (females/males: 7/6, mean age ± SD: 29.5 ± 4.9 years), not involved in the Main Experiment, were recruited in the Control Experiment.

Experimental procedures

Observers were involved in a single session experiment during which they were required to perform a video task lasting about 30 minutes. Video stimuli were the same used in the Main Experiment (Figure 1A). In a single trial, a video was shown and then observers were asked to rate the actor’s effort using a Visual Analog Scale (VAS). The VAS consisted of a line approximately 10 cm (378 pixels) long, with “No effort” and “Maximal Effort” as initial and final anchor points, respectively. The score was expressed on a scale from 0 to 100 (Delgado et al., 2018). Each video was displayed 10 times, in random order, for a total number of 140 trials (7 box weights, 10 repetitions, 2 conditions).

Statistical analysis

The difference between the perceived effort of the actor at each comparison stimulus and at the reference stimulus (i.e., 7.5kg), expressed as an absolute value (ΔVAS Effort; e.g., abs[VAS Effort(7.5kg) – VAS Effort(12.5kg)]), was calculated. Furthermore, based on the value of the comparison stimulus for which ΔVAS Effort was calculated, ΔVAS Effort values were classified as Light (0kg, 2.5kg, 5kg) and Heavy (10kg, 12.5, 15kg), and LHmean ΔVAS Effort was obtained for the two conditions. Shapiro-Wilk test was performed to evaluate if the parameters were normally distributed. Levene’s test was used to evaluate the equality of variances. ΔVAS Effort was analyzed via a 2 x 6 ANOVA with Condition (2 levels: Concentric and Eccentric) and Box-Weight (6 levels: 0kg, 2.5kg, 5kg, 10kg, 12.5kg, and 15 kg) as within subject factors. Also,LHmean ΔVAS Effort was analyzed via 2x2 ANOVA with Condition (2 levels: Concentric and Eccentric) and Amount-of-weight (2 levels, Light and Heavy) as within subject factors. Bonferroni post hoc tests were applied in case of significant interaction.