Introduction

Starting from the discovery of the mirror neuron system, many studies concluded that a common representation of human movement execution and perception exists (Bonini et al., 2022). The scientific literature refers to this mechanism as ”motor resonance”, which consists of the activation of the perceiver’s motor system when observing human movement (Rizzolatti et al., 1999). In a recent review paper, crucial factors modulating the mirror neuron system activity during action observation have been identified (Kemmerer, 2021). Some of them are specifically related to the features of the observed movement and the possibility for them to be mirrored in the observer’s motor repertoire. For instance, a stimulus that moves according to the biological laws of motion can be mapped into the observer’s motor programs (A. Bisio et al., 2010; A Bisio et al., 2012; Ambra Bisio et al., 2014; Gavazzi et al., 2013). Also, observing a specific motor skill will activate motor resonance if it is also possessed by the observer (Aglioti et al., 2008; Calvo-Merino et al., 2005, 2006). Thus, the way humans perceive external movement is shaped by their motor repertoire. Therefore, if there are differences among types of motion, it is likely that these differences will also affect motion perception. Human movements are implemented by concentric (i.e., the muscle contracts and shortens, such as the upward phase of a biceps curl) and/or eccentric (i.e., the muscle contracts and lengthens, such as the downward phase of a biceps curl) muscle contractions. Scientific literature provides evidence concerning the difference between these kinds of contractions (Duchateau & Baudry, 2014). For example, maximal voluntary force is greater during eccentric than concentric contractions, as are force fluctuations (Christou & Carlton, 2002; Fang et al., 2001; Grabiner & Owings, 2002; G. H. Yue et al., 2000) and the risk of injury (Shellock et al., 1991), while the electromyogram amplitude is either similar or greater during concentric than eccentric contractions (Duchateau & Baudry, 2014). The differences are also related to cortical activation (Gueugneau et al., 2023; Kwon & Park, 2011; Winstein et al., 1997; G. Yue & Cole, 1992). During eccentric contractions, greater cortical activity was detected through a fMRI investigation in the right inferior parietal lobe, the pre-supplementary motor area, the anterior cingulate cortex, the right prefrontal, and the left cerebellar hemisphere (Kwon & Park, 2011). In contrast, a greater BOLD signal intensity was observed in the left primary motor cortex and the right cerebellum and vermis during the execution of concentric contractions (Howell et al., 1995). Differences between these kinds of movements were found in the primary motor cortex activity and during the preparation phase. Canepa and colleagues showed a time-specific modulation of corticospinal excitability in the preparatory phase to an eccentric muscle contraction (Canepa et al., 2021). Given all these differences between the two contraction modes, and based on the motor resonance theory, one might hypothesize that there might be differences when a person observes a movement implemented by an eccentric or a concentric muscle contraction and is then asked to judge its features, such as the weight of the object being moved. Skilled and dexterous manipulation requires the ability to estimate the weight of an object accurately. Additionally, assessing the characteristics of the objects moved by other people can be a valuable source of information for planning an accurate response (Reichelt et al., 2013). According to action observation studies, the weight of the object involved in the action and the effort of the observed individual both affected the activity of the observer’s sensorimotor brain areas (Alaerts, Swinnen, et al., 2010; Meulenbroek et al., 2007). This suggests that the sensorimotor representation includes both characteristics, which are then used to plan the subsequent motor response. This has been suggested by Reichelt et al., who showed that after observing the handling and transfer of an object, the observer automatically adjusts the lift to the weight of the observed object (Reichelt et al., 2013). Previous studies by our group have shown that the ability to judge the weight of an object moved by an external agent is modulated by factors such as age-related decline in motor skills due to aging (Albergoni, Biggio, Faelli, Ruggeri, et al., 2023) and motor experience in the observed movement (Albergoni, Biggio, Faelli, Pesce, et al., 2023). In this study, in order to gain a deeper insight into how the characteristics of an observed movement influence the observer’s perception, based on the known effect of the sensorimotor repertoire on action perception, the observer’s discrimination ability was assessed while observing eccentric or concentric contractions. Since it has been shown that the activity of a fronto-parietal network involving areas that are part of the mirror neuron system is greater during eccentric than concentric contractions, we hypothesize that participants’ discrimination ability would be the highest when observing the eccentric contraction.