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.