Concentric and eccentric contractions show different patterns of neural activity at both peripheral and cortical levels, which are thought to influence the perception of action properties such as the weight of objects moved by others. The aim of this study was to investigate how the type of muscle contraction influences weight estimation during action observation. Forty-eight volunteers completed the Main experiment and the Control experiment. In the Main experiment, they performed a weight discrimination video task in which they watched videos of an actor moving two objects, a comparison and a reference box, executing concentric or eccentric contractions and they had to indicate which box was the heaviest. Sensitivity analysis and psychometric functions were used to analyze the data. In the Control experiment, observers judged the actor's effort in moving the boxes. The results of the Main experiment showed that the weight discrimination sensitivity was higher in the Eccentric condition for the Light boxes. Conversely, for the heaviest boxes, discrimination sensitivity was higher in the Concentric than in the Eccentric condition. These results were confirmed by the psychometric function analysis. The Control experiment showed that for Light stimuli, the perceived difference in effort between the comparison and reference stimuli was greater in the Eccentric than in the Concentric condition. These results showed that the ability to evaluate the weight of the object involved in the observed action was influenced by the type of contraction and the amount of weight. The effort attributed to the actor influenced the observer's perception.