Researchers generally agree that perceived heaviness is based on the actions associated with unsupported holding. Psychophysical research has supported this idea, as has psychophysiological research connecting muscle activity to the perceptions of heaviness and effort. However, the role of muscle activity in the context of the resulting motions has not been investigated. In the present study, perceptions of heaviness were recorded along with the electromyogram (EMG) of the lifting muscle and peak acceleration of the lift. Consistent with predictions derived from Newton's Second Law of motion (Force=Mass × Acceleration), normal and illusory perceptions of heaviness were a function of the ratio of muscle activity to lifting acceleration. These results identify a psychophysiological mechanism for heaviness perception based on the forces and motions associated with unsupported holding.

Research has shown that perceived heaviness is a function of the ratio of muscle activity (measured by electromyogram [EMG]) to the resulting acceleration of the object. However, objects will commonly be lifted at different speeds, implying variation in both EMG and acceleration. This study examined the effects of lifting speed by having participants report perceived heaviness for objects lifted by elbow flexion at three different speeds: slow, preferred, and fast. EMG and angular acceleration were recorded during these lifts. Both EMG and angular acceleration changed across lift speed. Nevertheless, despite these variations, perceived heaviness consistently scaled to the ratio of EMG to angular acceleration. The exponents on these parameters suggested that the saliency of muscle activity and movement changed across the three lift speeds.

Perceived heaviness is clearly a function of muscle activity: objects feel heavy, in part because they are lifted with more force than lighter feeling objects. Recent research showed that participants scale their perceptions to the ratio of muscle activity to lift acceleration during elbow lifts (Waddell et al. J Exp Psychol Hum Percept Perform 42:363-374, 2016). The current study sought psychophysiological functions relating perceived heaviness to EMG and peak lift acceleration across multiple lifts employing different muscles as prime movers. Participants lifted objects with three arm lifts-shoulder, elbow, and wrist-and reported perceived heaviness. In each lift, EMG was recorded from the anterior deltoid, biceps brachii, and forearm flexors, and peak angular acceleration was recorded about each joint. The resulting psychophysiological functions revealed the hypothesized ratio of muscle activity to peak lift acceleration in all lifts. Principal component regressions showed that the EMG of the forearm flexors and peak acceleration of the lifting joint were most relevant for perceived heaviness. The special role of forearm flexors in perceiving heaviness across different lifts was interpreted in terms of the invariant structure of the inertia tensor about the wrist.

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