Observing another person experiencing exogenously inflicted pain (e.g. by a sharp object penetrating a finger) modulates the excitability of the observer’ primary motor cortex (M1). By contrast, far less is known about the response to endogenously evoked pain such as sudden back pain provoked by lifting a heavy object.
Here, participants (n=26) observed the lifting of a heavy object. During this action the actor (1) flexed and extended the legs (LEG), (2) flexed and extended the back (BACK) or (3) flexed and extended the back which caused visible pain (BACKPAIN). Corticomotor excitability was measured by applying a single transcranial magnetic stimulation pulse to the M1 representation of the muscle erector spinae and participants scored their perception of the actor's pain on the numeric pain rating scale (NPRS).
The participants scored vicarious pain as highest during the BACKPAIN condition and lowest during the LEG condition. MEP size was significantly lower for the LEG than the BACK and BACKPAIN condition. Although we found no statistical difference in the motor-evoked potential (MEP) size between the conditions BACK and BACKPAIN, there was a significant correlation between the difference in NPRS scores between the conditions BACKPAIN and BACK and the difference in MEP size between these conditions. Participants who believed the vicarious pain to be much stronger in the BACKPAIN than in the BACK condition also exhibited higher MEPs for the BACKPAIN than the BACK condition.
Our results indicate that observing how others lift heavy objects facilitates motor representations of back muscles in the observer. Modulation occurs in a movement-specific manner and is additionally modulated by the extent to which the participants perceived the actor's pain. Our findings suggest that movement observation might be a promising paradigm to study the brain's response to back pain.