Video Game Reveals the Human Body is a ‘Slacker’

Robot-mediated therapy for motor rehabilitation following stroke remains a compelling complement to traditional physical therapy. It is well established that best practice for robot-mediated therapy is to engage patients in exercises that belong to either the active or active-assistive modalities. These modalities ensure that robotic assistants provide an appropriate amount of ‘assistance as needed’, helping the patient complete important functional movements while maintaining the patient’s engagement, which is necessary to optimally stimulate neuroplasticity. Working against a patient’s engagement in an exercise is the persistent tendency of their motor system to minimize its own effort—a process that has come to be known as ‘slacking’.

Smith et al. (2018b)

We used video games controlled by human muscle force to ask a simple question: “Is the human body a slacker?”. The strength of a participants grip force determined the vertical position of a cursor (or laser sword!) which they were asked to move to different target levels and hold at each for a few seconds. Sometimes, when the participant reached the target we tricked them, by locking the cursor on target, regardless of what they did afterwards. If the participant slacked, we figured, they would reduce their muscle force whenever we applied this trick, since the target position could technically be maintained without any effort at all!

We determined that the human body is indeed a ‘slacker’, in that it always and automatically reduces its own muscle contraction force, even when it feels like force is being held constant.

Our research implies that when humans produce sustained forces, such as when carrying a coffee mug or cellphone, the body relies on various modes of feedback, such as slip detection, to prompt regular corrections for slacking. Supposing those corrections can be interrupted, such as when we get distracted, this discovery might be the best evidence to date for the importance of a good cellphone case.


Smith, B. W., Rowe, J. B., & Reinkensmeyer, D. J. (2018). Real-time slacking as a default mode of grip force control: implications for force minimization and personal grip force variability. J Neurophysiology, 120(5), 2107–2120.

Smith B. W., Rowe J. B., & Reinkensmeyer D. J. (2018). Directly measuring the rate of slacking as stroke survivors produced isometric forces during a tracking task. 40th Annual International Conference of the IEEE Engineering in Medicine and Biology Society.