Humans interact with a variety of objects, many of which have dynamically complex properties, such as a cup of coffee. An efficient way to manipulate such objects is to drive them at a resonant frequency. This requires less effort due to dynamic amplification of control inputs. However, errors in control may also be amplified, increasing variability and making it difficult to achieve a kinematic goal. How humans control dynamically complex objects near a resonant frequency, while at the same time achieving kinematic goals, is unknown. To address this question, ten healthy subjects were asked to practice oscillating a cart and pendulum (simulated using a haptic display). Subjects had to move the cart between two spatial targets at the pendulum's resonant frequency. A visual display showed the pendulum bob moving in a semicircular cup (the cart was hidden), mimicking a ball rolling in a cup. Results showed that in early practice subjects moved below the resonant frequency and used an in-phase strategy — the cup and ball moved in the same direction. This was associated with large applied forces and high variability. With practice subjects moved above the resonant frequency and switched to an anti-phase strategy — the cart and pendulum moved in opposite directions. Concurrently, subjects' applied force decreased by half and the interaction force of the ball on the cup increased, which moved the cup to the spatial targets. Using this strategy, subjects became less variable and more accurate. Although the switch in phasing was in part dictated by the task dynamics, the direction of the shift is best explained by a controller that seeks to exploit the dynamics of an object to achieve task goals with small outlays of energy and low variability.
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