. It has been widely suggested that the many degrees of freedom of the musculoskeletal system may be exploited by the CNS to minimize energy cost. We tested this idea by having subjects making point-to-point movements while grasping a robotic manipulandum. The robot created a force field chosen such that the minimal energy hand path for reaching movements differed substantially from those observed in a null field. The results show that after extended exposure to the force field, subjects continued to move exactly as they did in the null field and thus used substantially more energy than needed. Even after practicing to move along the minimal energy path, subjects did not adapt their freely chosen hand paths to reduce energy expenditure. The results of this study indicate that for point-to-point arm movements minimization of energy cost is not a dominant factor that influences how the CNS arrives at kinematics and associated muscle activation patterns.
I N T R O D U C T I O NA fundamental question in motor control is how the CNS coordinates the many kinematic and mechanical degrees of freedom of the body to control posture and movement (Bernstein 1967). Several theories of motor control are based on or inspired by the idea that the CNS arrives at a set of muscle activation patterns for a given task by minimizing a cost function in which minimization of energy cost takes part (e.g., Alexander 1997;Cruse 1986;Engelbrecht 2001;Hatze and Buys 1977;Kashima and Isurugi 1998;Nelson 1983;Rasmussen et al. 2001;Soechting et al. 1995;Todorov 2004;Todorov and Jordan 2002;Torres and Zipser 2002). Such theories have been shown to predict kinematic features for movement tasks in the absence of external loads, like relatively straight hand paths and bell-shaped velocity profiles. However, at least for unperturbed arm movements, many motor control theories predict these same kinematics (e.g., d 'Avella et al. 2006; Flash and Hogan 1985;Uno et al. 1989). In addition, even if kinematics and muscle activation patterns are in accordance with energy minimization, it would not represent evidence that energy cost is minimized by the CNS to generate motor commands. For example, the motor commands that minimize energy cost could have been selected over an evolutionary timescale.The goal of the present study was to directly test the extent to which energy cost is minimized by the CNS in the context of a motor learning task. One of the most well studied motor learning tasks involves making arm movements while grasping a robotic device that applies forces to the hand that are perpendicular and proportional to the hand velocity (Caithness et al. 2004). Unfortunately, the mechanics of these so-called curl fields are such that the minimal energy path is similar to movements in the absence of forces (see METHODS). Therefore this motor learning task is unsuitable to investigate whether energy minimization plays a role in the selection of kinematics and/or muscle activation patterns by the CNS.In the present study we designed a novel force field ...