Older Adults Learn Less, but Still Reduce Metabolic Cost, during Motor Adaptation 2 3 Abbreviated Title: Older Adults Learn Less but Learn to Be Efficient 4 5

26 The ability to learn new movements and dynamics is important for maintaining 27 independence with advancing age. Age-related sensorimotor changes and increased 28 muscle coactivation likely alter the trial and error-based process of adapting to new 29 movement demands (motor adaptation). Here, we asked, to what extent is motor 30 adaptation to novel dynamics maintained in older adults (65+ years)? We hypothesized 31 that older adults would adapt to the novel dynamics less well than young adults. 32 Because older adults often use muscle coactivation, we expected older adults to use 33 greater muscle coactivation during motor adaptation than young adults. Nevertheless, 34 we predicted that older adults would reduce muscle activity and metabolic cost with 35 motor adaptation, similar to young adults. Seated older (n = 11, 73.8 ± 5.6 yrs) and 36 young (n = 15, 23.8 ± 4.7 yrs) adults made targeted reaching movements while grasping 37 a robotic arm. We measured their metabolic rate continuously via expired gas analysis. 38 A force field was used to add novel dynamics. Older adults had greater movement 39 deviations and compensated for just 65% of the novel dynamics compared to 84% in 40 young adults. As expected, older adults used greater muscle coactivation than young 41 adults. Lastly, older adults reduced muscle activity with motor adaptation and had 42 consistent reductions in metabolic cost later during motor adaptation, similar to young 43 adults. These results suggest that despite increased muscle coactivation, older adults 44 can adapt to the novel dynamics albeit less accurately. These results also suggest that 45 reductions in metabolic cost may be a fundamental feature of motor adaptation. 46