Introduction: Aging leads to alterations not only within the complex ă subsystems of the neuro-musculo-skeletal system, but also in the ă coupling between them. Here, we studied how aging affects functional ă reorganizations that occur both within and between the behavioral and ă muscular levels, which must be coordinated to produce goal-directed ă movements. Using unimanual reciprocal Fitts' task, we examined the ă behavioral and muscular dynamics of older adults (74.4 +/- 3.7 years) ă and compared them to those found for younger adults (23.2 +/- 2.0 ă years). ă Methods: To achieve this objective, we manipulated the target size to ă trigger a phase transition in the behavioral regime and searched for ă concomitant signatures of a phase transition in the muscular ă coordination. Here, muscular coordination was derived by using the ă method of muscular synergy extraction. With this technique, we obtained ă functional muscular patterns through non-negative matrix factorization ă of the muscular signals followed by clustering the resulting synergies. ă Results: Older adults showed a phase transition in behavioral regime, ă although, in contrast to young participants, their kinematic profiles ă did not show a discontinuity. In parallel, muscular coordination ă displayed two typical signatures of a phase transition, that is, ă increased variability of coordination patterns and a reorganization of ă muscular synergies. Both signatures confirmed the existence of muscular ă reorganization in older adults, which is coupled with change in ă dynamical regime at behavioral level. However, relative to young adults, ă transition occurred at lower index of difficulty (ID) in older ă participants and the reorganization of muscular patterns lasted longer ă (over multiple IDs). ă Discussion: This implies that consistent changes occur in coordination ă processes across behavior and muscle. Furthermore, the repertoire of ă muscular patterns was reduced and somewhat modified for older adults, ă relative to young participants. This suggests that aging is not only ă related to changes in individual muscles (e.g., caused by dynapenia) but ă also in their coordination.