Background This study was aimed at investigating the impact of assisted cycling therapy (ACT) on fine manual dexterity in adults with Down's syndrome (DS). Methods Nine persons with DS completed four different interventions: voluntary exercise (VE), voluntary with music (VEM), assisted exercise (AE) and no exercise (NE). Fine manual dexterity (e.g., Purdue Pegboard) was tested pre and post interventions. Results The results showed dominant and bimanual hands were improved after AE but no evident in either subtest after VE. The assembly subtest, processed by spatial‐temporal ability, was only improved after VEM and NE. Conclusions It is speculated that AE stimulated more peripheral sensory input to the motor cortex and neurotransmitters than the other interventions. In addition, the involvement of music may enhance spatial intelligence during VEM. Even though the exact mechanisms are still unknown, the implication of our results showed that AE and VEM can improve fine manual dexterity in persons with DS.