Preservation of motor flexibility in healthy aging: Flexibility in joint coordination is unaffected by age and task constraints in two fundamental activities of daily living

Humans can perform a given voluntary movement in many different ways. When reaching for an object or rising from a chair, we can set the position of the upper and lower extremity joints in an infinite number of combinations, providing a range of movement possibilities. Healthy adults make use of this rich movement repertoire to perform reaching and sit-to-stand movements under different environmental conditions. However, healthy aging impairs neuromuscular function. The present thesis aimed to address the unanswered question if such neuromuscular impairments interfere with healthy old adults’ ability to flexibly coordinate the available joint motions while reaching and rising from a chair especially when the environmental conditions become challenging. Based on the ‘principle of motor abundance’ and the ‘constraints to movement hypothesis’ the hypothesis was that the age-related decline in neuromuscular functions leads to an increase in motor flexibility to compensate for deficits in muscle strength, coordination and balance abilities. A mathematical model and four experimental studies revealed that adults aging healthily do retain motor flexibility during simple sit-to-stand and challenging reaching movements independent of the balance, force and accuracy requirements. When rising from a chair set to low heights old adults even increase flexibility in joint coordination. The possibility exists that flexibility in joint coordination, as a compensation for neuromuscular deficits, would increase only when old adults must execute daily tasks near the limits of the available muscle strength, coordination, and balance abilities.