Older and young adults adopt different postural strategies during quiet bipedal stance after ankle plantarflexor fatigue

The individual effects of fatigue and age on postural control have been extensively studied. However, there is a lack of knowledge about the combined effects of fatigue and age on postural control and posture. The purpose of this study was to investigate the influence of age and muscle fatigue on bipedal stance postural control and posture. Twenty-three participants, fourteen young adults and nine older adults performed 30-s bipedal postural trials before and after fatigue. The fatigue protocol consisted in repeating heel raises until exhaustion. Back, hip, knee and ankle joint angles and centre of pressure (COP) displacements were recorded with a 3D motion capture system and a force plate. Fatigue induced similar increases in sway area and COP velocity for both populations. This was associated with a forward displacement of the COP position for both populations but with different postural adaptations. Similar increased flexions (+1.7 to 1.8°) for all joint angles (ankle, knee, hip and back) joints were observed after fatigue for older adults, whereas young adults mainly showed increased flexion for the ankle joint (+3.1°) and decreased flexion at the hip (−1.2°). It can be argued that older adults act on all joints because they are too weak to sufficiently modify any joint individually, contrary to young adults. These posture adaptations may be strategies to cope with fatigue by limiting the reliance on fatigued plantarflexors, increasing ankle stiffness, increasing the sensitivity of muscle spindles, increasing the number of degrees of freedom of the task.