Muscle weakness and atrophy are well known consequences of disuse and ageing. Whereas reduced physical activity, hormonal, and possibly, nutritional factors are the primary causes of the loss of muscle mass in disuse; in ageing, neuropathic processes, as well as well as the above factors, contribute to the condition of sarcopenia. In both conditions, however, the loss of muscle mass exceeds that of muscle size and a decrease in force per unit of muscle cross-sectional area (F/CSA) is observed. Several factors contribute to this phenomenon but, generally, these may be of muscular, tendinous and neural origin. What seemed less known up to date, was the role of changes in muscle architecture and in tendon mechanical properties in the decrease of F/CSA with disuse and ageing. However, recent evidence shows that both muscle architecture and tendon mechanical properties are markedly modified in both conditions. MRI-determined muscle volume of the gastrocnemius medialis (GM) and ultrasound-based measurements of fibre fascicle length, pennation angle and GM tendon stiffness were obtained in vivo in: 1) a population of elderly individuals aged 70-81 years and compared to a group of height-matched young adults aged 27-42 years, and 2) in a group of 19 young adults undergoing a 90-day bed rest period (ESA LTBR 2001-2 study). The two studies received ethical approval by the host institutions were the experiments were performed. In ageing, sarcopenia, represented by a 25% loss of muscle volume, was associated both with a decrease in GM fascicle length (10%) and in pennation angle (13%). Also, tendon stiffness was 14% lower in the elderly. In disuse, the 90-day bed rest period resulted in a 9% and 14% decrease in GM fascicle length and pennation angle, respectively and, similarly to ageing, tendon stiffness was reduced by 32%. These myotendinous alterations are expected to influence the length-force relation of muscle fibres and, as such, may play a role in the decrease in intrinsic muscle force with disuse and ageing.