The Na+,K+-ATPase (NKA) is a key protein involved in the maintenance of skeletal muscle excitability and comprises 2 subunits (α and β), each of which express multiple isoforms at a protein level in skeletal muscle (α1-3 and β1-3). The fibre-specific expression, adaptability and roles of each isoform in human skeletal muscle are explored in this thesis. Research utilising muscle biopsies typically uses samples obtained from the vastus lateralis muscle, which in healthy young people comprises similar proportions of type I and II fibres. Analyses using whole muscle pieces don’t allow the detection of fibre-type specific differences and changes occurring at a cellular level. Hence analysis of skeletal muscle samples at the single fibre level offers important advantages in understanding NKA regulation. This thesis therefore investigated the isoform abundance of the NKA in human skeletal muscle single fibres and their adaptability following intense repeated-sprint exercise (RSE) training in healthy young adults (Study 1); with ageing (Study 2) and after high-intensity interval training (HIT) in the elderly (Study 3) and after voluntary inactivity and resistance training in healthy young adults (Study 4).