Introduction: Knee osteoarthritis (OA) is a debilitating disorder that is accompanied by loss of muscle mass and strength and is prevalent in older populations (Lawrence et al., 2008). In skeletal muscle the Na+,K+ -ATPase (NKA) regulates membrane potential and excitability and thereby directly influences skeletal muscle force generation (McKenna et al., 2008). This study examined the effects of both OA and ageing on muscle NKA content in old participants. Methods: Thirty-six older adults participated (range: 55-81 years old); nineteen had OA (69.9 ± 6.5 yr, mean age ± SD) and 17 were asymptomatic age-matched controls (CON; 66.8 ± 6.4 yr). A vastus lateralis muscle biopsy was taken and analysed for NKA content ([3H] ouabain binding sites), NKA α1-3 and β1-3 isoform protein abundance (immunoblotting) and NKA isoform mRNA content (real time RT-PCR). Participants also underwent knee extensor muscle strength testing and completed a physical activity questionnaire. Differences in NKA between OA and CON were assessed using independent t-tests, and association between age and NKA content was investigated using bivariate correlation in both the OA and CON group individually, and in the pooled data. Results: OA participants had 40.8% lower strength (p = 0.005), but 34% higher NKA α2 (p = 0.006) and 1 fold greater NKA α3 (p = 0.016) abundance compared to CON. OA participants performed more incidental physical activity (p = 0.035). No significant differences were found between groups for total NKA content, NKA α1-3 and β1-3 isoform mRNA content, and NKA α1, β1, β2, and β3 isoform abundance. There was an inverse correlation between age and [3H] ouabain binding sites with both participant groups combined (r = -0.467, p = 0.038, Figure 1), and in the OA participants (r = -0.626, p = 0.03). Conclusions: NKA content was not decreased in OA compared to asymptomatic controls. We also reported that age is negatively correlated with [3H] ouabain content independent of disease state which may suggest that age but not knee OA is related to decreased muscle NKA content.