Osteoarthritis (OA) is characterized by pain, and problems with activities of daily life, especially if the hip or knee joint is affected. The aim of this project was to study associations between strength, voluntary activation and functioning in elderly patients with OA. People with OA of the knee often have lower muscle strength, but also a lower ability to voluntarily activate their knee extensors. In Chapter 2 we investigated the effects of relatively low stimulation currents on the assessment of VA of the knee extensor muscles. We concluded that by using submaximal muscle stimulation overestimation of VA may even be less compared with maximal nerve stimulation. In Chapter 3, we investigated physical performance longitudinally in a large cohort of participants with and without self-reported hip or knee OA. Physical performance was tested with a short battery consisting of a chair stand test, a balance test and a 6 meter walk test, performed in the participants' home. Chair stand and walking performance were lower in participants with OA 3 to 6 years after OA was reported for the first time, and men were more affected than women. In the laboratory, more elaborate lab tests can be done, such as strength testing, standardized stair climb tests and longer walk tests. Such lab tests may be more sensitive to detect impairments. In Chapter 4, we compared home tests used in Chapter 3 with a set of frequently used lab tests to investigate whether home tests are indeed a good alternative if time and space are limited. Also we investigated differences in functioning in a sample of participants with and without knee OA. Home tests were significantly related to lab tests and showed reasonable relationships. Only the battery of home tests showed lower performance in participants with OA. In Chapter 5, we investigated the feasibility and effectiveness of 6 weeks of preoperative training for elderly OA patients undergoing total knee arthroplasty. Pre and post operative outcome measures were not different compared to a standard training group. We conclude that physical function, but not VA is impaired in older people with OA and that strength and physical function is more impaired just before total knee arthroplasty. When assessing function in older participants or patients with musculoskeletal disorders, home tests are a good alternative to lab tests to obtain a representative sample. Preoperative training before total knee arthroplasty can prevent the decline in function often observed before surgery, but there were no additional effects of intensive strength training.