Osteoarthritis is one of the most prevalent degenerative diseases in the elderly that affects the structural and functional integrity of the musculoskeletal system. Currently, hydrotherapy is taking relevance for osteoarthritis treatment due to its possible efficacy on pain relief and functionality. This therapy can be evaluated by means of an electromyographic analysis. The objective is to show a database of lower limbs muscles electrical activity in charge of postural stability, comparing the difference between hydrotherapy and conventional therapy through surface electromyography. A signal acquisition methodology was developed measuring electromyographic activity of the Tibialis Anterior, Soleus, Vastus Medialis, Biceps Femoris, Medial Gastrocnemius and Lateral Gastrocnemius muscles, before and after applying a specific exercise routine for knee osteoarthritis. This routine consisted of 12 therapy sessions spread over 4 weeks and could be done in water and on land. The data is organized according to the attendance of 28 patients to the therapy sessions. There are two records per person, before and after each hydrotherapy or physiotherapy session, obtaining a total of 1344 records at the end of the 12 sessions, and 56 data for each session. However, a total of 864 complete records were obtained for both therapies, withdrawing those who missed any of the sessions. There is a personal information database in .CSV format and electromyographic records with a .MAT extension.