Sentence:  Serum potassium at this time was 3.8 mEq. per liter , and the hemoglobin was 13.9 gm. By Dec. 1 , 1958 , the weakness in the pelvic and quadriceps muscle <WSD>groups</WSD> was appreciably worse , and it became difficult for the patient to rise unaided from a sitting or reclining position . 
Meaning of the word marked with <WSD> tags:  any number of entities (members) considered as a unit