ABSTRACT
Thigh muscle cross-sectional area (CSA) and maximum voluntary isometric strength of knee extensor and flexor muscles were measured in 97 men (2065 years) and 162 women (2069 years) in sedentary Japanese adults. Each subject was assigned to one of five age groups (3<SUP>rd</SUP> 4<SUP>th</SUP> 5<SUP>th</SUP> 6<SUP>th</SUP> and 7<SUP>th</SUP> decade) . Thigh muscle CSA was estimated by our developmental measuring system using an ultrasonographic device, which was connected to a PC for graphical processing. Muscle CSA for the 7<SUP>th</SUP> decade in men was significantly smaller than that for the 3<SUP>rd</SUP> decade. For women, muscle CSA were no significant from the 3<SUP>rd</SUP> to the 7<SUP>th</SUP> decade. The isometric knee extensor strength showed a significant decline with age from the 7<SUP>th</SUP> decade in men, whereas there was no significant change with age in women. Isometric strength of knee flexors in men showed a gradual decline from the 5<SUB>th</SUB> decade. The aging-associated reduction of muscle strength per muscle CSA in the extensors and flexors started from the 6<SUB>th</SUB> decade in men. It was concluded that the size and strength of the thigh muscles begin to decrease simultaneously by approximately the 6<SUB>th</SUB> decade in men, whereas there are no change until the 7<SUP>th</SUP> decade in women.