From the previously published reports by the department of physical education in the ministry of education and the experiments which consist of the mesurements of back and abdominal muscle strength and trunk flection and extension, and the electromyogram of back, abdominal, and leg muscle, trunk flection and extension, which were well recognized as the indicator of the body flexibility, were re-examined. The obtained results were as follows;
1) Trunk flection tended to decrease from 1964 to 1988. As the reason for this the development of physique, especially lower limb length are major factor. It is not right to recognize that the body became stiff.
2) There was no relation between abdominal muscle strength and trunk flection, but significant reletion was observed between back muscle strength and trunk extension.
3) The muscle pain from lower limb, not back mainly prevented the performance of trunk flection. The inhibition mechanism such as Ia inhibition in neuro-muscular system were not observed.
4) The isometric sit-up exercise in 30 seconds increased the performance of trunk flection. It might be considered that Ib inhibition mechanism partcipated.
5) In trunk extension the development of body hight and sitting hight were major factor, because the ratio, trunk extension to hight or sitting bight, didn't change in relation to age.

DESCENTE SPORTS SCIENCE Vol.12/THE DESCENTE AND ISHIMOTO MEMORIAL FOUNDATION FOR THE PROMOTION SPORTS SCIENCE