Sorry, I should have quoted Go-Fast’s original message.
Charlie suggested that Asafa’s large psoas major may be the result of hill sprints. As far as I know, Stephen Francis’ group do their hill sprints on concrete. The tendon stiffness is thus more likely a result of the large volume of training they do on grass.
Regarding psoas size, the study group compared Asafa to Asahara, a Japanese sprinter. Studies comparing psoas size in black and white individuals have shown that this muscle is substantially larger in blacks. The finding of this study group thus probably reflects a genetic/racial difference rather than the result of any particular type of training.
See, for example:
Anatomical differences in the psoas muscles in young black and white men
PATRICK HANSON a1 c1 , S. PETER MAGNUSSON a1 a2 , HENRIK SORENSEN a1 and ERIK B. SIMONSEN a1
a1 Department of Medical Anatomy C, Laboratory for Functional Anatomy and Biomechanics, The Panum Institute, University of Copenhagen, Denmark
a2 Team Danmark Test Center/Sports Medicine Research Unit, Bispebjerg Hospital, Copenhagen, Denmark
Abstract
The anatomy of the psoas major muscle (PMA) in young black and white men was studied during routine autopsies. The forensic autopsies included 44 fresh male cadavers (21 black, 23 white) with an age span of 14 to 25 y. The range for weight was 66–76 kg and for height 169–182 cm. The PMA was initially measured in its entire length before measuring the diameter and circumference at each segmental level (L1–S1). At each segmental level, the calculated anatomical cross-sectional area (ACSA) was more than 3 times greater in the black group compared with the white (P<0.001). The psoas minor muscle (PMI) was absent in 91% of the black subjects, but only in 13% of the white subjects. These data show that the PMA is markedly larger in black than white subjects. The marked race specific difference in the size of the PMA may have implications for hip flexor strength, spine function and race specific incidence in low back pathology, and warrants further investigation.