Effect of lumbar spine motion on psoas major length during running at different speeds.

Among the paraspinal muscles, the psoas major is essential for leg swing during running. As its sarcomere length indicates lower force-generating capacity at a lengthened position, greater hip extension at higher speeds may limit its force. Meanwhile, the lumbar spine becomes more lordotic and tilts anteriorly as running speed increases, which may shorten the psoas major. However, lumbar spine motion has been neglected when estimating psoas major length. We examined the effect of lumbar spine motion on psoas major length, comparing models with (MOVING) and without (FIXED) considering lumbar spine motion. Psoas major length was estimated during treadmill running at four different speeds (4.0-8.5 m/s). The main effect of running speed on PM length was not significant in MOVING (length normalised to standing: from 102.1±1.7 to 102.3±2.0%), whereas it was significant in FIXED (from 102.2±0.8 to 104.1±0.9%). A strong linear relationship between hip angle and psoas major length was found in FIXED (r ²>0.92), but was compromised in MOVING (r ²=0.74±0.17). Lumbar spine motion alters psoas major dynamics and prevents psoas major elongation at higher speeds. This may facilitate rapid leg swing by preserving the force-generating capacity of the psoas major.