Journal of Applied Biomechanics, 2012, 28, 542-550 © 2012 Human Kinetics, Inc. An Official Journal of ISB www.JAB-Journal.com ORIGINAL RESEARCH 542 Erik B. Simonsen (Corresponding Author), Katrine L. Cap- pelen, Ragnhild í Skorini, and Tine Alkjær are with Department of Neuroscience and Pharmacology, University of Copenhagen, Copenhagen, Denmark. Poul Dyhre-Poulsen, now deceased, was also with the Department of Neuroscience and Pharmacol- ogy, University of Copenhagen, Copenhagen, Denmark. Peter K. Larsen is with the Laboratory of Biological Anthropology, Department of Forensic Medicine, University of Copenhagen, Copenhagen, Denmark. Explanations Pertaining to the Hip Joint Flexor Moment During the Stance Phase of Human Walking Erik B. Simonsen, Katrine L. Cappelen, Ragnhild í Skorini, Peter K. Larsen, Tine Alkjær, and Poul Dyhre-Poulsen University of Copenhagen A hip joint fexor moment in the last half of the stance phase during walking has repeatedly been reported. However, the purpose of this moment remains uncertain and it is unknown how it is generated. Nine male subjects were instructed to walk at 4.5 km/h with their upper body in three different positions: normal, inclined and reclined. Net joint moments were calculated about the hip, knee and ankle joint. The peak hip joint fexor moment during late stance was signifcantly lower during inclined walking than in the two other conditions. During normal walking the iliacus muscle showed no or very weak activity and frst at the transition from stance to swing. When walking reclined, a clear but rather low activity level of the iliacus muscle was seen in the frst half of the stance phase, which could contribute to the hip moment. In the inclined condition the iliacus showed much increased activity but only in the swing phase. It is concluded that the hip fexor moment in question is largely generated by passive structures in the form of ligaments resisting hip joint extension. Keywords: walking, inverse dynamics, hip joint moment, EMG, iliacus muscle Both during walking and running, a rather large fexor moment about the hip joint in the last half of the stance phase has been repeatedly reported (e.g., Pedotti, 1977; Mann and Sprague, 1980; Winter 1983, 1988; Winter et al., 1990; Simonsen et al., 1997). Obviously, this moment seems to oppose horizontal progression, but it has been argued that it should serve the purpose of bal- ancing the upper body (Mann & Sprague, 1980; Hunter et al., 2004). The obvious reason for this would be not to release the upper body backward, especially during push- off. It has also been argued that the hip fexor moment decelerates the extension of the hip joint and pulls the hip joint into fexion, which progresses into the swing phase (Winter, 1988). In elderly the moment has been observed to increase (DeVita & Hortobagyi, 2000) as well as in patients with hip dysplasia (Pedersen et al., 2004). A possibility exists that the hip joint fexor moment in question could partly be due to passive elastic struc- tures, e.g., ligaments resisting hip joint extension (Yoon & Mansour, 1982; Riener & Edrich, 1999; Silder et al., 2007). However, if the hip joint fexor moment is generated partly or completely by muscles, the primary movers have to be the psoas major, iliacus (IL) and/or the rectus femoris (RF) muscle, the tensor fasciae latae and the sartorius muscle presumably being too week for such a task (Wickiewicz et al., 1983). Andersson et al. (1997) recorded fne wire intramuscular EMG from the iliopsoas and the RF during level walking. At the speed of 3.6 km/h, the IL and the psoas major were activated very close to the end of the stance phase and at 5.4 km/h the activity started a little earlier but still too late to generate the hip joint fexor moment during the stance phase. The RF was active in the frst part of the stance phase and during the swing phase at 3.6 km/h while at 5.4 km/h it only showed one burst about the transition from stance to swing (Andersson et al., 1997). Cappellini et al. (2006) observed very low or no IL activity (surface electrodes) during walking at 3 km/h or 5 km/h while the RF seemed to have a low burst of activity at the very end of the stance phase. Gottschall and Kram (2005) reported IL and RF activity by surface EMG at the transition from stance to swing at 4.5 km/h. Among several others, Hof et al. (2005) reported a low RF burst at the time of toe-off but with a much variable size. Accordingly, the activity peri- ods reported for the hip joint fexor muscles were situated too late in the stance phase and the muscular explanation for the relatively large hip joint fexor moment during the last half of the stance phase is unknown. The purpose of the current study was to investigate whether the hip joint fexor moment is generated to bal- ance the upper body and fnd possible explanations for the