A hip joint flexor 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 flexor moment during late stance was significantly lower during inclined walking than in the two other conditions. During normal walking the iliacus muscle showed no or very weak activity and first 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 first 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 flexor moment in question is largely generated by passive structures in the form of ligaments resisting hip joint extension.

Running on a lower-body positive-pressure (LBPP) treadmill allows effects of weight support on leg muscle activation to be assessed systematically, and has the potential to facilitate rehabilitation and prevent overloading. The aim was to study the effect of running with weight support on leg muscle activation and to estimate relative knee and ankle joint forces. Runners performed 6-min running sessions at 2.22 m/s and 3.33 m/s, at 100%, 80%, 60%, 40%, and 20% body weight (BW). Surface electromyography, ground reaction force, and running characteristics were measured. Relative knee and ankle joint forces were estimated. Leg muscles responded differently to unweighting during running, reflecting different relative contribution to propulsion and antigravity forces. At 20% BW, knee extensor EMG peak decreased to 22% at 2.22 m/s and 28% at 3.33 m/s of 100% BW values. Plantar flexors decreased to 52% and 58% at 20% BW, while activity of biceps femoris muscle remained unchanged. Unweighting with LBPP reduced estimated joint force significantly although less than proportional to the degree of weight support (ankle). It was concluded that leg muscle activation adapted to the new biomechanical environment, and the effect of unweighting on estimated knee force was more pronounced than on ankle force. Keywords: rehabilitation, body weight support, EMG, estimated joint load, runningEarly mobilization after injury and surgery on the lower extremities is essential to reduce adverse effects of immobilization and to facilitate rehabilitation. Walking or running with weight support on a lower-body positive-pressure (LBPP) treadmill is a new training and rehabilitation modality. 1,2 A recent study indicated strong correlation between tibiofemoral force and treadmill ground reaction force in a small group of elderly patients during slow walking. 3 Another study showed that runners can reduce peak vertical ground reaction forces while maintaining the aerobic stimulus when running on a LBPP treadmill. 4 Even training at high speeds and high aerobic stimuli with the benefit of low vertical ground reaction force and near-normal movement pattern can be performed on the LBPP treadmill. 5 Furthermore, no adverse impact on systemic and head cardiovascular parameters has been found. 6 However, the effect of different levels of LBPP weight support during running on neuromuscular adaptations and their possible associations to joint forces have not been studied systematically.During level running, leg muscles support body weight (BW) against gravity and accelerate/decelerate the body mass in a forward direction. BW, as measured in Newton, is directly related to gravity, while body mass (kg) is independent of gravity. Therefore, by unweighting in a LBPP chamber, it is possible to create a new biomechanical environment where the vertical forces are reduced while forces associated with horizontal acceleration/deceleration of the body remain unchanged. This allows systematic study of individual leg muscle contribution to body support against grav...