Generation and modification of human locomotor EMG activity when walking faster and carrying additional weight

In activities of daily living, people walk at different speeds with or without carrying additional loads. In this study, we sought to examine how human adults manage these commonly encountered additional demands during walking. We measured electromyography (EMG), triceps surae H-reflexes, joint motion and ground reaction forces (GRF) while participants walked at 1.0 m/s and 1.5 m/s with or without an additional 20.4 kg of weight (the equivalent of 23-36% bodyweight). Faster walking was accompanied by a universal increase in burst EMG amplitude across flexors and extensors of upper and lower leg muscles (with most notable increases found in the plantarflexors) while burst patterns of activity were maintained. In addition, the range of motion increased at the ankle, knee and hip joints, while the step cycle duration was shortened. In bearing additional weight, upper and lower leg extensor activity, especially early stance quadriceps activity, was increased while joint motion was minimally affected at the ankle and knee (but not hip). When walking faster and carrying additional weight, changes in locomotor EMG (except for plantarflexors) and knee and hip joint motion displayed combined features of those two additional demands; changes in plantarflexor activity and ankle joint motion were more complex. Locomotor H-reflexes were larger at 1.5 m/s than at 1.0 m/s only when carrying additional weight. In generating plantarflexor activity and controlling ankle joint motion for propulsive force generation when walking faster and carrying additional weight, multiple mechanisms of both spinal and supraspinal origin may be involved.