Kinematic and electromyography (EMG) aspects of running on a firm surface and on soft, dry sand were studied to elucidate mechanisms contributing to the higher energy cost (EC) of sand running. Eight well-trained males (mean VO(2max) 64.3+/-8.6 ml.kg(-1).min(-1)) performed barefoot running trials on a firm surface (wooden floor) and on a soft, dry sand surface (track dimensions 8.8 mx60 cm; depth 13 cm) at 8 and 11 km.h(-1). Kinematic and EMG data were collected simultaneously using an integrated six-camera 50 Hz VICON motion analysis system, an AMTI force-plate and a 10-channel EMG system. Running at 8 km.h(-1) on sand resulted in a greater (P<0.05) stance time (t(s)) compared with the firm surface. At 11 km.h(-1), sand running resulted in a greater stance-to-stride ratio (P<0.005), a shorter stride length (SL) (P<0.05), and a greater cadence (P<0.001) compared with the firm surface values. Hip and knee flexion at initial foot contact (IFC), mid-support (MS) and flexion maximum were greater (P<0.001) running on sand compared with firm surface values at 8 and 11 km.h(-1). Over duration of stride, Hamstring (semimembranosus and biceps femoris) EMG was greater running on sand compared with the firm surface at 8 (P<0.001) and 11 (P<0.05) km.h(-1). During the stance phase in the 8-km.h(-1) trials, EMG in the Hamstrings (P<0.001), Vastii (Vastus lateralis and Vastus Medialis) (P<0.02), Rectus femoris (Rec Fem) (P<0.01) and Tensor Fascia Latae (Tfl) (P<0.0001) were greater than the firm surface measures. During stance in the 11-km.h(-1) trials, Tfl EMG was greater (P<0.02) running on sand compared with the firm surface. At IFC and MS, Hamstrings' EMG was greater on sand at both running speeds (P<0.001). For the Vastii (P<0.02), Rec Fem (P<0.0001) and Tfl (P<0.0001) muscles, the EMG at MS running on sand at both speeds was greater than the firm surface values. The increased EC of running on sand can be attributed in part to the increased EMG activation associated with greater hip and knee range of motion compared with firm surface running.