Spontaneous kicking in infants is one of the earliest displays of motor skills. Abnormalities observed in these displays are an important indicator of later abnormal neuromotor function. However, these abnormalities are not well defined and difficult to detect outside of direct clinical observation. To allow for extended, non-clinical observation of spontaneous kicking, IMU sensors are attached to the limb segments of the infant's legs. An activity detection algorithm is then used to quantify kicking activity derived from collected measurement data. This paper presents our method in detail and discusses results from kicking data acquired from term and low-risk preterm infants.