## 4 Discussion  
This is the first study to measure head stability in children with CP when completing a targeted stepping task. Children with diplegic CP exhibited greater head instability than children with hemiplegic CP and TD children during the task, but similar foot placement error to TD children. Children with hemiplegic CP exhibited similar head stability to TD children but greater absolute anteroposterior foot placement error. Children with diplegic CP showed greater head-to-lab and head-to-trunk instability than other groups, whilst children with hemiplegic CP behaved similarly to TD children. One explanation of this global instability may be the challenge for children with diplegic CP to attenuate accelerations travelling up the body to the head via active or passive methods when impaired by reduced bilateral lower limb control and differing gait patterns compared to unilateral impairment in children with hemiplegic CP. This is consistent with previous findings [21; 32]. Children with diplegic CP exhibited a significantly greater downward (sagittal) angular velocity of the head relative to the lab and trunk, compared to children with hemiplegic CP and TD children. This may be because children with diplegic CP tilt their head to look down at the ground more to gather additional information about the target location in an attempt to reduce foot placement errors [15; 33; 34]. Children with diplegic CP may also look down at the ground more due to their reduced visual acuity, which could make it more difficult to see the target clearly on the floor, although their visual acuity still fell within typical ranges for children [35]. Capturing gaze behavior with an eye-tracker alongside head segment kinematics may provide further insight into whether children with diplegic CP utilize movement of the head to maintain visual focus at the expense of previewing what is in front of them, though this could lead to an increased fall risk due to reduced detection of upcoming obstacles along their travel path. Children with CP have previously exhibited an 'en-bloc' stabilization strategy by locking (i.e., reducing the degrees of freedom between) neighboring segments together under challenging circumstances [5; 36]. In contrast during the current stepping task, children with diplegic CP showed greater head-to-trunk angular velocity. This suggests that the task may not have produced a significant threat to postural control and future gait studies could focus on higher risk activities such as obstacle clearance or stair descent. Children with hemiplegic CP exhibited greater anteroposterior foot placement error despite no differences in head instability, suggesting the inaccuracies in foot placement are driven by factors other than the impaired gathering of accurate sensory information because of head instability. One possible explanation may relate to asymmetry between the affected and unaffected limb. The previous literature has reported step length differences between limbs in children with hemiplegic CP [37; 38]. However, since targets were positioned within the regular footfall of each child during familiarization trials, this is unlikely to be the case. Instead, the difference is likely to be caused by the low difficulty/demand of the task and verbal instructions provided by the investigator. Due to the construction of the targets, the task presented no significant consequence if the child failed to place their foot within the target, and no specific instructions were provided to the participants regarding placement of the foot within the target, as long as the foot was inside the rectangle. No instruction was given to limit potential visual targeting by participants, but may this have resulted in participants simply placing the foot within the borders of the target. More complex and unpredictable travel paths are required to determine how accuracy of movement control changes when there are greater consequences for fall risk.