## 4 Discussion  
Turning is a vital motor skill required in the performance of daily living activities and human mobility. Previous investigators have reported that impaired turning may increase the risk of falls [24; 25] and fall injuries in several populations such as PD [26] and stroke [27]. To our knowledge, this is one of the first studies to investigate turning using motion-capture data and characterizing turning into turn cycles similar to gait cycles. This characterization into turn cycles makes it easier to study complicated movements such as 360\({}^{\circ}\) turns. This study aimed to characterize the turning ability of patients utilizing kinematic data from infrared markers. This would allow clinicians to assess new parameters of turning disability, which can identify the effectiveness of rehabilitation and possibly identify which persons after stroke are at risk of falling. Figure 5: (**a**) Average number of turn cycles in stroke and healthy groups. (**b**) Critical time in seconds in stroke and healthy adults. (**c**) Number of critical phases during 360\({}^{\circ}\) turn in stroke and healthy adults. Where * represents significant different among groups. As expected, we found individuals after stroke took significantly more time and required a greater number of steps compared to age-matched healthy older controls. Stroke-affected participants faced difficulty turning due to the effects of hemiparyalysis and impairments in balance, muscle weakness, and coordination [1; 6]. The slowness of turning in those after stroke can be linked to their level of impairment [2; 28] or increased fall risk [24; 29]. Healthy control participants have been reported to complete a 360\({}^{\circ}\) turn in under 4 s [3]. However, the method for how the turning speed was measured was not reported, and no normative data exist. In this study, we defined turn cycles, their phases, and how they are measured. Individuals after stroke took 1.88 \(\pm\) 2.05 s to complete a turn cycle, whereas healthy older adults took 1.08 \(\pm\) 0.28 s to complete the turn cycle. We found participants after stroke took significantly more turn cycles compared to healthy individuals (F (1, 19) = 13.8, with \(p\) = 0.0014 (5.4 \(\pm\) 0.37 s versus 3.45 \(\pm\) 0.36 s)) (Figure 5a). As a turning strategy, individuals after stroke had longer critical times (rests) on the trailing foot, whereas healthy adults had longer critical times on the leading foot. We also found that the stroke group had longer double support times than the healthy adult group, approximately five times more when feet were apart and six times more when feet were together. We found that stroke participants spent on average 8.74 s (SD = 12) and healthy participants 1.27 s (SD = 0.44) on double support (both feet apart and feet together) when completing a 360\({}^{\circ}\) turn (Table 2). During double support phase, proprioceptive inputs from skin mechanoreceptors, spindle fibers, and joint receptors provide loading information to our central nervous system [30; 31]. The increased double support phase during turning in stroke survivors may be due to neuromuscular dysfunction relating to reduced proprioceptive signals and increased time to attain more signals and stabilize during double support phase. We also found that stroke survivors had significantly longer critical time than the healthy adult group (F (1, 19) = 9.83, with \(p\) = 0.0054). The critical time is an indicator of high instability, which may lead to falling. Since during single support phase, one of the feet is in the air and the other on the ground, stroke survivors have to (i) modulate the pace of the swing foot (either affected limb or opposite limb), requiring an appropriate motor control strategy that results in stable landing of the foot and (ii) counter for reduced proprioceptive signals from the stance foot (either affected limb or opposite limb). Thus, this complex SS critical phase may require more time for those after stroke due to neuromuscular deficits compared to healthy controls.