# Perturbations During Gait on a Split-Belt Treadmill: A Scoping Review
## 4 Discussion
### Terminology of Slip-Like and Trip-Like Perturbations  
[PERSON], et al. [62] explained that trips occur when the swinging leg collides with an external object, hindering its forward movement. This collision creates a destabilizing scenario because the CoM is ahead of the CoP and outside the base of support during single support [63]. According to [PERSON] [64], individuals are most vulnerable to trips during the MSw phase due to minimum toe clearance, increasing the risk of hitting obstacles. However, trips can also occur during the early swing phase and cause forward destabilization [62].  
Another event that can lead to a fall due to a loss of balance is a slip. [PERSON] [65] explained that slips happen when the coefficient of friction between the foot and the floor is too low, such as on wet or icy surfaces. Slips are usually classified based on the direction of foot movement, like forward or backward [66; 67]. A forward slip occurs when the backward horizontal frictional force is less than the forward horizontal reactive force at IC [66], causing the foot to slip forward and the CoM to fall backward [68]. Conversely, backward slips occur when the horizontal propulsive force at push-off exceeds the surface's coefficient of friction [67], causing the foot to slip backward and the CoM to fall forward [69].  
To simulate tripping and slipping, researchers use split-belt treadmills or manipulate ground surfaces [41; 66; 67]. Ground protocols use obstacles to induce trips or lubricants to create slippage, while treadmills offer a safe environment. On split-belt treadmills, trips are simulated by suddenly decelerating or momentarily stopping one belt during the IC phase, unlike natural trips that occur during the swing phase [70; 71]. In contrast, slips on treadmills are triggered by accelerating one belt forward or backward [32; 35; 71]. For a forward slip, the belt under the foot in the IC phase accelerates forward, causing the foot to slide rapidly forward. For a backward slip, the belt under the foot in the push-off phase decelerates or stops, causing the foot to slide backward. Therefore, it is crucial to precisely define the treadmill belt's actions and the gait cycle phase [4] rather than using general terms like slips or tripping. Additionally, differences between treadmill and over-ground perturbations can impact reactive stepping and balance responses. Understanding these variations is essential for comprehending fall etiology and improving intervention strategies.  
This paper takes a narrower approach, linking treadmill acceleration solely with forward slips [31; 32; 33; 34; 35; 36; 37; 45] and deceleration with trips [19; 38; 39]. Slip-like perturbations increased step width while decreasing step length and time. Participants exhibited increased muscle activation, particularly of the tibialis anterior, gastrocnemius, rectus femoris, biceps femoris, and ventral muscles. Slip-like perturbations also reduced the MoS, indicating decreased stability and necessitating gait adjustments.  
Trip-like perturbations caused a significant increase in step width, prolonged step time, and altered joint angles, especially at the knee and hip. These perturbations led to higher muscle activation and a greater MoS, indicating a higher risk of falls. Consequently, slip-like perturbations resulted in faster reactions and less destabilization, while trip-like perturbations were more destabilizing and required broader gait adjustments.