## 2 Materials and Methods
### Data Collection  
A Noraxon TeleMyo wireless EMG System (Noraxon, Scottsdale, AZ, USA) was used to detect muscle activity signals. Noraxon adhesive pre-gelled Ag/AgCl surface EMG electrodes (40 \(\times\) 21 mm, inter-electrode distance: 20 mm) were bilaterally placed on the tibialis anterior (TA), medial gastrocnemius (mGas), rectus femoris (RF), biceps femoris  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Characteristics** & **Older Adults (_n_ = 13)** & **Younger Adults (_n_ = 13)** & _p-Value_ \\ \hline Anthropometric & & & \\ Age (years) & 77 \(\pm\) 7 & 22 \(\pm\) 3 & \\ Sex (female/male) & 6/7 & 6/7 & \\ Height (cm) & 170.8 \(\pm\) 5.32 & 173.0 \(\pm\) 7.77 & 0.75 \\ Weight (kg) & 74.33 \(\pm\) 11.21 & 73.74 \(\pm\) 16.55 & 0.74 \\ BMI (kg/m\({}^{2}\)) & 23.02 \(\pm\) 0.46 & 22.33 \(\pm\) 3.02 & 0.19 \\ Gait Speed (cm/s) & 111.23 \(\pm\) 21.13 & 122.14 \(\pm\) 14.30 & 0.25 \\ Gait Initiation step length (cm) & 51.33 \(\pm\) 5.33 & 55.90 \(\pm\) 5.40 & 0.10 \\ Step length (cm) & 59.24 \(\pm\) 5.96 & 69.32 \(\pm\) 4.10 & 0.03 * \\ Single stance duration (s) & 0.45 \(\pm\) 0.12 & 0.44 \(\pm\) 0.02 & 0.49 \\ H–H Base of Support (cm) & 10.29 \(\pm\) 3.41 & 11.89 \(\pm\) 3.16 & 0.15 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Anthropometrics and spatiotemporal gait characteristics of study participants across age groups. Values are presented as mean \(\pm\) standard deviation. Single-stance duration is equal to the swing time of the opposite foot. H–H (heel-to-heel) base of support is “base width”, which is the vertical distance from the heel center of one footprint to the line of progression formed by two footprints of the opposite foot. * represents a significant difference between the two groups.  
(BF), gluteus medius (Gmed), and erect spinae (ES). The electrode positioning followed the guideline provided by SENIAM [22]. Thirty-nine reflective markers were placed on the body in accordance with the standard Plug-In-Gait full-body model (Vicon Nexsus 2.12, VICON Motion Systems Ltd., Yarrton, UK). A 10-camera motion capture system (VICON Motion Systems Ltd., UK) was used to record body kinematics.  
The surface drop perturbation was applied at heel strike during gait initiation to standardize the timing of the perturbation and normalize kinematic responses to the same phase of the gait cycle. Prior to the start of testing, spatiotemporal gait data (usual walking speed, step length) during unperturbed gait were collected from three trials using the GaitRite mat (Protokinetics, Havertown, PA, USA). Step length was used to determine the starting position for each participant, ensuring that the leading leg's heel strike occurred at the center location on the unexpected surface drop platform. As our focus was on capturing participants' initial naive response to the ground walking balance challenge, only the first exposure to the surface drop balance perturbation was measured.  
During testing, participants were fitted with a safety harness that was connected to an overhead pulley system with minimal resistance. The length of the rope was adjusted to match each participant's height, allowing them to walk freely. Participants were instructed to walk at their normal pace along a 30\({}^{\prime}\) (9.14 m) walkway (U.S. Provisional Pat. Ser. No. 62/949,184). The walkway was modular, and the first 3\({}^{\prime}\) module was located before the surface drop module. Spring hinges held the drop panel (44\"W \(\times\) 24\"H) along the boardwalk in place until activated via heel strike of the leading limb (Figure 1). The instructions provided to the participants were as follows: \"Walk to the end of the walkway at your normal walking pace. Your balance may or may not be challenged. If your balance is disturbed, react naturally and continue walking to the end of the walkway\".