## 2 Materials and Methods  
The participant groups consisted of a young adult group (YG) and an elderly group (EG). Both participants had not experienced medical issues in their lower extremity musculoskeletal function in the last year and were able to walk for more than 10 min. The YG group was comprised of 15 male college students in their 20s (24.7 \(\pm\) 0.8 years; 173.5 \(\pm\) 4.2 cm; 76.2 \(\pm\) 9.8 kg) and the EG group was made up of 14 healthy male seniors (78.4 \(\pm\) 5.5 years; 162.4 \(\pm\) 6.8 cm; 65.3 \(\pm\) 8.9 kg), where community-dwelling senior individuals recruited from the local senior citizens' center at Chungju city in South Korea participated in this study. They were determined as subjects of analysis considering their medical history (i.e., presence of cardiovascular problems, neurological disorders of the vestibular or cerebellar systems, history of lower-limb or spinal surgery in the previous 12 months), problems in communication and cognition indicated by a Mini-Mental State Examination-Korean (MMSE-K) version score >24, and no requirement to use any type of assistive walking device such as cane, crutch, or walker.  
The protocol of this study was approved by the Ethics Committee of Konkuk University. The purpose and methodology of the study were fully explained to all participants, who were enrolled in the study after they gave their informed consent.  
All experiments were conducted on a self-paced treadmill, where the belt speed was controlled by the participants [16]. The gait experiment, which was repeated three times, involved walking with the metronome (MET) and walking with the metronome while counting backwards (MET + BC). The experiment sequence was randomized, and the participants were required to match the metronome beat with their heel contact while walking. Backward counting was performed by consecutively subtracting three from the number 100 (e.g., 100, 97, 94, 91, and so on) [17]. Before each experiment, all participants had an adaptation period of 3 to 5 min on the treadmill. After the adaptation, the preferred walking speed of each participant was estimated from the average walking speed for one minute at the beginning, and the metronome beats per minute (BPM) for each individual was determined accordingly. Using this BPM, gait experiments under MET and MET + BC conditions were performed.  
For both gait experiment conditions, a system implemented with an inertial sensor and LabVIEW programming was used to measure the temporal error between the metronome beat and the heel contact [15]. The system was configured to present the metronome rhythm synchronized to the participant's preferred walking speed, and measure the temporal error of the heel contact from presentation of the metronome rhythm. The metronome rhythm transmitted through speakers was implemented using LabVIEW software (National Instruments Corp., Austin, TX, USA). To record the heel contact events, a Delsys Trigno inertial sensor was attached to the participant's instep, and the data were stored using the Delsys SDK (software development kit). For convenience, the heel contact before and after the metronome beat was expressed as a negative (\(-\)) value and positive (\(+\)) value, respectively, and the absolute value was used when calculating the mean of the temporal error. The coefficient of variance (CV = standard variation/mean \(\times\) 100) reflecting the variation (or distribution) in the mean value was used to compare the temporal errors.  
The repeated measures ANOVA was used to determine differences in temporal error according to the gait conditions by age groups. SPSS software ver. 25 (IBM SPSS Statics Inc., Chicago, IL, USA) was used for statistical processing, and a significance level of 0.05 was applied.