# Criterion-Related Validity and Reliability of the Front Plank Test in Adults: The ADULT-FIT Project
## 2 Materials and Methods
### Procedures
#### 2.3.5 Electromyography  
To quantify muscle activity, sEMG data were acquired using the Trigno Wireless System (Delsys, Natick, MA, USA) and the Trigno Avanti Sensor (Delsys, Natick, MA, USA), with a sample rate of 1950 Hz for the EMG signal and 148 Hz for the accelerometer. Before sensor application, the skin was carefully shaved, abraded, and cleansed with alcohol. Following the surface EMG for the Non-Invasive Assessment of Muscles (SENIAM) recommendations, electrodes were placed on the upper (2 cm above the umbilicus) and lower (2 cm below the umbilicus) regions of the rectus abdominis (URA and LRA, respectively); the external oblique (EO) (lateral to the rectus abdominis and directly above the anterosu-perior iliac spine, midway between the ridge and the ribs at a slightly oblique angle); the multifidus (MF) (aligned with a line from the caudal tip of the posterior superior iliac spine to the space between L1 and L2 at the level of the L5 spinous process, i.e., about 2-3 cm from the midline); and the longissimus subdivision of the erector spinae (ES) (two fingers width lateral to L1) on the dominant side [26]. Notably, for sensor placement in muscles not specified in the SENIAM protocol, the locations recommended in the study by [PERSON] et al. (2019) were followed [35].  
The sEMG signals from each electrode were amplified with an input impedance of 120 k\(\Omega\), a signal-to-noise ratio of 750, and an inter-electrode distance of 40 mm. The gain range was set between 500 to 5000. Surface electrodes were connected by WI-FI to the  
Figure 1: Set up of the front plank test.  
Trigno Base Station and streamed continuously to a computer through an analogue-to-digital converter (G-42, HP notebook computer, Palo Alto, CA, USA). The sEMG data were managed using EMGworks(r) software (Delsys 4.1.7, Natick, MA, USA). All data were centered and filtered with a second-order IIR Butterworth filter with a 10 Hz high-pass and a 500 Hz low-pass.  
To assess the sEMG recorded during testing, the root mean square (RMS) was utilized. A 60 ms moving window was used to calculate the data, and the onset and end of muscle contraction were identified through data analysis. The RMS was evaluated following previous recommendations [36].  
\[RMS=\sqrt{\frac{1}{n}}\underset{n}{\sum}x_{n}^{2}\]  
where \(\mathrm{Xn}\) is the value of the sEMG signal and \(\mathrm{n}\) is the sample number. During muscle contraction, sEMG activity was recorded and coupled with an accelerometer to detect the onset and end of the contraction. To achieve this, a sensor was placed on the knee, and behavior analysis of the acceleration signal was conducted in a pilot session before testing sessions. A distinct initial peak was observed at the beginning of the test, corresponding to the start of muscle contraction. The onset was visually identified as an increase in the amplitude of the acceleration signal above the baseline. Another peak was observed at the end of the test, indicating the end of contraction. Each RMS sEMG data were expressed as a percentage of the MVIC [37]. The reliability of these measures was previously established [36].  
To perform MVIC, a partial curl was executed for the RA (Figure 2A), a lateral plank for the EO (Figure 2B), and trunk extension for the MF and ES muscles (Figure 2C). During these exercises, an assessor applied counter-resistance to achieve the maximum voluntary contraction of the muscles involved [38; 39].