Muscle activity pattern
CONTRACTION
Surface electromyography (Biometrics Ltd, Nine Mile Point Ind. Est. UK) was bilaterally recorded during static and dynamic balance tests, using Biodex Balance System, in terms of SENIAM recommendations for rectus abdominus (2 cm lateral to the umbilicus), erector spine (2 fingers width lateral to the L3 spinous processes), rectus femoris (at 50% on the line from the anterior superior iliac spine to the superior part of the patella), biceps femoris (at 50% on the line between the ischial tuberosity and the lateral epicondyle of the tibia), tibialis anterior (one third of the distance between the tip of the fibula and the tip of the medial malleolus), and medial gastrocnemius (on the most prominent bulge of the muscle) [The myoelectric signals were recorded with a 16-channel electromyography device at the sampling rate of 1000 HZ and then processed with 50 HZ notch filter and 20 to 450 Hz band pass filter. Filtration was applied post-recording using MATLAB® software (the MathWorks, Inc., Ver. 2015a, USA). The ground electrode was placed over the lateral malleolus using a conductive gel. Pairs of Ag/AgCl bipolar surface electrodes were fixed along the long axis of the muscles with 2 cm center-to-center distance [Raw electromyography (EMG) data were full-wave rectified, and then the root mean square was calculated for each trial. The onset time was considered while the values, in millivolts, exceeded the mean level of the baseline activity by two standard deviations for a minimum of 30 ms. To normalize the EMG data, the root mean square of reference voluntary contraction was determined using the following methods: the middle three seconds of trunk reference voluntary contraction, and the five seconds of lower extremity reference voluntary contraction with no resistance: 5-sec supine isometric trunk curl-up, 5-sec prone isometric trunk extension for rectus abdominus and erector spine respectively, and 10-sec upright standing posture for rectus femoris, biceps femoris, tibialis anterior, and medial gastrocnemius. Data were averaged over three trials and then reported as reference voluntary contraction % for both groups.