Fig. 1

Static-dynamic extension exercise at 40 % of 1RM. A, start position, this is the beginning of the concentric phase (and the end of the eccentric phase); B, the holding phase. Subjects were in prone, with a trunk flexion of 45°. The legs were strapped to the table and the hands of the subject were placed on the ipsilateral shoulders. They raised the upper body in 2 s, holded it for 5 s at the horizontal position and lowered the upper body to the start position in 2 s. The performance was adjusted by use of tactile feedback of the assessor and a metronome was used to ensure appropriate timing (60 beats/minute). Exercise volume and load were set at 10 repetitions of 40 % of the subjects’ personal one repetitions maximum (1RM) (Fig. 1). The 1RM was indirectly determined from the maximum amount of trunk extensions performed with the subjects own upper body weight, which was assessed at least 2 h before the EMG measurement in order to avoid muscle fatigue. The Holten diagram was used to calculate the exercise weight corresponding to the personal 40 % of 1RM. The exercise weight was lower compared to the subject’s trunk weight. Therefore a load-pulley system assisted in performing the trunk extensions by use of assisted weight [22]