## 1 Introduction  
Studies have revealed that awkward posture, excessive exertion and repetitive work are the primary causes of musculoskeletal injuries [1]. This includes exposure of the static upper trunk of humans to deeper flexion postures or prolonged stooping in many workplaces [2; 3; 4]. Such postures are commonly observed in agriculture, manufacturing, mining and construction industries, which are more likely to employ males than females in Taiwan [5]. Although experts in occupational medicine or ergonomics have called for the use of squatting rather than a stooped posture, a field survey has shown that the stoop is considered to be one of the most effective postures for frontline workers [6; 7]. When considering the work experience, however, novices are more inclined to use the stooped working postures than the experienced workers, and this may cause higher injury risks [8; 9]. The mechanism of lower back loading is associated with the flexion-relaxation phenomenon (FRP) of the back muscle, which is generated when the human trunk is bent forward. [PERSON] and [PERSON] [10]were the first to propose the existence of FRP. The reason for the occurrence of FRP is that when the trunk is flexed deeply, the role of back muscles (especially lumbar erector spinae, LES) in balancing the torque generated by the trunk on the lumbar region shifts to the passive tissues of the adjacent lumbar spine. Because identical torque needs to be balanced with a shorter arm, it generates more force on the tissues. FRP generally leads to elongated and even damaged passive tissue of the lumbar spine [11], which may cause lower back injury [12; 13]. When FRP occurs as a result of trunk flexion, it may be a warning of lower back injury. To determine the occurrence of FRP, an absolute threshold using maximum voluntary contraction (MVC) from electromyography (EMG) is commonly adopted [2; 12]. A threshold of 5% integrated EMG (IEMG) is a typical threshold for MVC, which is then employed for all testing trials to identify the silences in EMG signals. [PERSON] et al. [14] developed an algorithm to identify the onset and cessation of FRP in lower back muscle. However, a generally accepted protocol is that when the trunk is flexed forward, the LES EMG value begins to decrease, and thus FRP occurs [15; 16; 17; 18]. When a person bends the trunk forward from a standing position, the LES and hamstring (HMS) are gradually stretched, which results in FRP. This may imply that individual flexibility in these muscle groups is closely related to FRP. It has been found that when performing identical deep trunk flexion, flexible people exhibited relatively delayed FRP compared with less flexible people [3; 19; 20]. [PERSON] et al. found that participants with high toe-touch test (TTT) scores had to flex their trunk and rotate their hip more than others, and then back muscle FRP was observed [21]. This may be because flexibility provides spinal stabilization in back muscle recruitment patterns and the central nervous system strategy. In a recent study, [PERSON] et al. found that participant's TTT scores significantly affected the FRP of back muscles. When the trunk was bent close to 90\({}^{\circ}\), flexible participants produced a greater ES contraction than less flexible participants. The authors speculated that this was because flexible people have a larger range of motion in the trunk, and thus lumbar lordosis could be maintained, resulting in a slighter degree of FRP [20]. Although the effect of flexibility on FRP has been preliminarily confirmed, flexibility is commonly examined using TTT. Different flexibility measurements, however, can be used to evaluate different stretches of body parts. Generally, TTT and sit-and-reach test (SRT) are primarily used to assess the stretch ability of the HMS muscle, although the two involve different postures. Therefore, although the correlation between them is high, they cannot completely substitute each other [22].