The results of the present study indicate that fatigue of the hip extensor and ES muscles modifies lumbopelvic rhythm and, consequently, FRP parameters. In healthy participants, fatigue of these muscles led to reduced hip flexion angle (increased L/H ratio) and decreased FRP onset angle. However, fatigue did not modify EMG activity during the flexion-extension cycle. Previous data [
8] suggested that ES muscles, in a state of fatigue, were unable to provide sufficient stabilization to the vertebral units, transferring load-sharing to passive structures earlier in trunk flexion. Alternatively, the present findings indicate that lumbar-stabilizing mechanisms, when hip extensor (gluteus or hamstring muscles) and ES muscles are fatigued, may remain unchanged. An apparent earlier onset of myoelectric silence in total trunk flexion angle seems to be caused by a change in lumbopelvic dynamics. Lumbopelvic dynamics during a flexion-extension cycle have been studied in the past [
11,
13,
17], and it has been shown several times that the first half of the flexion phase occurs primarily at lumbar spine segments with the pelvis remaining relatively fixed, whereas the final half of flexion is accomplished primarily by forward pelvic rotation. A reverse mechanism has also been described during extension from a flexed posture where initial movement is achieved primarily at the hip level with an increasing contribution from the lumbar spine in the later stages of extension [
18]. In the present experiment, it is possible that fatigue probably augmented stiffness in the hip extensor muscles. Heightened passive tension in the hamstring muscles has been suggested to limit pelvic movement during flexion and to facilitate it in early extension [
18]. Hashemirad et al. [
19] recently reported that subjects presenting less general flexibility (toe-touch test) showed decreased FRP onset and cessation angles. They suggested that when sufficient passive tension values are reached (monitored by spine mechanoreceptors), the central nervous system deactivates the active controlling element of movement (ES). Alternatively, recent studies showed that immediately following static and cyclic loading of the spine, laxity can develop in the associated viscoelastic structures without neuromuscular compensation mechanisms [
20,
21]. In the present experiment, such responses to loading (fatigue task) may have occurred and may have led to increased displacement and tension neutral zone and subsequent changes in lumbopelvic dynamics. Whether the modulating effect of hip and lumbar extensor muscle fatigue derives from changes in soft tissue mechanical properties or from alterations in neuromuscular strategies remains to be determined. Nevertheless, although it has often been suggested that hip extensor muscles can generate tension in passive lumbopelvic structures, such as the sacrotuberous ligament and the thoracolumbar fascia [
22], the results of the present study indicate that hip and lumbar extensor muscle fatigue may challenge spinal stability requirements by changing lumbopelvic dynamics. Repeated trunk flexion and extension as well as lifting tasks have been previously targeted as potential causes of work related low back pain [
21]. Therefore, repeated trunk movement or sustained static posture leading to muscle fatigue of back or hip extensor muscles may alter usual spinal loading and stability mechanisms therefore putting, the lumbar spine at risk of injury or reinjury.
Finally, the addition of a load anterior to the trunk modified the FRP response. FRP onset and cessations angles were increased in loading conditions. Several authors have reported a similar effect of load positioned either anteriorly or posteriorly to the trunk [
2,
8,
23]. Such a decrease in the EMG silence period during flexion reflects the need for additional muscular contraction to counteract the increased flexion moment generated by the load, but can also be explained by the increment of total flexion angle during loading conditions. This augmentation of FRP onset and cessation angles was accompanied by heightened ES muscle activity through all phases of the flexion/extension cycle. Increased loading of the spine also led to a greater contribution of the hip in both flexion and extension movements, again illustrating a change in lumbopelvic dynamics when stability requirements are modified.