Fatigue can be defined as, decrease in the power generation capacity of the muscles from the beginning of the movement due to internal homeostasis breakdown caused by an increase in energy production demanded by an external stimulus that lead to general decrease in physical performance related to a rise within the real/perceived difficulty of a task or exercise, as well as the inability of the muscles to keep up with the specified level of strength during exercises. Either Adolescents spending long time using smartphone, they are more likely to have their spine in an incorrect posture causing damage to the surrounding skeletal structures as well as to ligaments which affect the proprioception input of the spine. Muscle fatigue is an important condition that impairs sensorimotor integration, proprioception, and thus postural control. Muscle fibers and Golgi tendon organ activity decrease with fatigue, resulting in changes in joint proprioception and afferent output from joint and muscle receptors. Fatigue of postural muscles such as lumbar extensor and abdominal muscles impairs postural control and may form the basis for injuries. Fatigue can also impair the proprioceptive and kinesthetic properties of the joints. this study aims to investigate how the trunk extensor muscles respond to a fatigue inducing exercise, especially concerning balance and trunk lumbar position sense for understanding and preventing musculoskeletal injury and lowering risk in health adolescents in primary care and to contribute to preventive rehabilitation. Sixty-eight normal adolescent boys will participate in this study
