Flexibility is the ability of the muscle to move in a single or a series of joint planes within an unrestricted range of motion. Muscle flexibility is an important element of muscle functionality and has a very important place in ensuring the biomechanical function of the muscle. One of the advantages of flexibility in the context of literature is its ability to enhance athletic performance, decrease the likelihood of injury, alleviate or minimize post-exercise discomfort, and improve coordination. Furthermore, it serves as a crucial aspect for injury prevention and rehabilitation. Limited flexibility causes the person to be prone to various musculoskeletal system overuse injuries and significantly affects functionality. Flexibility is different for each muscle. This difference depends on the adaptation of the muscle to the stresses placed on the muscle. Decreased flexibility is usually seen in fast-twitch muscles that span two joints. Hamstring muscles are a typical example of this. In normal daily activities, the physiological amplitudes of the hamstring muscles are rarely fully realized. For this reason, hamstring muscles are one of the muscle groups that tend to shorten. Hamstring shortness occurs in almost all populations worldwide, in both symptomatic and asymptomatic individuals. The causes of hamstring shortness include genetic factors, adaptation of the muscle to acute or chronic injury, sitting in a sitting position for a long time, etc. countable. Some studies have stated that decreased hamstring flexibility may cause patellar tendinopathy, patellofemoral pain, hamstring strain, and muscle damage after eccentric exercise.Therefore, the flexibility of the hamstring muscles must be maintained. Flexible hamstrings can prevent acute and chronic musculoskeletal injuries, low back pain, posture deviations, gait limitations and the risk of falls. Stretching exercises are often performed to increase hamstring flexibility . There are also different methods to increase hamstring flexibility. A widely used approach is manual therapy. According to Greenman, this method can strengthen weak muscles by restoring their normal length when they are shortened, contracted, or affected by spasticity. In addition, manual therapy can help reduce edema by promoting the lymphatic system's pumping action. This approach also enhances the mobility of joints with limited movement.

Another alternative method is massage application. Massage is believed to increase blood flow to the muscles and increase muscle temperature, thus improving performance. In addition to the effects mentioned above, massage is believed to reduce the risk of injury by increasing flexibility and reducing stiffness.

The rate of use of massage guns has increased significantly, especially in recent years, by athletes, coaches and physiotherapists. Perhaps the most important reasons why it is so popular are that it helps to increase sports performance and increase the level of flexibility. According to the literature, applying the sacroiliac joint muscle energy technique and percussion massage to the hamstring muscle has been shown to increase hamstring flexibility. However, there is no study available that compares the immediate effects of applying the sacroiliac joint muscle energy technique versus percussion massage on hamstring shortness. Therefore, the aim of our study is to compare the instant effects of sacroiliac joint muscle energy technique and percussion massage applications on hamstring flexibility.