One of the reasons for muscular imbalances is related to the fact that muscles are divided into 2 categories called Tonic and Phasic muscles. Tonic muscles tend to shorten, this is secondary to overuse, underuse, or trauma. Conversely, Phasic muscles tend to lengthen, weaken and easily become inhibited. A muscle that has become inhibited is not able to perform the function it is designed for. This leads to compensation, soft tissue strain and joint instability. In the hip/pelvic region the hamstring, iliopsoas, and TFL muscles are Tonic muscles, while Gluteal and Rectus Abdominus muscles are primarily Phasic muscles. Therefore, the Hamstring muscles will shorten and tighten when muscular imbalances affect the pelvic girdle. The malalignment occurs during periods of muscular overuse, underuse, and trauma. Extended periods of sitting will cause hip muscles to be underused and therefore shorten or lengthen depending on the type of muscle they are.
It is easy to see when someone has muscular imbalances in the hip region as one hip will be elevated compared to the other, or looking from the side the hips will be significantly in front of the heels. The Central Nervous System assesses the muscular imbalances and in the case of the hamstring muscles sends signals that tell them to shorten and tighten. The person feels the tension and attempts to stretch the hamstring muscles, but can never successfully return them to ideal length. The way to successfully reduce hamstring tension is to improve the pelvic stability and alignment. As mentioned, one common example occurs when the hips are in front of the heels. In this case the hips need to be retrained to stabilize above the heels which is called frontal plane alignment or stability. The way to do this is to have the hips perform hip flexion exercises such as a squat, counter stretch, static extension, or downward dog. In doing this the Gluteal and Abdominal or Phasic muscles will be strengthened in the process, and the tension within the Tonic muscles will return to normal.
As the balance of the hip and pelvic muscles improves and the hip alignment returns to neutral the tension will reduce in the hamstring muscles.