According to various sources, you get an imbalance between the hip extensor muscles (gluteals and hamstrings) and the hip flexor muscles (iliopsoas, pectineus and rectus femoris of the quadriceps group) from running. According to these running specialists, weak gluteals and hamstrings do not allow you to “swing through” your stride. The hip flexors are then recruited to pick up the slack. They pull the pelvis down making the lower back arch excessively.
If you study these statements carefully they are difficult to understand. How can weak gluteals and hamstrings prevent you from “swinging through”? If anything, they should enhance it! Understand that these muscles are not involved in the pushoff as they remain relaxed and under slight eccentric/isometric tension.
Emg’s of the muscle activity show that the swing-through of the thigh is made possible by contraction of the hip flexors with concurrent relaxation of the gluteal and hamstring muscles. The hip extensor muscles contract eccentrically as the range of motion increases and develop sufficient tension at the end of the movement to stop the thigh when it is well in front of the body. It is at the end of the forward thigh drive that the glutes and hamstrings kick in to pull the leg down and back in the pawback movement.
The statement that the hip flexors pull the pelvis down (anterior rotation) is accurate, but only in a standing stable position, not during a dynamic movement. During the swing-through of the thigh, the pelvis remains in its normal anatomical position and even rotates backward (posterior rotation) as the swing leg thigh continues moving forward, not anterior as stated. Anterior rotation occurs during the push-off to allow the pushoff leg to move well behind the body. This is why we see an arch in the lower back during the push-off in high-level sprinters. This is a good sign. The pelvis rotates forward to allow for an effective push off to occur well behind the body. This is not opinion, it can be verified and seen on high-speed video tape of all world class and elite runners.
To compare the hamstrings to the quadriceps is tricky at best. The hamstrings can be considered powerful hip extensors but they are secondary to the gluteus maximus. The hamstrings are also powerful leg (knee joint) flexors. The quadriceps, however, are mainly knee joint extensors and only act secondarily as hip flexors. Thus when comparisons are made between these two muscle groups to determine muscle imbalances, it is important to equate them on opposite and equal actions (together with the other muscles involved).
It is common to hear people talk of a one-to-one ratio between these two muscle groups. However, the quadriceps is much bigger and more powerful than the hamstrings in their actions at the knee. Thus we should see approximately a 3 or 4:1 ratio rather than 1:1. In regard to the hip joint then it may be said that the hamstrings should be more powerful than the quadriceps but how much is unknown—at least according to the present information available. It may be trendy to talk about muscle imbalances but until we define exactly what we mean by an imbalance and what constitutes an imbalance, all discussions are moot.
Most important for runners should be strengthening of both of these muscle groups so that they can act effectively in running. They are the key muscles that make your run faster, more economical and help prevent many common running injuries.
Runners can also benefit greatly from a better understanding of the muscle actions involved in running based on effective running technique. Explosive Running is the main book that fully explores this topic. This knowledge is very important in order to create specialized exercises to strengthen the muscles in exactly the same movements that they execute during a run. This is a new concept that has been shown to improve running performance much more so than any other method.
For more information on this topic and running technique to improve running with running specific exercises see Explosive Running and Build a Better Athlete.