It is generally thought that there are 4 main phases to the gait cycle when running:

Inital strike, stance, take off and forward swing.

Breaking this down into 2 separate sections, we can firstly look at the strike and stance phases. As the foot makes contact with the ground and rectus femoris (one of the quadriceps group of thigh muscles) proactively fires. As the foot touches the ground, the subtalar joint inverts or everts, the midfoot abducts or adducts and the forefoot plantarflexes or dorsiflexes. All these subtle movements combine to allow a small amount of pronation to occur, maximising the foots ability to dissipate shock.From this, we can see that a tight foot that underpronates limits this ability, which may lead to achillies problems, calf strains, lateral knee pain and illiotibial band issues. Conversely, overpronation can lead to tibial strain, anterior calf injuries and medial knee pain, due to the medial rotation occurring in the tibia. 

Swing phase happens at the same time on the opposite leg. The pelvis rotates forwards, with hip flexion initiated by the iliospoas. The Hamstrings start to lengthen, limiting the extension in the lower leg, caused by the quadriceps.The lower leg decends, hitting the surface as the body accelerates, ideally creating a vertical line between head and toe on impact.

During both phases, the core provides stability for the upper body, allowing the forces to be shared and transferred correctly. As the spine can be considered by some researchers a store and transfer for the energy contained within the running motion, core integration is important and if not present, an indicator and predisposition of other issues that need to be addressed.