What is walking is a question everyone needs to ask themselves because there is a correct or precise way to walk and I meet very few people who know that. Almost everyone takes walking for granted which amazes me considering that the way we walk has a profound effect on the way we feel and the way we age.
Walking correctly follows a specific pattern that is predicated on the upright body making use of gravity as if falls forward through space. When one foot is in front of the other there is a moment when the upper body is directly on top of that forward leg and the pelvis. In the next moment the upper body begins to move past that front leg and the brain senses that the body is about to fall at which point it tells the psoas major of the back leg to pull it forward catching the body, becoming the front leg as it does, and the brain tells the back psoas major to pull the leg forward and the same thing happens on the other side over and over and over again.
When the back leg is furthest back (which is not far) the big gluteus maximus is working (a very little bit unless you are walking uphill) along with the hamstrings. The tensor fasica latae stabilizes the knee in the back plane of the body while the calf muscles and hip flexors are fully extended priming them for the contraction of the next step. If the leg moves straight forward the muscle work is minimized and if the foot falls successfully under the knee the arches of the feet can work correctly providing the spring in the step that propels us forward.
As your brain registers the need for the back leg to come forward the push off of the back foot helps the psoas major initiate the movement of the femur bone. The psoas major, due to its connection at the back of the inner thigh externally rotates the inner thigh and the inner knee out to the side slightly. This external rotation is balanced by a number of factors all resulting in the leg moving straight forward and the foot falling near parallel relatively in line with the ischial tuberosity or sit bone.
One of the inner thigh muscles (adductor magnus) attaches to the base of the pelvis as well as the pubic bone and this provides a check on the psoas externally rotating too much. The same side hip rotates forward through tone in the iliacus, and the internal oblique and the opposite external oblique. The gluteus medius and minimus work with the psoas to move the leg forward.
Tone in the trunk stabilizes the spine on top of the pelvis allowing for a balanced twist through all of the abdominal muscles as well as the muscles of the upper back and chest. A level head finishes the picture sitting directly on top of the pelvis. If you learn to walk correctly you can work out every muscle in your body with each step you take.