Muscle activity is typically studied using electromyography (EMG). EMG records differ between individuals, and differ for a single individual according to variables such as velocity. The following summary draws on the findings of reliable investigators.
Loading Response (0 to 12 percent of gait cycle)
This is a period of extensive muscle activity. The ankle dorsiflexors act eccentrically to prevent slapping of the foot on the ground. The quadriceps act eccentrically to control knee flexion. Hip flexion is controlled by isometric action of the hamstrings (primarily biceps femoris) and gluteus maximus (primarily its lower portion).
In the frontal plane, activity in the hip abductors, tensor fascia lata, and upper portions of the gluteus maximus control drop of the contralateral pelvis, which is relative hip adduction. While activity in the the anterior gluteals (gluteus medius and minimus) might appear eccentric, these muscles simultaneously move the hip joint into internal rotation. In a closed chain, this hip rotation causes the pelvis to rotate forward on the opposite side. Thus, gluteus medius activity may be nearly isometric. Also contributing to both internal rotation and extension of the hip joint are the muscles of the adductor group.
The erector spinae are also active during loading response. Their activity during this period has been characterized classically as a mechanism to stabilize the trunk during weight transfer, and to prevent its forward flexion during the rapid slowing of forward movement which occurs at initial contact. Recent theory (Gracovetsky 1988) attributes to the paraspinal muscles a more active role in producing important trunk and pelvic rotation.
Midstance (12 to 31 percent of gait cycle)
The quadriceps act concentrically to initiate knee extension, and the hip abductors continue their activity, becoming isometric as they halt contralateral pelvic drop.
Terminal Stance (31 to 50 percent of gait cycle)
Similarly, the hip abductors move from eccentric to isometric to concentric activity, elevating the pelvis in preparation for swing. The iliopsoas becomes active, eccentrically controlling the rate of hip extension.
The quadriceps are inactive during this phase, as ground reaction forces, as well as activity in the plantar flexors, maintain knee extension.
Preswing (50 to 62 percent of gait cycle)
At typical to faster walking speeds, the rectus femoris also acts in a nearly isometric fashion, to limit knee flexion and augment hip flexion. Only at slower walking speeds, when ground reaction and joint reaction forces are too small to initiate knee flexion, must knee flexors like the short head of the biceps femoris, or the gracilis, actually work to flex the knee directly.
The erector spinae are active on the preswing side, and produce greater EMG activity than during their previous period of activity during loading response; a vital debate concerns whether this assymetrical activity functions simply to control unwanted trunk movement or if it helps initiate forward pelvic rotation, through the mechanism of coupled motion, and thereby helps drive the extremity into swing.
During this very brief phase, the hip flexors and knee extensors (primarily rectus femoris) continue their preswing activity. The dorsiflexors act concentrically to permit the forefoot to clear the ground. While their activity varies widely among individuals, the hip adductors can also assist during preswing and initial swing to assist in hip flexion.
Muscle activity virtually ceases except for the dorsiflexors as the extremity's inertia carries it through swing like a pendulum.
The hamstrings (primarily the medial group) act eccentrically to decelerate the swinging extremity, while the dorsiflexors hold the ankle in position for initial contact. Just before the foot touches the ground, the quadriceps and the hip abductors initiate activity, disclosing the existence of a feedforward mechanism by which the body prepares for the large ground reaction its joints will encounter at initial contact.
Inman, V.T., Ralston, H.J., & Todd. F. (1981). Human Walking. Baltimore: Williams and Wilkins.
Rodgers, M.M. (1995). Dynamic foot biomechanics. Journal of Orthopedic and Sports Physical Therapy, 21, 306-316.
Winter, D.A. (1987). The Biomechanics and Motor Control of Human Gait. Waterloo, Ontario: Univ. of Waterloo Press.