Insights Into Human Gait – Walking
Human gait, also known as ambulation or locomotion, are terms that refer to walking. At first glance this form of physical activity appears quite simple in nature. However, when studied in details, it reveals its true complexity. A large number of studies focusing on walking have been conducted, which showed that during waking different muscle and joints are involved, and forces are produced. Here walking is discussed in depth, and all the different phases, forces and motions are described in details.
Walking is the major achievement of human locomotor life. It takes approximately one year for humans to reach the first stage and about six years to reach full maturity. Bipedal motion (walking on two feet) frees the hands from locomotion and provides man with structures for manipulation, carrying and throwing of objects, protection and communication. Human locomotion is a typical example of complex movement of the body where efforts of the central nervous system (CNS), peripheral nervous system (PNS), and muscular-skeletal system are combined. The CNS and PNS coordinate the movements by sending signals to the correct skeletal muscle at the correct moment. After receiving a signal, muscle initiate movement by exerting force to various body segments. For locomotion the contribution of a large number of muscles of different size and shape is necessary. With a feedback system, consisting of numerous specialized sensors, the CNS evaluates the initiated movements. In healthy humans all these complex coordinated actions lead unconsciously to smooth movements.
The term "gait" is used to describe a particular manner or style of walking, and the term "normal gait" is used to present those parameters that have been generalized across sex, age, genetic predisposition and anthropometric variables. The effectiveness of walking is a matter of distributing the mechanical equilibrium of the body, pushing the body forward, and forming a successive new base by moving the legs forward alternately. A gait cycle is defined as the time between two successive occurrences, e.g. from a right weight acceptance period to the next right weight acceptance period. It has been well established that dynamic ambulation involves a stance phase (foot is in contact with the supporting surface), which comprises about 60% of the entire cycle, and a swing phase (foot is not in contact with the supporting surface), which comprises about 40% of the cyclic event.
The support phase includes step, and stride, where the step is the period of time between the heel strike (HS, the touches the ground) of one foot to HS of the opposite foot (controlateral). Whereas the stride is the period of time between HS of one foot and HS of the same foot (ipslateral). In the act of walking each leg alternates between a supporting and swinging phase.
The eight sub-phases of "normal" gait have been well described and include: initial contact (IC), loading response (LR), midstance (MSt), terminal stance (TSt), preswing (PS), initial swing (ISw), midswing (MSw) and terminal swing (TSw). However, without technological assistance, these eight sub-phases of gait cannot be reliably distinguished. Four divisions for functional gait assessment procedures are necessary to accumulate relevant information in the analysis of normal gait. These include: 1) Weight Acceptance (initial contact and loading response), 2) Stance (midstance and terminal stance), 3) Forward Progression (terminal stance and preswing) and 4) Swing (initial swing, midswing and terminal swing). Furthermore, in walking there is a period of double support, where both lower extremities are in contact with the supporting surface (12% of gait cycle), however, such double support period is not present during running.
The efficiency of the gait cycle essentially depends up on the ability of the neuromuscular system to produce coordinated and smooth movements by generating tensile force, and absorbing forces resulting from locomotion. The following section of this article will describe, and analyse both kinetics (branch of biomechanics that deals with action of forces) and kinematics (the branch of biomechanics that deals with motion) characteristics of gait. As soon as the heel strikes (HS) a ground reaction force (GRF), equal in magnitude and opposite direction is generated (1.5 BW). This cause a moment (tendency to produce motion) at different joints; on the ankle joint the GRF tends the produce a sudden plantar flexion. The tibialis anterior muscle contract eccentrically, generating an opposing force to GRF, and accepting, and lowering the body’s centre of gravity (COG) gradually. Simultaneously, the GRF causes a moment on the knee joint, which tends to flex, the quadriceps muscles contact eccentrically, accepting, and lowering slowly the COG.
In addition, during HS the GRF tends to cause a sudden and excessive tilting of the pelvis, at that instant the muscle gluteus medius (part of the buttocks) of the striking leg, and erector spinae and oblique muscles (opposite side) of the trunk, contract stabilising the pelvis. Furthermore, since the foot strikes the ground ahead of the body there is a forward component of the force in the thrust of the foot against the ground. This results in a backward counterpressure of the ground against the foot, which controls the forward momentum of the body.
During load response (LR) and mid stance (MS), the body weight (BW) is immediately spread over the foot, by “rolling” it across the foot, from heel to toe, thus, reducing the shock of impact by providing gradual force absorption. This is accomplished by the concentric contraction of tibialis anterior (muscle located in the lower leg), hip, and knee extensors, which propel the body’s COG forward. Additionally, as soon as the body’s COG has moved forward of the supporting foot, the thrust against the ground caused by the extension of the hip, knee, and ankle has a backward component and, therefore, the counterpressure of the ground pushes the body forward (Newton’s 3rd law).
Subsequently, during heel raise (HR) and toe off (TO) the gatrocnemius, and soleus (calf muscles) contract concentrically, causing initially the heel raise (HR), and then toe off (TO), the latter occur with the contribution of GRF push. Furthermore, as the heel raises, the backward component of force increase rapidly, reaching its maximum as the ball of the foot is leaving the ground, and becomes zero when the foot has cleared the ground. The vertical component of the force supports the body against the gravity force (GF), however, if such component is too large in proportion to GF and horizontal component, an ineffective bouncing walk will occur as a result. Additionally, during gait the arm swing is largely the product of inertia and GF acting on the weight of the arms. However, when the speed augments, voluntary swinging of the arms helps to carry the upper trunk forward. Any bending of the arms at the elbow shortens the lever, and thereby facilitating the swing acceleration. The speed of walking can be increased either by increasing stride length or cadence (steps per minute), or both together. However, usually this will result in an increased vertical displacement of the body’s centre of gravity (COG).
The swing phase starts as soon as the supporting leg has exerted its push against the ground. This begins with the flexion at the hip joint, which is followed by knee and ankle flexion. The flexion of all the leg joints shortens the lever so that the foot clears the ground, and also it facilitates the leg acceleration since the leg’s centre of gravity is closer to the axis of rotation (moment of inertia law). The swing motion is initiated by muscular contraction (flexion of hip), but both GF and momentum contribute in its continuance. The hip continues to flex as the knee begins to extend after the foot has passed under the body. As soon as the foot approaches the ground, the hip extensor muscles contract to decelerate the forward movement.
One of the most important aspects of ambulation is the COG displacement (which in normal walking is situated just in front the 2nd sacral vertebrae) as it is an important indicator of efficiency of motion, and energy sparing. In essence, the main purpose of an efficient gait cycle is to coordinate both kinetics and kinematics in order to maximise the forward COG displacement (sagittal plane) and minimise vertical, and lateral displacement, which would constitute a waste of energy. Furthermore, this would lead to a smoother motion, diminishing the ground reaction force (GRF), and the inherent stress that the body tissues have to absorb, hence reducing the likelihood off strain and injury. Most movement therefore takes place in the sagittal plane (plane of progression). However, some movement is required in the frontal planes for balance, and in the transverse plane to improve energy efficiency. According to Saunders (1953), 6 factors are important in determining efficiency during gait, and these are: pelvic rotation, pelvic list (also pelvic side bend or pelvic rotation in the frontal plane), knee flexion, foot and ankle mechanisms, and lateral displacement of the body. During double limb support, that time when the centre of gravity is at its lowest point, joint angles in both lower extremities are such that their effective lengths are maximized.
In addition, rotation of the pelvis causes the femur, fibula, and tibia (thigh and leg bones) to rotate about the long axis of the limb, and the magnitude of such rotational motion increases from pelvis to tibia. Furthermore, the pelvis is rotated forward on the side where the limb is in loading response, and backward on the limb, which is in preswing. This functional lengthening of the limbs minimizes the vertical drop in the centre of gravity. During midstance and midswing, when the centre of gravity is highest, the stance limb's hip, knee, and ankle are all flexed 50. In addition, the pelvis is tilted downward laterally toward the swing limb. This minimizes the centre of gravity's upward excursion, keeping it lower than if the individual were standing erect. Throughout the gait cycle pelvic rotation is matched by a counter rotation of the trunk and shoulder. While the pelvis rotates in one direction 7-11 degrees the shoulder girdle rotates in the opposite direction 7-11 degrees.
Human ambulation pattern is rich and complex interrelationships between physiology, physics, biomechanics and neuroscience. The harmonious interactions of these factors conduce to an enhancement of the efficiency of motion, energy sparing and injury prevention during human locomotion.