PRINCIPLE 1. Equitable Use
5.4 MECHANICAL CONCEPTS PERTINENT TO UNDERSTANDING GAIT
Pathological gait results in progress with adequate support; however, the means by which these are achieved are altered. Before the mechanics of gait can be reviewed, the mechanical concepts of loading, the kinematic chain and effi ciency are briefl y outlined.
5.4.1 LOADING
Given the nature of walking, the body is loaded with each ground contact. The over-all load on the body is refl ected in the ground reaction force (GRF), which indicates the force on the CoM. The direction and magnitude of this force can be represented as a vector (Figure 5.1) known as the ground reaction vector (GRV), which has its origin at the center of pressure (CoP).
As the joints and tissues are loaded deformation results. The extent of the defor-mation is dependent on the properties under load, the size and shape of the tissue,
environmental factors and specifi c to the force—its magnitude, direction, point of application, duration, and frequency. The resultant load on the CoM in able-bodied walking is not particularly high, reaching no more than a little over one body weight in the loading and propulsion phases—up to nine times body weight have been recorded in some jumping activities. As such, the load on the joints and tissues is well within their mechanical limits and, under normal circumstances, tolerance is easily achieved. This may not be the case under extreme conditions.
For instance, stress fractures are known to occur in the military as a result of walk-ing long distances, carrywalk-ing heavy loads and/or repeated marchwalk-ing. Army recruits have shown that the knee effects substantial compensations during backpack loaded marching. These high loads have been associated with shock attenuation or the reduction of excessive loads elsewhere. Interestingly, after marching for 40 min, the altered knee mechanics were not sustained, suggesting the fatigue may occur and the compensations could not be sustained [10]. Carrying a load on the back also has an effect on normal walking gait. Adolescent girls walked more slowly and with decreased pelvic motion with increased load. Hip fl exion and extension increased as did the joint moments and power with increased mass [11]. Research indicates that backpack loading be limited to 10% body weight to avoid excessive loading on the musculoskeletal system. Lateral bending of the trunk to counteract the asymmetric placement of a load (e.g., a shoulder strap over one shoulder) has been suggested as a risk factor for a number of low-back disorders. Asymmetric loading has been FIGURE 5.1 Skeleton at initial contact. The GRV is represented by an arrow. The GRV originates at the CoP and indicates the magnitude and direction of the overall force act-ing on the body. The force vector can cause, or tend to cause, fl exion or extension at a joint depending on where it passes relative to the joint. An internal moment, caused by muscular contraction, will facilitate or counteract the tendency toward fl exion or extension caused by the external force.
shown to affect posture, with increased forward lean through thoracic adjustments and lateral bending through lumbar adjustments [12]. High heels distort the biome-chanics of the foot and lower limbs to redirect the forces in unusual directions and may result in injury. Ankle plantarfl exion, knee fl exion, the timing of subtalar and knee joint action and the vertical and braking forces have been shown to be affected by increased heel height. Poor alignment may result in other complications. The resultant abnormal direction of the loading may be implicated in the development of secondary injuries or the inability to walk without aids, or not at all.
In pathological conditions, even though the loading may not be much higher than normal, the consequence of an altered magnitude (GRF) or direction of the load (GRV), or the anatomy experiencing the load may be more extreme. Stress fractures can be caused by the GRV with reference to the position of the body segments. For instance, if dealing with osteoporosis or osteoarthritis in the lower limb joints, the loading may result in joint pain or fractures. Postarthroplasty assistive devices such as canes are used in rehabilitation to reduce the force on the prosthetic device and on the incised muscles. The use of an assistive device such as a cane, held contralater-ally for a hip arthroplasty, can also help in preventing a lurching gait by reducing weight-bearing pain and by assisting the weakened hip abductor muscles.
5.4.2 KINEMATIC CHAIN
Fundamental to understanding the biomechanics of gait, and specifi cally the com-pensatory mechanisms if a pathology is introduced, is the mechanical concept of the linked segment. The mechanical fl exibility of the locomotor system is as a result of the fact that the human body is made of a system of levers, which do not act inde-pendently of one another. The linked segments form a kinematic chain, which can enhance the range of motion and the overall load-bearing capacity of the system.
In gait, when the limb is in stance, it is a closed chain as the distal segment (the foot) is fi xed. In this situation, movement at one joint has an effect on the more proxi-mal joints and this motion is relatively predictable. For instance, at initial contact, the foot strikes the fl oor and the kinematic chain becomes closed. If wearing high heels, the foot is plantarfl exed and to achieve foot fl at, the knee, hip or pelvis will have to compensate. In a novice high heel wearer, the compensation is typically achieved through increased knee fl exion similar to crouched gait or through trunk lean. In more experienced heel wearers, hip abduction and pelvic rotation are the compensatory mechanisms. Specifi cally the increased knee moment compensations have been indi-cated as relevant in the development and/or progression of knee osteoarthritis [13].
Pathologically, if there is limited dorsifl exion, at midstance, the knee and hip will compensate. The hip may fl ex, externally rotate and abduct, the knee may fl ex or even hyperextend to facilitate foot-fl at. In early hip osteoarthritis, reduced hip extension is associated with kinematic changes in the pelvis to maintain effective extension of the lower limb at push-off.
When the limb is in swing, it is an open chain and the joints have more freedom to move independent of each other, but can still compensate. For instance, with lim-ited dorsifl exion, toe clearance at midswing can become an issue, but the knee or hip can fl ex or the pelvis can hike to ensure that the person does not trip. These modifi ca-tions are detailed further in the description of walking below.
So, when one joint or a muscle/muscle group affecting a joint is not functioning correctly, compensations will be evident at adjacent joints in the kinematic chain, and depending on the extent of the problem at more than one joint. If trying to cor-rect an abnormal mechanism at one joint, it is important to remember the effect this manipulation will have on the remaining joints.
5.4.3 EFFICIENCY
Walking gait should be effi cient as walking usually needs to be sustained for a period of time. In many pathological conditions, if effi ciency is not feasible then walking will not occur and wheelchair locomotion will be preferred.
Mann [14] states that the act of walking is a result of the blending and compro-mising of physical and biological forces in order to achieve maximum effi ciency at minimum cost. For effi cient progress to occur, energy must be conserved so that the act of walking can continue over a prolonged period of time in steady state. To increase effi ciency, the body must (a) maximize the use of gravitational force, which it does through converting potential energy to kinetic energy, (b) minimize the excur-sion of the CoM and control momentum, and (c) minimize the use of muscles.
To minimize the excursion of the body’s CoM, it travels in a three-dimensional sinusoidal pathway. As shown, the CoM is farthest to the right and high at right mid-stance, central and low at double support and farthest left and high at left midstance.
It is useful to think of this pathway as a three-dimensional “~” with the highest and widest position at midstance and the lowest and most central position at double sup-port. If this movement of the CoM is deviated from the “~” through deformity, pain or impaired control, or if the excursion is jagged as a result of muscle weakness, then this is energy expensive and therefore undesirable.
Momentum is used in such a way that for natural walking speed the least energy per meter traveled is expended through the conservation of momentum. At slow and fast speeds the energy expended increases [15]. Walking slowly requires that momentum is removed from the system by eccentric muscle contractions to maintain the slow pace. At fast speeds, momentum, through energy, must be continually added to the system through larger concentric muscle contractions to maintain the pace.
Pathological gait is often slower as the mechanics required for effi cient momentum use are disrupted through muscle weakness and impaired control. Deformity and pain can result in the ineffective raising and lowering of the trunk and the subsequent requirement for additional momentum to be added to the system.
Minimizing the use of muscles is achieved by taking advantage of the human body’s ability to transfer energy passively from one segment to another. Winter [4]
describes two major energy-saving mechanisms used by the body. One uses the passive fl ow of energy across a joint, obvious in the terminal swing phase of walk-ing when the swwalk-ingwalk-ing foot and leg transfer energy through the thigh to the trunk.
The trunk then conserves the potential energy and converts it to kinetic energy to accelerate the head, arms, and trunk (HAT) in the forward direction. During terminal stance, the power generated by the plantarfl exor muscles also transfers through the knee to the thigh to help lift the stance limb into swing. In pathological gait, if the limb does not have suffi cient energy, then the reduced energy fl ow will have to be supplied through active contraction, or the gait will be slower. In stroke
or aged gait, the energy in the swinging limb is reduced due to reduced hip fl exor and ankle plantarfl exor activity. As a result the energy available to be transferred to the trunk is reduced and slower gait follows. The other mechanism involves the active energy transfer across muscles when the adjacent joints are rotating in the same direction. A further mechanism, which reduces energy expenditure, is to take advantage of the stretch-shortening cycle. Here, the muscles function such that they tend to stretch in gait and the “stretch energy” is returned without the need for active contraction. As detailed above, Fonseca et al. [7] illustrated that the altered mechanism evident in CP gait takes advantage of the stiff plantarfl exors to store more stretch energy to compensate for the reduced capacity to actively contract these muscles, thereby taking advantage of this energy-saving mechanism to achieve progression.
It is through understanding the process by which we walk and the loading pat-tern of the movement that we can understand the adaptations and consequences of pathological walking. When these compensatory mechanisms are understood, it is generally easier to understand the role and development of devices or rehabilitation to functionally restore capacity.
5.5 FUNCTIONAL RESTORATION: DEVICES AND TRAINING