PRINCIPLE 1. Equitable Use
5.9 PHASES OF THE GAIT CYCLE
5.9.1 L OADING : 0%–10% C YCLE
Aims of this phase: Safe contact with the fl oor; ensure stable limb and absorb force/
shock; smooth transfer of body weight from trailing limb; active bearing of weight.
Overall, the role of the foot is to absorb impact and adjust to surface irregularities.
This is achieved by absorbing shock in the heel pad, eccentrically contracting the dor-sifl exors, and pronating the subtalar joint to make the foot compliant and fl exible [1].
The support of the trunk is primarily provided by the ankle dorsifl exors [33].
The role of the knee is to absorb shock by the eccentric contraction of the knee extensors, which also maintains weight-bearing stability at the knee [2]. The role of the hip, through an eccentric contraction of the hip abductors and the activation of the core muscles, is to maintain stability of the pelvis and to ensure that an erect posture of the trunk is preserved. A concentric contraction of the hip extensors (H1S) helps to pull the trunk over the supporting limb.
As load is transferred to the stance limb, compression begins and deforma-tion occurs. Under normal condideforma-tions, this deformadeforma-tion is well within the critical limits of the whole body and the individual joints and tissues. Weight is taken on the new stance limb, though at the early stage (0%–2%) in the cycle the muscles have not “taken control” of the movement and there can be a peak in the Fz, often known at the impact peak. It is a matter of debate as to whether this shock does any harm [3,39].
FIGURE 5.3 Photographs of normal gait indicating start and end of right limb stance phases.
The phase is the interval between these images. Loading is the period between image 1 and image 2, weight is borne, the foot is lowered to the fl oor and the contralateral limb is in propul-sion. Support/progression is the period between image 2 and image 3, the trunk passes over the supporting limb and the contralateral limb is in swing. Propulsion is the period between image 3 and image 4, the limb is propelled into swing and contralateral stance loading occurs.
Loading Support/progression Propulsion
In the sagittal plane, the GRV is directed behind the ankle and knee and in front of the hip (Figure 5.4). This causes an external plantarfl exor moment at the ankle and fl exor moment at the knee and at the hip. The ankle dorsifl exors contract eccentrically to control the lowering of the foot to foot-fl at and providing smooth transition to stance. The knee extensors activate to control knee fl exion allowing up to 20° of fl exion (K1S). This movement helps to control the downward acceleration of the CoM. A hip extensor moment helps to control hip fl exion and maintains 30°
of fl exion. Once the hip is stabilized, it moves from fl exion to extension through a concentric contraction of the hip extensors (H1S) (Figure 5.4).
In the frontal plane, the GRV is directed medially and a strong adduction moment is exerted at the hip and knee that follows the rapid transfer of body weight onto the limb (Figure 5.5). The contralateral pelvis drops as the limb is being unloaded is limited to 5° by this strong hip abductor moment, which generates power through a concentric contraction (H1F). The external adductor moment that acts on the knee is largely controlled by the iliotibial band. The vector causes subtalar valgus, which is restrained by the tibialis anterior and posterior.
In the transverse plane, the subtalar valgus causes internal rotation at the talus and the accompanying rotation of the tibia results in an internal torque at the knee.
The external rotators of the hip prevent the femur from internally rotating and this prevents excessive internal rotation at the knee. The limited internal rotation, which occurs at the hip, assists advancement of the contralateral limb in progression phase.
5.9.2 EFFECTSOF JOINT PATHOLOGIES
Due to the kinematic chain, if a pathology exists at one joint, it can effect other joints in the chain and compensations can also occur at different points in the chain.
FIGURE 5.4 External GRV causes an external plantarfl exor, knee fl exor, and hip fl exor moment. This is counteracted by an internal dorsifl exor, which controls the lowering of the foot to the fl oor, and a knee extensor and hip extensor moment, which stops the limb from collapsing as it is loaded.
A common pathology evidenced in this phase is that initial contact is made with the forefoot. Pathologies at the foot, knee, and hip can cause and effect this condi-tion. With the foot in equinus forefoot initial contact occurs and the GRV passes anterior rather than posterior to the ankle. Due to the location of the CoP a large external dorsifl exor moment occurs. The cause of the equinus could be an ankle plantarfl exor contracture. Poor alignment of a prosthesis can also result in a similar plantarfl exor position. To resolve the forefoot strike, if the person tries to contact in footfl at then the compensation may be knee hyperextension and possibly increased foot (if present) pronation. Pseudoequinus can occur if the knee is excessively fl exed at initial contact. Here, the ankle is normal, but the fl exed knee enforces forefoot contact. Higher up the chain, if there are fl exor hip contractures present, the impli-cations for this phase of the gait cycle mean that the pelvis tilts anteriorly and the knee can be excessively fl exed to ensure the heel contact the ground. This requires increased quadricep activity and results in crouched gait. For all these conditions, the initial contact is poorly aligned to absorb the subsequent load, which cannot be easily accommodated by the eccentric contraction at the hip and knee. Further, H1S is inhibited and so raising the trunk to the high point of midstance is diffi cult. As a result, anterior trunk lean can compensate to move the CoM forward. Reduced step length is evident with hip and knee fl exor contractures.
At the ankle, loss of dorsifl exor function results in uncontrolled plantarfl ex-ion and footslap occurs. If the knee at the end of the previous swing phase cannot absorb the momentum of the swinging limb prior to initial contact, then the force FIGURE 5.5 External GRV passes medially and the internal abductor hip moment, caused by an eccentric contraction of the muscles, controls the drop of the pelvis.
experienced at initial contact may be high, and the orientation of the limb may cause higher loads. If the knee extensors are weak, excessive knee fl exion can be seen, though more often the knee will remain fully extended throughout early stance and the gait will be abrupt in this phase.
In pathologies where the ankle and knee mechanisms do not work effectively, an excessive vertical displacement of the CoM can result, which is energy expensive.
This is often seen in TT amputees and those with a peronial nerve disorder.
If the knees are in valgus or varus, the GRV may pass inside or outside the knee joint and if the knee abductor or adductor muscles are insuffi ciently strong to balance the load then the knee may collapse. A leg length discrepancy results in an asym-metrical pelvis and asymasym-metrical loading on the limbs.
Wearing high heels results in a larger plantarfl exor moment as a result of the increased moment arm. The pretibial muscles have to generate a larger eccentric contraction to control the lowering of the foot to foot fl at.