NORMAL GAIT IS CYCLICAL AND SYMMETRIC

The principal goal of locomotion is to propel the body forward as efficiently as possible. The most natural way to accomplish this task is to employ a bipedal gait pattern, where the base of support alternates from one leg to the other.

Stance period includes two intervals of double limb support at the stance/swing transitions, each lasting approximately 10% to 12% of the gait cycle at typical walking speeds. These are generally described as initial and final double support, but can also be identified in the context of the leading limb as right or left double limb stance period. The duration of the double limb support periods decrease with increasing walking speed, reaching zero at the moment running begins. The time interval between the initial and dou­ble support periods is defined as the single support period, and is the same duration as the swing period of the opposite limb. Assuming normal symmetry, any reduction in double limb support time is absorbed by a proportional increase in single limb support time, but since single limb support always corresponds to the contralateral swing period, the overall stance period decreases, reaching 50% at the initiation of running when double limb support reaches zero. When both limbs' primary temporal events of foot-strike (initial contact) and foot-off (terminal contact) are repre­sented on the same time scale, the duration of each time period is easily illustrated. These general terms for temporal events are applicable, regardless of gait pathology. Other terms are routinely used to identify temporal events marking the transition from swing period to stance period (heel strike, forefoot initial con­tact, foot flat) and stance to swing (toe-off, push-off), but should only be used when they clearly describe the observed foot/floor contact pattern.

While period durations relative to a single side are easily described when the gait cycle is represented on a linear scale, left/right symmetry may be more easily conceptualized when the gait cycle is wrapped around a unit circle (2,4,5), as shown in Figure 16.2.

Figure 16.1 A typical gait cycle normalized in time, and represented on a linear scale from 0-100% of the total stride. This repeating cycle begins with initial contact and ends with the next initial contact of the same foot. The stick figures shown on top represent temporal events associated with foot-to-floor contact. They divide the cycle into swing and stance periods, one period of single support and two equal periods of double support.

Figure 16.2 A typical gait cycle normalized in time, but wrapped around a continuous unit circle to illustrate symmetric phase relationships of temporal events and time periods. The beginning and end of the cycle occur at the 12 o'clock position. The temporal events of initial contact and foot-off for each leg are typically opposite each other on the unit circle, and the single support period of one limb is equal to the swing period of the opposite limb.

that the resulting time periods must be of equal dura­tion for left and right single support, initial and final double support, and left and right swing periods. Any disruption in the natural sequence of temporal events anywhere along the cycle as a result of physi­cal impairment, weakness, or spasticity will result in incorrect timing for the events that follow. This nec­essarily leads to a loss of symmetry that can be quan­tified by comparing the timing of temporal events between sides. Changes in symmetry reflected in the gait period durations is an index of gait pathology, and measuring this simple quantity can be quite useful for evaluating treatment performance over time.

Since the duration of the swing period and leg length determine the distance covered by the swing­ing limb, deviation from normal symmetry and tim­ing will give rise to differences in step length on each side, and subsequently total distance traveled per gait cycle. By definition, step length and stride length are not synonymous. Step length is the distance (in the direction of progression) from a point of ground con­tact of the trailing foot to the next occurrence of the same point of ground contact with the leading foot. It is measured during initial double support and named for the leading limb. In contrast, stride length is the distance from initial contact of one foot to the next initial contact of the same foot, corresponds directly to the stride period, and is equivalent to the sum of suc­cessive left and right step lengths. Recognizing that speed is defined as the ratio of distance per unit time, step length, stride length, cadence (steps per minute), and walking speed are mathematically related by simple formulae:

walking speed (m/s) = (cadence ? stride length)∕120 or

step length (m) = (walking speed ? 60)/cadence

These basic outcome measures of overall gait per­formance, including the timing measures previously described and other quantities such a stance/swing ratio, are collectively known as temporal-distance or temporal-spatial parameters. They can provide con­siderable insight into the overall effect of subtle gait abnormalities on walking performance. For example, children with cerebral palsy may experience foot clear­ance problems during limb advancement due to exces­sive ankle plantar flexion or decreased knee flexion during swing period. Evidence of this could be found in prolonged single support times on the more normal or less involved side, and a reduced stance period, step length, and stance/swing ratio on the more involved side (6). If the source of the limb advancement prob­lem can be attributed solely to the excess plantar flex­ion, the simplest intervention would be to prescribe a solid or leaf-spring ankle foot orthotic (AFO) with a rigid plantar flexion stop to restrict excess plantar flexion during swing.

While clinical motion laboratories routinely com­pare a patient’s temporal-spatial measures to age- matched normative values, caution should be used when interpreting these results. Temporal-spatial parameters of cadence and stride length are directly related to walking speed (7), and since humans rou­tinely walk at a variety of speeds, simple deviations from reference values alone may not be indicative of gait pathology. Rather, reduced values for these mea­sures may simply reflect the need to adopt a speed appropriate to the terrain, the required task, or the size of the room (8). A person’s natural gait is also dependent on the environment, with studies showing that subjects walk faster on a long walkway compared to a short one, and typically walk faster in outdoor studies compared to indoor studies (9). This lack of consensus regarding normal values supports the con­vention adopted by most clinical laboratories to com­pare patient results to their own laboratory-collected references, where these environmental factors can be consistent for all subjects. Nevertheless, while it is “normal” to walk at a variety of speeds, it clearly is abnormal to walk asymmetrically, so side-to-side dif­ferences in temporal/spatial measures within a partic­ular patient should always be investigated.

When comparing temporal-spatial parameters in children, even greater care must be exercised, since several age-related differences arise from the close relationship of these measures to leg length and gait maturity (10). Sutherland has shown that in typi­cally developing children, heel-first initial contact, sagittal plane knee flexion wave, reciprocal arm swing, and an adult joint angle pattern are acquired prior to the development of mature temporal-spatial parameters (11). All of these adult gait characteristics arise before the age of 3 years in most children (6).

Because normal gait should be cyclical and sym­metric, the existence of even small amounts of step-to-step variability may be an indication of gait pathology. Gait is most variable in the toddler, but grad­ually stabilizes as the child reaches adolescence (9). Hausdorff and colleagues have shown that the coeffi­cient of variation for stride time in typically developing 3-4-year-olds is approximately 6%, but decreases to 2% in 11-14-year-olds (13). In the elderly, increased vari­ability is associated with increased risk of falling, with speed variability the single best predictor of falls (9). These examples provide further evidence of the impor­tance of a cyclical and symmetric gait pattern and how variations in symmetry and cycle times reflected in the temporal-spatial parameters of gait may be associated with gait pathology.