CRITICAL EVENTS DURING THE SINGLE LIMB SUPPORT TASK
As shown in Table 5.3, there are three critical events during the single limb support task. These are controlled tibial advancement during mid-stance, controlled ankle dorsiflexion with heel rise (heel-off) during terminal stance, and a trailing limb posture during terminal stance (39). During the two phases of this task (mid-stance and terminal stance), the responsibility of the stance limb is to simultaneously provide support against gravity without losing balance and contain the forward momentum built up by the contralateral swinging limb. Both of these objectives can be accomplished by controlling tibial advancement in the first half of single support and controlling ankle dorsiflexion in the second half. This will lead to the trailing limb posture (body COM forward of the base of support) necessary to permit a sufficient step length on the opposite side. If, at the end of loading response, the foot has achieved foot-flat, then during mid-stance, the ankle becomes the axis of rotation for the body's forward progression. This is referred to as the ankle or second rocker, and this mechanism continues until maximum dorsiflexion is achieved in terminal stance (Figure 5.13). With the heel and forefoot firmly planted, the tibia can rotate over the talus smoothly under the selective control of the soleus, later assisted by both heads of the gastrocnemius, which simultaneously limits knee extension. The slow-twitch, fatigue-resistant muscle fibers of the soleus are usually well suited to the sustained eccentric contractions required to control tibial advancement. Weakness in the triceps surae, however, results in the tibia advancing too quickly, which prematurely allows the tibia to move past vertical and leads to sustained knee flexion during mid-stance, and premature or excessive dorsiflexion and lack of knee extension at terminal stance. In this circumstance,
TABLE 5.4 GAIT MEASUREMENTS USEFUL FOR IDENTIFYING THE CAUSE OF AN ABSENT OR ABNORMAL CRITICAL EVENT DURING THE WEIGHT ACCEPTANCE TASK
a rigid AFO or, in extreme cases of weakness, a floor-reaction AFO can effectively supplement the weak plantar flexors, restore a more normal plantar flexor moment, and control tibial advancement during mid-stance and dorsiflexion during terminal stance. Gage also suggests using a rear-entry hinged, floor-reaction AFO in these circumstances (12), which permits ankle plantar flexion but resists dorsiflexion in mid-stance and terminal stance.
In normal adults and typically developing children, the forward progression of the body causes the origin of the GRF vector (center of pressure or COP) to move forward to the metatarsal heads, causing the heel to rise at the beginning of terminal stance. Now the axis of rotation for the body's forward progression is the metatarsophalangeal (MTP) joint, giving rise to the forefoot or third rocker (31) (see Figure 5.13). While the first two rockers were constraining forward progression using eccentric plantar flexor contractions, the forefoot rocker is an accelerating rocker, as evidenced by the large ankle plantar flexor moment and transition from power absorption to power generation (36). With the help of strong concentric contraction of the fast-twitch fibers of the gastrocnemius, the ankle is stabilized and continued dorsiflexion in terminal stance is halted. By the end of terminal stance, the ankle is plantar flexing in preparation for initial contact on the other side, which yields the trailing limb posture necessary for maximum step length. When there is plantar flexor weakness, the third rocker is ineffective, which fails to control continued dorsiflexion, allows the knee to prematurely drop into flexion, reduces trailing limb posture, and shortens the opposite side step length. All of these factors reduce overall walking performance. AFOs that store energy in the structure of the orthosis as the ankle dorsiflexes (rigid, leaf-spring, floor-reaction) can provide a plantar flexion assist as the foot is unweighted in early pre-swing, depending on the amount of stiffness and energy storage built into the custom orthotic. This assist can return some of the reduced plantar flexor moment that would occur without orthotic use, and evidence of this can be found in the plantar flexor moment curve comparing orthotic and barefoot conditions.
If, at the beginning of mid-stance, the foot has either not achieved or is past foot-flat (equinus, early heel-off, spring-foot), the normal ankle and forefoot rocker mechanisms may not be effective, and the three critical events of single limb support will not be achieved. In toe-toe gait (equinus) or jump knee gait (forefoot initial contact and excessive knee flexion at loading response, followed by rapid knee extension and ankle plantar flexion in mid-stance), plantar flexors that are tight or have increased tone overly constrain forward tibial advancement in mid-stance and dorsiflexion in terminal stance, leading to excess knee extension and early heel rise. While the mechanism is different from the case of weak plantar flexors, the end result is the same: reduced effectiveness of second and third rockers and inability to achieve the three critical events. In these cases, Botox injections into the triceps surae, tendo-Achilles lengthening, or intramuscular triceps surae lengthening (Strayer procedure) can be effective in restoring second and third rockers, depending on severity. Ankle plantar flexion moments and powers, and dynamic EMG recordings are quite useful in selecting which procedure is most appropriate (22). Another example is crouch gait deformity, where hip and knee contractures combined with weak or overlengthened plantar flexors lead to early heel rise and premature forward advancement of the tibia in mid-stance, and premature and excessive dorsiflexion in terminal stance. In this case, the same impact on the second and third rockers described previously for weak plantar flexors will often occur. Dr. Gage has long been a proponent of performing single-event, multilevel (SEML) soft tissue and bony surgery for this deformity to restore the proper rocker mechanisms and, with the proper orthotics, the plantar flexion/knee extension couple that allows the patient to stand more erect and walk more effectively (12). Other centers have taken a more conservative approach of staging the procedures, which has the advantage of reducing the surgical impact at the time of the procedure, but may cause muscle imbalances at other joints, leading to additional surgeries down the road. In either case, or when nonsurgical interventions are warranted, the goal should be to restore the rocker mechanisms so that the three critical events of single limb support can be realized. Table 5.5 summarizes many of the gait measurements that are useful in identifying causes for absent or abnormal critical events during the single limb support task.