Walking is the most convenient way to travel short distances. Free joint mobility and appropriate muscle force increases walking efficiency. As the body moves forward, one limb typically provides support while the other limb is advanced in preparation for its role as the support limb. The gait cycle (GC) in its simplest form is comprised of stance and swing phases. The stance phase further is subdivided into 3 segments, including (1) initial double stance, (2) single limb stance, and (3) terminal double limb stance.
Each double stance period accounts for 10% of the GC, while single stance typically represents 40% (60% total). The 2 limbs typically do not share the load equally during double stance periods. The swing phase for this same limb is the remaining 40% of the GC. Ipsilateral swing temporally corresponds to single stance by the contralateral limb. Slight variations occur in the percentage of stance and swing related to gait velocity. Duration of each aspect of stance decreases as walking velocity increases. The transition from walking to running is marked by elimination of double support period(s).
A stride is the equivalent of a GC. The duration of a stride is the interval between sequential initial floor contacts by the same limb. A step is recognized as the interval between sequential floor contacts by ipsilateral and contralateral limbs. Two steps make up each GC, which is roughly symmetric in normal individuals.
A consistent sequence of motions is performed at each of the lower extremity joints during locomotion. Each stride contains 8 relevant phases. Stance is comprised of 5 gait phases (ie, initial contact, loading response, midstance, terminal stance, preswing), with the remaining 3 phases occurring during swing.
The first 2 gait phases (0-10% GC) occur during initial double support. These phases include initial contact and the loading response. Initial contact often is referred to as heel strike. While this term is appropriate in normal gait, many patients achieve heel contact later in the GC, if at all. The joint motion during this phase allows the transfer of weight onto the new stance phase leg while attenuating shock, preserving gait velocity, and maintaining stability.
Swing phase by the contralateral limb corresponds with single support by the ipsilateral limb to support body weight in the sagittal and coronal planes. The first half of single support is termed midstance (10-30% GC) and is involved with progression of the body center of mass over the support foot. This trend continues through terminal stance (30-50% GC). This phase includes heel rise of the support foot and terminates with contralateral foot contact.
The final stance element, preswing (50-60% GC), is related functionally more to the swing phase that follows than to the preceding stance phase events. Preswing begins with terminal double support and ends with toe-off of the ipsilateral limb.
Three unique phases characterize swing, including initial swing (60-73% GC), mid swing (73-87% GC), and terminal swing (87-100% GC). The swing phase achieves foot clearance and advancing of the trailing limb.
Shock absorption and energy conservation are important aspects of efficient gait. Altered joint motion or absent muscle forces may increase joint reaction (contact) forces and lead subsequently to additional pathology. In early stance, nearly 60% of one’s body weight is loaded abruptly (less than 20 milliseconds) onto the ipsilateral limb. This abrupt impact is attenuated at each of the lower extremity joints. Loading response plantar flexion is passive, substantially restrained by eccentric work of pretibial muscles. The absorptive work by pretibial muscles delays forefoot contact until late in the initial double support period (7-8% GC).
At initial contact, external (ground reaction) forces applied to the contact foot produce a tendency toward knee flexion. Repositioning the knee (recurvatum) increases knee mechanical stability, but at the cost of increased contact forces and shock generation. A balance between knee stability and shock absorption is achieved by eccentric quadriceps contractions during loading response. The impact of loading is minimized at the hip during single support through hip abductor muscle contraction.
A study by Ricci et al demonstrated the importance of the pubic rami in maintaining pelvic integrity during loading in the gait cycle. Evaluating pubic ramus fractures, the investigators found that if the anterior pelvic ring rami are completely disrupted, load redirection causes significantly greater posterior pelvic stress.
Ambulation always is associated with metabolic costs. These costs are relatively minor in normal adults performing free speed level walking. The self-selected walking speed in normal adults closely matches the velocity that minimizes metabolic work. This association does not apply with gait pathology. Walking velocity, energy cost per time, and energy cost per distance are considerations when the patient is making choices about walking versus wheelchair mobility. Gait velocity typically decreases with all neuromuscular pathology, and the reduction is related to the magnitude of the pathology. Energy cost per unit of time may not change substantially, even with severe involvement. Energy cost per unit of time is maintained by decreasing walking velocity considerably. Energy cost per unit of time does not change markedly following stroke, as compared to changes associated with aging; however, the energy requirement per distance traveled is more than 3 times normal.
In this same population, wheelchair use cuts energy cost per distance in half and decreases cost per minute slightly, while preserving ambulation velocity. Similar trends are observed when examining various energy cost parameters in individuals with spinal cord injury, myelomeningocele, and increasing levels of amputation. Energy cost to travel a prescribed distance increases (greater than 500% increase in myelomeningocele with bilateral knee-ankle-foot orthoses), while oxygen cost per minute is maintained by decreasing walking velocity substantially. Often the critical factor in selecting a wheelchair for mobility is the energy requirement to traverse a given distance. Most individuals self-select wheelchair mobility when cost per distance exceeds 300% of normal values.