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Phases of the Normal Gait Cycle
Walking can be defined as a method of locomotion involving the use of the two legs, alternately, to provide both support and propulsion, with at least one foot being in contact with the ground at all times.2 While the terms gait and walking are often used interchangeably, the word gait refers to the manner or style of walking, rather than the actual walking process.
The gait cycle is the time interval between the exact same repetitive events of walking. The defined cycle can start at any moment, but it is generally begun when one foot contacts the ground. If it starts with the right foot contacting the ground, then the cycle ends when the right foot makes contact again.
There are two phases of the gait cycle. Stance phase is the part of the cycle when the foot is in contact with the ground. It comprises 62% of the cycle, beginning with initial foot strike and ending with toe-off. Swing phase occurs when the foot is in the air and comprises 38% of the cycle, beginning with toe-off and ending with second (ipsilateral) foot strike.
While there is debate as to how many sections the cycle should be divided into, we will present the cycle as consisting of 6 major periods:
1. Initial Double Support
2. Single Limb Stance
3. Second Double Support
4. Initial swing
6. Terminal swing
Periods 1-3 make up the stance phase, while periods 4-6 make up the swing phase. The stance and swing phases of both feet and their relative time occurrence are illustrated in Figure 1.
Figure 1: The Normal Gait Cycle.3
Notice that the opposite limb repeats the same sequence of events, but is displaced by 50%. For example, while left toe off occurs at 12%, right toe off occurs at 62% of the gait cycle.
Here are the six periods of one limbís gait cycle described in more detail:
Figure 2: Periods of normal gait cycle with body position for sagittal plane motion.4
Period 1: Initial Double Limb Support (0-12% of gait cycle)
This period begins with the first foot strike and ends with the opposite footís toe-off. The center of mass (COM) begins to rise during this period as the shift to a single limb occurs. It is characterized by rapid loading with shock absorption (mostly from the knee) and slowing the bodyís forward momentum. The hip begins flexed and then extends as the opposite foot comes off the ground. On the contrary, the knee begins fully extended and then flexes during this period. The ankle goes from a neutral position to plantarflexed until the foot is flat on the floor.
Period 2: Single Limb Support (12-50% of gait cycle)
This period begins with the opposite toe off and ends with opposite foot strike. As the body passes over the grounded foot, the COM reaches its peak height while both the forward and vertical velocity decrease. The COM then falls until the end of this period at opposite foot strike, as forward and vertical velocity increase again and forward shear reverses to aft shear (at ~30% of cycle). The hip continues to extend throughout this period as the limb progresses from an anterior to a posterior position in relation to the body. The knee extends again until it hits a maximum extension and starts to flex. The ankle dorsiflexes throughout the phase but actively plantarflexes towards the end to resist this dorsiflexion.
Period 3: Second Double Limb Support (50-62% of gait cycle)
This period (also called pre-swing) begins with opposite foot strike and ends with toe off. The COM is at its lowest level during the beginning portion of this period as both feet are in contact with the ground. The COM then starts to rise again as the opposite limb is in its initial double limb support. As body weight is transferred to the opposite limb, the trailing limb ends its extension and prepares to swing in front of the body. As a result, the hip and knee flex and the ankle plantarflexes during this time in order to prepare for the limbís lift off.
Period 4: Initial Swing (62-75% of gait cycle)
This is the beginning of the second single limb support interval. It begins when the foot is lifted off the ground (toe-off) and ends when the swinging foot clears the ground and is opposite the standing opposite foot (feet adjacent). The COM reaches its peak height towards the end of this period as the opposite foot is in the middle of single limb support. The ankle continues to plantarflex but then begins to dorsiflex in order to clear the ground. The knee continues to flex rapidly to its peak at the end of this period, which largely results from the hip flexing as well.
Period 5: Mid-Swing (75-85% of gait cycle))
This is the middle third of the swing phase. It begins with foot clearance where the feet are adjacent and ends when the swinging foot is in front of the body and the tibia is vertical. The COM falls during this time as the opposite limb is in the middle portion of the single limb support period. The legís advancement is carried out by continued hip flexion. The knee begins to extend quickly due to momentum and in response to gravity. The ankle continues to dorsiflex to neutral.
Period 6: Terminal Swing (85-100% of gait cycle)
This is the last third of the swing phase. It begins when the tibia is vertical and ends with initial contact (foot strike) of the swinging foot. The COM reaches its lowest level at the end of this period as double limb support begins. The knee continues to extend in order to complete limb advancement. The hip reaches maximum flexion at the beginning of this period and tends to extend slightly before initial double limb support. The ankle remains dorsiflexed to neutral.
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