Human gait or locomotion can be described as translatory progression of the body produced by co-ordinated , rotatory movements of body segments i.e. the forward movement of body which is produced by co-ordinated movement between different body parts such as the head, arm, trunk and legs. The alternating movements of the legs or lower limbs carry the weight of the Head, Arm and Trunk. One of the main purpose of gait cycle is to maintain weight of head, arm and trunk on lower limbs thereby maintaining an upright posture and balance of the body. The three main tasks of gait cycle as described by experts  are :

  1. Weight acceptance
  2. Single leg support
  3. Swing leg advancement

A single gait cycle consists of two successive events of the same lower leg, usually beginning at initial contact or heel strike/heel contact of the lower limb with the contact surface. During one gait cycle , each of the lower leg passes through two major phases – the stance phase (weight bearing phase) and the swing phase (non-weight bearing phase). As the words are self explanatory – the stance phase is the weight bearing phase when the leg (in consideration) is on the ground. This phase comprises of 60% of the gait cycle. The second phase or the swing phase is the one when the same lower leg is in non weight bearing position and is in air. This phase comprises of 40% of the gait cycle. In between the stance and swing phase of the gait cycle, there are two periods of double support (when both the legs/limbs are on ground for a brief period of time) occurring between the time one limb makes initial contact and the other one leaves the floor at toe off.

https://www.physio-pedia.com/Gait

Both the stance and the swing phase can be subdivided into various phases. According to RLA (Ranchos Los Amigos) institute  the sub-phases of stance phase are:

  1. Heel Strike or Heel Contact – begins when the heel of the reference/leading leg strikes the ground
  2. Foot Flat – occurs when the foot of the leading leg is flat on the ground
  3. Midstance Phase – is the point at which the body weight passes directly over the supporting lower extremity
  4. Heel-Off – when the heel of the reference/leading extremity leaves the ground
  5. Toe-Off – is the instant at which the toe of the foot leave the ground

Stance Phase of Gait Cycle

The sub-phases of swing phase are :

  1. Acceleration or Early Swing – begins when the toe leaves the ground and continues until midswing
  2. Mid Swing – when the extremity passes directly beneath the body
  3. Deceleration or Late Swing Phase – occurs after midswing when the limb is decelerating in preparation for heel strike

Swing Phase of Gait Cycle

Some Important Terminology Related to Gait Cycle:

  1. Stance Time – the amount of time that elapses during the stance phase of one extremity in a gait cycle
  2. Single- support Time – the amount of time that elapses during the period when only extremity is on the supporting surface in a gait cycle
  3. Double- Support Time – the amount of time spent with both feet on the ground during one gait cycle. This time is increased in elderly and persons with balance issues and is decreased during running.
  4. Stride Length – is the linear distance between two successive events that are accomplished by the same lower extremity during gait, i.e. the distance from the point of one heel strike of lower extremity to the point of next heel strike of the same lower extremity.
  5. Stride Duration – the amount of time it takes to accomplish one stride
  6. Step Length – is the linear distance between two successive points of contact of opposite extremities
  7. Step Duration – the amount of time spent during a single step
  8. Cadence – the number of steps taken by a person per unit time

https://en.wikipedia.org/wiki/Bipedal_gait_cycle

Reference- Levangie KP, Norkin CC. Joint Structure and Function – A Comprehensive Analysis. Fourth Edition. F. A. Davis Company, Philadelphia; 2005.