May29
2012

By: Chris Webster                Categories: Animation

The passing position describes the moment in the walk cycle when the body is supported by only one of the legs, which is held in a vertical position with little or no bend at the knee. At this point the leg is positioned directly below and in line with the torso. The other, nonsupporting leg is positioned alongside the weight-bearing leg and swings forward past the supporting leg—hence the term passing position. The knee of this leg is bent at this stage to allow the foot to be raised to clear the ground. During the passing position the kinetic energy of the figure’s forward movement is converted into gravitational potential energy as the figure rises slightly. This energy is then released as the leg swings forward and the body drops and completes the stride.

If we now take a look at the two passing positions and both the strides as a sequence of keyframes, we can see how the entire cycle works and can analyze it in its simplest form.

walk cycle phases

The walk cycle keys. From left to right: stride, passing position, stride, passing position

The leg movement is clearly the primary action of a walk cycle, but the walk action is not limited to the movement of the legs alone. In the following phases of the walk, we can see how all parts of the body work together to produce a complete sequence. The phase order in this instance is an artificial one and quite arbitrary, since there is no distinct beginning or end to this cyclical movement. However, the sequence holds as a general guide, though variations will occur in different walks.

Passing Motion

Walk cycle; Phases 1–5. Left to Right

Phase One: The Stride

  • The left leg is thrown forward, the knee is straight, and the foot has just made contact with the ground.
  • The right leg extends backward behind the body, the knee is straight­ened, and there is a bend in the foot as it remains in contact with the ground.
  • The right arm is in a forward position, with a slight bend in the elbow and the forearm held in front of the body. The right shoulder is rotated forward.
  • The left arm is extended in a backward position prior to its movement for­ward. There is a slight bend at the elbow and at the wrist. The left shoulder is rotated to a backward position.
  • The body is at the lowest point in the cycle. The angle between the legs is at its widest, which places the hips at their closest position to the ground.

Phase Two: Squash

  • The left leg moves into a more upright position and begins to take the weight of the body, the foot is flat on the ground, and there is a slight bend at the knee.
  • The right leg is now lifted from the ground. There is a slight bend at the knee that ensures the foot clears the ground as the leg begins to swing forward.
  • The right arm begins to move backward as the right shoulder rotates backward. The arm is held in front of the body and has a bend at the elbow.
  • The left arm begins to move forward as the shoulder rotates. The bend at the elbow and wrist remain. There is slight drag on the hand.
  • The body remains upright.

Phase Three: The Passing Position

  • The left leg now takes the entire weight of the body. It is straightened at the knee and held in a completely upright position directly below the torso.
  • The right leg has swung forward to its position alongside the supporting left leg. The knee bends increasingly to ensure that the foot clears the ground.
  • The right arm continues to swing backward; the elbow remains slightly bent, with the forearm extending in front of the upper arm.
  • The left arm moves forward and is now positioned alongside the body. The elbow remains bent and the hand is located directly below the shoulder.
  • The body, head, and hips have risen to their highest positions in the walk cycle. The supporting leg and the torso are vertically aligned.

Phase Four: Stretch

  • The left leg now begins to extend backward and is positioned slightly behind the body; the knee remains straightened.
  • The right leg swings forward. A bend in the knee means that the lower leg moves ahead of the upper leg in anticipation of the foot contacting the ground.
  • The right arm moves backward, with the elbow and forearm extending behind the body. The right shoulder rotates backward.
  • The left arm continues to swing forward ahead of the shoulder, which also rotates to a forward position. The bend in the elbow increases slightly.
  • The body begins to fall forward from its highest position in the cycle.

Phase Five: The Stride

  • The right leg is thrown completely forward; the foot has just made contact with the ground and the knee has straightened.
  • The left leg is extended backward behind the body and there is a bend in the foot as it prepares to be lifted from the ground. The knee is straightened.
  • The left arm is now in a forward position, with the forearm held in front of the body, a slight bend in the elbow, and the left shoulder rotated forward.
  • The right arm is extended backward prior to it moving forward. There is a slight bend at the elbow and wrist. The right shoulder is rotated backward.
  • The body once again reaches the lowest point in the cycle, with the angle between the legs placing the hips closer to the ground.
  • It is important to realize that the sequence described here makes up only half of a complete walk cycle. The second half simply reflects the first, with the opposite limbs undertaking the same actions with the same timings.

Excerpted from Action Analysis by Chris Webster.

Chris Webster is an animator who has worked for 20 years in the industry and has extensive experience as an educator teaching across a broad range of levels from schools, higher education and professional training programs and within the studio environment. He is currently Head of Animation at the University of the West of England.

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