CBMP, Contra Body Movement Position

Yes, it's a strange name! It refers to the position of the moving foot at the end of a step.

Lets consider the Feather Step โ€“ Step 1 (Leader)

โšก TL;DR โ€“ CBMP CBMP describes a foot position where the moving foot is placed in front of or behind the standing foot. It is a consequence of the previous two or more steps, and cannot be manufactured by โ€œjust putting your foot in front of the other.โ€ Well... you can โ€” itโ€™s just really ugly and doesnโ€™t feel good.

Step Description

RF Forward, CBM during 1

โœ… Clarified Technical Details

Term Clarification
CBM on 1 โŒ Incorrect โ€” implies a discrete moment
CBM during 1 โœ… Correct โ€” rotation develops over the step
Arc of the figure โŒ Misleading โ€” Feather Step is a straight-line figure
Trajectory setup โœ… The step sets internal frame rotation, not external path curvature

๐Ÿงฎ Biomechanical Vectors

๐Ÿ”น Vt โ€” Travel Vector

  • Represents the motion of the stepping foot
  • Lies in the XZ plane
  • Directionally straight โ€” the Feather Step is a linear figure
  • Magnitude reflects speed and drive \(V_t = (ฮ”x, 0, ฮ”z)\)

๐Ÿ”น Vf โ€” Frame Vector

  • Represents rotational change of the upper body (CBM)
  • Originates from rotation around the Y-axis during the step
  • Varies over time: dฮธ/dt (rotational velocity)
  • Causes tangential alignment changes to set up for Outside Partner \(\vec{V_f} = \frac{dฮธ}{dt} ร— r\)

Where:

  • ฮธ = torso yaw
  • r = lateral partner offset (approx. 1 unit)

๐Ÿง  Key Insight

  • CBM is not a discrete action, but a rotational development during Step 1.
  • There is no arc in the floor path โ€” the trajectory is linear.
  • However, the rotational gearing of the torso (Vf) shapes what will be required next.
  • Step 1 does not include CBMP. But it plants the biomechanical seed that will make CBMP necessary in Step 3.

CBM 'On 1' (or on the beat). It's CBM DURING the entirety of the beat

  • Step 2: LF forward Left Side Leading: The swing forward of the left leg provides more clockwise rotational inertia requiring sway to absorb it. Step 2 is accomplished on 1 beat of music (0.5s for Foxtrot) following the slow which took 1 second to accomplish.