Important concepts

Cycle and phases

The locomotor cycle is the fundamental rhythm of movement. In quadrupeds (like cats) and bipeds (like humans), it refers to the cyclical sequence of movements a limb goes through during one complete step.

Each cycle is typically divided into two main phases:

These phases alternate rhythmically between limbs, allowing smooth and coordinated movement. The timing and duration of these phases can vary with speed, surface, or after injuries like a spinal cord lesion.

Muscles: Who Does What, and When?

Now that we know the structure of the locomotor cycle, let’s talk about the muscles involved and how they coordinate to make movement possible. 

• During the stance phase, extensor muscles (like the quadriceps or gastrocnemius) are primarily active. They support the body, resist gravity, and help push the limb backward to generate propulsion.

• During the swing phase, flexor muscles (like the iliopsoas or tibialis anterior) take over. They lift the limb off the ground and move it forward to position the foot for the next contact.

What’s fascinating is how precisely this activation is timed and how inhibition and excitation of muscle groups are carefully balanced to avoid co-contraction and allow smooth transitions between phases. This coordination is tightly controlled by central pattern generators in the spinal cord and modulated by sensory feedback and brain input.