The first year of motor development consists of a crawl which takes about seven months to complete and creeping which takes about five months. These motions consist of a flexion and extension phase. During the extension phase the arms and legs are going away from each other. During the flexion phase the arms and legs are going towards each other. For clarity, we will describe the extension phase with the right arm reaching up and the flexion phase as the right knee coming up.
| Extension | |||||||
|---|---|---|---|---|---|---|---|
| Muscle | Crawl | Creep | Initial Walking | Step | Dia Tuck | Stride | Drop |
| Infraspinatus | Con D2 Flexion | Closed Hor Ab | Balance | off | Open Hor Ab | ||
| Piriformis | Con D2 Flexion | Closed Hor Ab | Balance | off | Open Hor Ab | ||
| Trapezius | Hor Ab/fl open | Swing only | Balance | off | off | off | off |
| Quadratus Lumborum | Pelvis to spine | shut off replaced by TFL/psoas (swing) | Balance (hurdle) | off | back hip? | off | off |
| Rhomboid | Pull: Scapula to Spine with pec minor, subscapularis, pec major, Lats, triceps long head, then has to let go and replaced by pec minor as arm behind back | Shut off, replaced by triceps | Balance (hurdle) | off | front arm | off | off |
| Medius/Minimus | Hor Ab/fl open | Minimized | Primary Stabilizer | off | Prime Actor | Stabilizer | off |
| Medius/Minimus | Hor Ab/fl open | Minimized | Primary Stabilizer | off | Prime Actor | Stabilizer | off |
| Pec Minor | Closed pull stabilize, at end tilts scapula forward | Stabilize scapula, initial swing prior to trapezius | off | off | off | Main function, initiates swing after | Primary function |
| TFL | Hip Flexion/Abb Hor | Prime Initial Swing | Activates as piriformis in inhibited | off | Prime Actor | Main function, initiates swing after | Primary Function |
Crawl
Extension Phase
Right Shoulder
Muscles
Shoulder: Rotator Cuff: The infraspinatus horizontally abducts (needs 3/5 strength) and function exactly the same as the piriformis muscle of the hip. The supraspinatus is positioned as an abductor and only needs a 2/5 strength. The middle and posterior deltoid are also functioning to drive abduction/horizontal abduction.
Scapula: The trapezius completely facilitated and acts as one unit during the reach phase. The serrates anterior is inhibited from the trapezius so likely minimally activated, so does not have to be tested initially.
Spine: The thoracic spine is rotating to the right as it side-bends to the back left. This specifically activates the multifidus along the spine on the left side.
Creep
Transition Phase
The transition phase is when weight is placed on the hand and right knee. From there the right arm and the left knee will swing forward.
Left Shoulder
Muscles
Rotator Cuff: The teres minor is active here which acts as an extensor. The infraspinatus acts as a stabilizer and requires 4/5 strength with full eccentric control. The supraspinatus is inhibited meaning the infraspinatus which as linked to the supraspinatus during crawling is now separated. Also, the deltoid is filtered out here.
Scapula: The serrates anterior dominates here at the same time the trapezius is inhibited. 4/5 strength is necessary at this phase. Of special note is the fact that the trapezius is now separated from the infraspinatus. This lack of separation is a common reason for shoulder impingement.
Right Shoulder
Muscles
Rotator Cuff: The supraspinatus is positioned as a flexor as the right arm swings forward.
Scapula: The trapezius is isolated here and clearly separated from the serrates anterior on the same side. The trapezius is now associated or linked with the serratus anterior on the opposing arm which is later used for swinging motions.
Spine: Same as crawling.
Right Hip
During crawling, the right hip flexex/horizontally abducts as the knee comes up, and when pushing off of the right hip it adducts/extends. The creeping position introduces two completely different patterns. As soon as the right knee is on the ground and the left knee lifts up, the piriformis now acts as a horizontal abductor. It helps to see that the the piriformis acts very similarly to the infraspinatus of the shoulder, and other areas seem to be a common source of pain as they insert on the trochanter/tubercle. The piriformis is specifically strengthened isometrically/eccentrically just like the infraspinatus for five months. At the same time, the pelvis is driven into the ground which is a right rotation of the pelvis. This rotation is specific to the multifidus on the left side side. Note that just like with walking, the multifidus acts on the same side in the upper and lower half. The horizontal abduction/extension of the right hip should completely shut off the psoas on the right side (which contracts on the left side)
Left Hip
The left leg swings forward which is specific to the psoas which helps create a new pattern on the left in which the left psoas is in concert with the left multifidus.
Initial Walking
Left Hip:
During crawling the abductors were active on the right hip as the leg moved upwards. During creeping the abductors were mildly active on the left leg while the piriformis on the left leg dominated. Now, during initial one legged standing on the left the abductors are the primary actors. So the key theme for the plant leg is separation of of the piriformis from the abductors. Note that the gluteus medius and minimus do not attach to the IT band, so if someone has a short IT band it is likely related to weakness in the abductors and compensation of the glutes maximus which should be more of a power actor, but can become a standing muscle if standing in hip flexion. Just standing on one leg actively for the first time or in soft stand will trigger a larger response of the abductors and the external rotators like the piriformis, (frisbee)but as the action is done with less overflow from above the abductors and be contracted with a letting go of the piriformis. To summarize, initial standing will be a balance reaction, and eventually more upright.
Right Hip
Initially the weight on the right hip is used for balance. If the child was to veer to the left their right hip would perform the crawling motion of flexion/abduction/external rotation. During this time the quadratus lumborum is activated and lifts the pelvis up on the right. As the balance is improved, a similar motion is performed after a delay. It is identical to the creep motion of with weight bearing hip in which the piriformis is isolated from the psoas and quadratus lumborum and associated with the multifidus on the left.
