Stance & Squatting

Definition of Stance

- The position and manner in which a person stands

• Single Stance = standing on one foot

• Double Stance = standing on two feet

Definitions of CoG, CoP, CP & BoS

1. Contact Patch (CP)

The CP = the parts of your body touching the ground.

2. Base of Support (BoS)

2) BoS = the entire area inside and around those contact points.

3) CoG → balance point of your body mass

4) CoP → balance point of your force on the ground

Support Moment

Definition: Support Moment

• all moments of force that support the body during stance against gravity

• Almost entirely generated by muscle forces

The equilibrium of stance

When gravity pulls your body downward, so your muscles creates opposing moments by pulling you back upright for your stance.

Equilibrium is the constant velocity in stance

Acceleration and Deceleration in Stance

• Moving DOWN + acceleration DOWN → you speed up ⬇⬇

Ex. Falling down, gravity pushing you down

• Moving DOWN + acceleration UP → you slow down ⬇⬆

Ex, Going up, gravity pushing u down

• Moving UP + acceleration UP → you speed up ⬆⬆

Ex. Going up, gravity pushing you up

• Moving UP + acceleration DOWN → you slow down ⬆⬇

Ex. Going up, gravity pushing you down

Moment of gravity in sagittal planes

gravity naturally tries to bend your body into flexion when you’re standing or squatting

• Hip: gravity tends to make the hip flex

• Knee: gravity tends to make the knee flex

• Ankle: gravity tends to make the ankle move toward dorsiflexion (often described as a flexion tendency)

Sagittal Plane Muscles

Stance Using Only Uni-Articular Extensors

Uni-articular means a muscle crosses ONE joint and can change length

• isometric standing still

• shorten during ascent

• lengthen during descent (bc muscle stretches)

Stance with Bi articular extensors

• A bi-articular muscle crosses two joints and have opposite actions at each joint

• Act as a flexor at one joint and an extensor at the other

• During ascent (both joints extend) or descent (both joints flex), the effects on muscle length cancel each other out

• Therefore, these muscles stay isometric and don’t change length

When are your muscles activating the most?

1) Mechanical work

When you stand up (ascent) or squat down (descent), the forces are similar if the movements are equal and opposite.

But when you’re deep in a squat, the extensor muscles need to produce more force.

2) Neural effort / muscle activation

Muscles generally produce force more easily when they are:

• lengthening while producing force → eccentric contraction

(It’s easier to carry smth heavy with long muscles)

Joints of Ankle

Talo-crural joint

Between: leg (tibia/fibula) ↔ talus

• Type: hinge (ginglymus)

• 1 DoF

• Movement: dorsiflexion (toes up) ↔ plantarflexion (toes down)

2. Sub-talar joint

Between: talus ↔ calcaneus/navicular

This is below the talo-crural joint.

• Type: roughly a hinge

• 1 DoF

• Movement for closed chain: pronation (roll inwards) ↔ supination (roll outwards)

• Movement for open chain: inversion and eversion

Sub-taler Motion

Pronation = 3 movements together

1. Abduction

• Foot/heel moves outward

• Heel goes into valgus

2. External rotation

• Foot turns outward

• Toes point more outward

3. Dorsiflexion

• Foot moves upward

• The long arch flattens

Supination = opposite pattern

Axial-Coronal Linkage at Rearfoot and Hip

Linked: movement of a joint makes other joint move

Rearfoot:

• Axial plane motion is linked with coronal plane motion.

• This is because the subtalar joint has an angled/diagonal axis of rotation.

Hip:

• Axial and coronal plane motion are linked when the foot is on the ground (closed chain / stance).

• When the foot is off the ground (open chain), axial and coronal hip movements are not linked.

Axial Plane Linkage of Hip-to-Rearfoot at Knee

• Knee: physiologically has only 1 DoF in the sagittal plane → flexion/extension.

• Even though the knee does not have an axial-plane DoF, it acts as an axial-plane link between the thigh and leg.

• In stance (closed chain), axial rotation of the hip or rearfoot causes opposite-direction axial rotation at the other end.

  • Hip rotates outward → rearfoot rotates inward

  • Hip rotates inward → rearfoot rotates outward

Axial-Coronal Collapse in Gravity

• Gravity can lower the body toward the ground without flexing the joints

• Instead, the hip and rearfoot move in the coronal plane.

• Rearfoot movement → navicular drops (bone in arch of feet) → medial arch lowers (arch of feet)

• Hip coronal movement → pelvis lowers

Coronal Plane Moment of Gravity at Hip

• Single stance (1 leg):

  • Gravity creates a hip adduction moment.

  • Adduction = thigh moves toward the body’s midline bc gravity pulls you to the CoP

• Double stance (2 legs):

  • Gravity moments at the hips partly cancel each other out.

Rearfoot moments of gravity

• Gravity creates a pronation moment at the rearfoot.

• Rearfoot pronation → medial arch lowers → navicular drops

  • This is called “arch collapse.”

  • Can lower the body’s CoG by ~1–2 cm.

• Navicular drop = a way to measure how much the arch lowers.

Axial-Coronal Collapse in Stance

• Collapse of the hip (AdIR) and

rearfoot (pronation) during stance