Resolution of Forces

• combine two forces (load and shear)
• parallelogram or triangle of forces
• resultant (the GRV) is longer than each of the components
• slopes at an angle depending on shear

tan A = opposite / adjacent = Load / Shear
by pythagoras: total force (resultant) = Ö(Load² + Shear²)

What is the resultant GRV if the load is 400N and shear 300N?100N350N500N2500N

• body accelerates up and down
• accelerations add to and subtract from the body weight:

• load rises from zero at heel-strike
• increases to slightly more (about 20%) than body weight in early stance
• falls to less than body weight during mid-stance (30% cycle)
• at toe-off (push-off phase) it rises above body weight

What is the acceleration of a 60kg woman if the vertical GRF is 720N?+2ms^-2-2ms^-2-60ms^-2+120ms^-2

What is her acceleration if her vertical GRF falls to 540N?1ms^-2 upwards1ms^-2 downwards10ms^-2 upwards10ms^-2 upwards

Estimating Muscle Activity from the GRV

• external moment due to GRV = internal Moment due to muscle
• active muscle is on opposite side of joint to GRV
• GRV on extensor side of the joint => flexor muscle active
• GRV on flexor side of the joint => extensor muscle active
• muscle (or ligament) moment = size of GRV x lever arm lever arm (perpendicular distance between the GRV and joint)

What muscles are active if the GRV is posterior to the knee and anterior to the ankle?dorsiflexors & quadsplantarflexors & quadsplantarflexors & hamstringsdorsiflexors & hamstrings

Center of Pressure

• starts at the lateral border of the heel at initial contact
• moves forward through stance to big toe (hallux) at toe-off
• CoP velocity not constant - moves quickly over heel and toes

Plantar pressure measurement (pedobarography)

• Emed
• Pedar

Abnormal pressures

Flat foot (everted, pronated)

Cavus foot (inverted, supinated)

Equinus (tiptoe)
Calcaneal (on heels)