Gait

• Stance phase

• Swing phase

• Running speed = stride length • stride rate

(frequency)

Lowering CG during the last 2 strides before takeoff

• Places joint at more optimal angles to produce torque

• Stretches muscles to be used during takeoff

– Increases passive tension

– Increases active tension

• Increased # of actin-myosin cross-bridges

• Muscle spindles stretch reflex

• Increases muscle calcium levels

– Impulse: F•t = m•v

Measure of Metabolic Efficiency

• O2 cost of locomotion

– Requirements:

• Steady state measure

• Energy utilization almost 100% aerobic

• Valid, reliable system of measure

Applications for measuring gait efficiency (oxygen cost of locomotion)

• 1. Improve athletic performance?• 2. Improve quality of life

– A. aging• Lower work and aerobic capacity• Less efficient gait

– walking/running at a given speed requires a higher % of work capacity

– B. stroke (rehab)– C. orthopedic problems

• Joint injury/surgery• Arthritis• Bone fractures

• 3. Minimizing injury risk at the workplace– Lifting, walking with a heavy load

Categorization of the factors that affect running economy

• External energy– Age– Segmental mass distribution– Biomechanical variables

• Internal energy– Heart rate– Ventilation– Temperature

• Others– VO2max– Training status– Fatigue– Mood state

Bailey and Pate, 1991

Physical constructs contributing to efficiency of locomotion(not mentioned b Bailey and Pate)

• 1. Muscle fiber type– Slow twitch fibers are more efficient than fast twitch fibers

• 2. Internal work of muscles and joints– A.V. Hill’s concept of the oxygen cost of shortening (each stride

consumes a quantifiable and predictable amount of energy)– He stated that 3 contractile properties held true for all vertebrate

striated muscle:• 1) maximal force per cross-sectional area• 2) maximal work per gram of muscle during a contraction• 3) maximal efficiency of chemical energy mechanical work

– Influence of running gait: metabolic energy consumed per stride per mass of muscle: 5 J/stride/kg

3. Complex pendulum swing of limbs

f = 1/(2)(ag/l)

Where: ag = acceleration due to gravity l = distance from axis of rotation to center

of mass (gravity) Logically, the most efficient running speed will match

the dynamic pendulum frequency of the limbs Keep in mind that this is a dynamic frequency which

changes as joint angles change in a multi-segmented limb

Physical constructs contributing to efficiency of locomotion(not mentioned b Bailey and Pate)

4. Strain energy return• Arch of the foot• Achiles’ tendon• Raped stretch of muscles

Up to 50% of mechanical energy needed for running can be stored in these structures (Bennet, M.S. Biomechanics in Sport, 1988)

For a 50kg man running at 4.5 m/s, each arch stores approximately 17J of energy at midstance…an additional 35J can be stored in the Achilles tendon

Physical constructs contributing to efficiency of locomotion(not mentioned b Bailey and Pate)

Spring oscillation frequency

f = 1/(2)(k/m)

Where

f = frequency

k = spring constant (stiffness)

m = unit mass

• Remember: connective tissue and skeletal muscle are viscoelastic

– They store and return energy well when stretched (or otherwise deformed) rapidly

– They dissipate energy when stretched slowly

Economic runners have:

• 1. Lower impact forces/kg mass• 2. Shank (tibia) angle of ankle closer to vertical at

heel strike– Little valgus or varus– Less pronation or supination of ankle during

stance phase• 3. Smaller plantar flexion angle during at end

puss-off phase• 4. Lower velocity of knee during foot plant

Kayano, 1986

Kayano, 1986

Mean patters of the arch of the footMeasured in different areas

Ker et al., 1987

• 70 kg man

• 17 J/step : Arch

• 42 J/step: Achilles’ tendon + gastroc

• Estimated total work needed / step: 100J

Saibene, 1990

Rate of energy expenditure and rate ofmechanical work -

walking at 1 km/hr

Kubo et al, 1999

Ideal (fantasy) linear oxygen cost of running data

We can TRY to make oxygen costof locomotion curves linear

Dose of Reality

Alinearity of O2 cost (C.R.Taylor et al.)

O2 costandstride length

Preferred gait in locomotion (walk, trot, canter, running, gallop) is usually one at which the oxygen cost is lowest when

expressed against running speed:(ml O2/kg/min) / (m/sec)

Body weight and O2 cost of locomotion

Taylor’s lab - Harvard measured 100s ofanimals O2 cost oflocomotion

Suni, dik dik, AfricanGoat, sheep, waterbuck,Eland, Zebu cattle

Exceptions:

Kangaroos, ducks, geese, lions

VO2 --> ml O2/kg 0.70/min

• resting metabolic rate • stride cost

5 J/stride/kg - A.V. Hill

Emet/mb = 10.7• Mb-0.316 + 6.03• Mb-0.303

VO2 --> ml O2/kg 0.70/min

• resting metabolic rate • stride cost

• oxygen cost of running for child higher than adult

• mechanics less efficient up to age 7

Size a factor up to age 16-18…

Applications: running velocity = SL • SR

Gait Strategies

• Expert sprinters – high stride rate• Expert speed skaters – high stride rate• Expert marathon runners – long stride length• Expert cross-country skies – long stride lengths

• A) pendulum f 1/L

• B) T (torque) = F * d = I *

where I = m*r2 (rod, cylinder)