Date post: | 11-Jan-2016 |
Category: |
Documents |
Upload: | corey-barnett |
View: | 243 times |
Download: | 0 times |
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)