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Outline Introduction Mechanisms of shock absorption during locomotion/gait active mechanisms passive mechanisms Artificial shock attenuation insoles/orthotic materials surfaces shoes Do more expensive athletic shoes reduce injuries? Heel strike transient Upon heel strike, a ground reaction force (GRF) is transmitted from the ground to the foot which travels up through the lower extremity The sharp irregularity in the GRF at heel strike (first 5-25 ms) is known as the heel strike transient Measurement techniques Force platforms and accelerometers form the foundation of skeletal transient investigation Other techniques used to analyse impact situations and their mechanical effects include Foot sensor systems Inverse Dynamics Simulation Models The approaches and limitations of each of these techniques are discussed elsewhere (Nigg BM et al. J Applied Biomechanics 1995: 407-432) Force platforms Haven't consistently shown a heel strike transient Heel strike transient may be observed for the vertical component of the force platform readings when platform settings are correct This requires a force platform of sufficiently high natural frequencies, sampled at a high rate and without excessive low pass filtering Units of measurement - Newtons (N) Accelerometers Measures acceleration changes at their point of application following heel contact. magnitude and frequency of components of the heel strike transient at a particular body segment can be quantified. Units of measurement - gravities (g), where 1 g = 10 ms-2 Accelerometers Shock attenuation in the human body during locomotion At heel strike during normal walking, peak forces at the tibia and skull have been recorded at 5 and 0.5 gravities (g) respectively. Peak magnitudes of the transient stress wave generated at heel strike are reduced by a factor of 1.45 across the knee joint and by a factor of 3.29-3.74 from the tibia to the forehead. This suggests that there is shock attenuation along the musculoskeletal system. Definitions Shock attenuation versus cushioning Definitions vary
Mechanisms of shock attenuation in the human body Active mechanisms
Passive mechanisms
Active mechanisms of shock attenuation Joint motion combined with eccentric contraction of muscles F = ma (where a = change in velocity/time) Implies that the rate at which the body segment changes velocity (negative acceleration) at initial contact will influence the magnitude of the GRFs and heel strike transient produced Less force - smaller magnitude of acceleration At initial contact, an alteration in joint position increases the time over which the velocity of a body segment will change, thereby reducing the magnitude of forces produced at impact. Magnitude of acceleration is reduced (When muscles lose their ability to absorb energy, more energy is transmitted to skeletal system. This is thought to contribute to the higher number of stress-related bone problems during the later part of a marathon when muscles are presumable fatigued) Active shock attenuation during the gait cycle Contact period motion of
Subtalar joint Shock attenuation is a primary function of subtalar joint pronation following heel strike Subtalar joint pronation disperses forces
Shock attenuation STJ Reducing subtalar joint pronation is associated with an increase in mechanical shock transmitted through tibia (Byrne & Cox, 1997; Perry & La Fortune, 1995) Correlation between lack of STJ pronation and shock induced musculokeletal pathology? Implications for manufacture of foot orthoses Shock attenuation Ankle joint Eccentric contraction of anterior compartment muscles combined with plantarflexion during contact period (heel-toe walking) Effect of foot position at initial contact on heel strike transient Maximum amplitude of the transient stress wave produced at initial contact during running (accelerometers attached to tibia) is reduced by 33% for toe striking compared to heel striking. Thought to increase the duration/time over which the velocity of body mass changes at initial contact Mechanism of shock attenuation: toe-striking Toe striking (compared to heel striking) causes more inverted foot position/STJ and plantarflexed ankle joint at initial contact At forefoot contact, posterior leg muscles undergo eccentric contraction (to slow the velocity of the heel hitting floor). Hence, rest of body descends more slowly towards ground under control of anti-pronators/posterior leg muscles. Force produced at initial contact is dissipated over a longer period of time Transient stress wave produced through tibia is attenuated Other considerations Toe striking: facilitates increased velocity of running and shock attenuation BUT.. muscular demand (fatigue) and possible pathology within foot (forefoot strikes ground and is decelerated abruptly) ?Accelerometers placed along metatarsal shafts? Passive mechanisms of shock attenuation Body tissues acts as passive shock attenuating devices through their viscoelastic behaviour (Pratt ; Clin Biomech 1989; 4(1): 51-7)
Calcaneal fat pad
Effect of foot type on heel strike transient Ogon et al. (1999)
Artificial shock attenuation 'The support surface, footwear/insoles, and the body constitute a three component system of which the body is the dynamic adaptive component...Physical tests of shoe components including innersoles are not valid predictors of their effects on human locomotion. This is true of shock attenuation materials in which case wide variations in density fail to have predicted effects on shock attenuation.' (Frederick (1986). Journal of Sports Sciences 4: 169-184)
Effects of:
Artificial Shock Attenuation: Insoles Poron (PPT) & Plastazoate® are commonly used as top covers for foot orthoses or as shoe inserts PPT reduces transient stress wave by 15% Plastazoate® is ineffective (Pratt. Long term comparison of some shock attenuating insoles. Prosthetics & Orthotics International 1990: 59-62) Viscolelastic shoe inserts have been shown to reduce
Artificial Shock Attenuation: Surfaces The transient stress wave produced at heel contact is influenced by the surface Accelerations (Ac) grass < Ac asphalt = Ac synthetic track. Accelerations measured at the hip and head remained constant across surfaces showing body protects the trunk and head from large shock waves by dampening the applied load in the lower extremities (Unold. Acceleration on different surfaces. Track Technique 1977; 69:211) Artificial Shock Attenuation: Shoes Amplitude of transient stress waves (accelerometers attached to tibia) reduced by 36% in shod (leather soled shoes, athletic shoes) conditions compared to the barefoot condition (LaFortune & Henning. Clin Biomech 1992; 7: 181-184). halved (from 5 g to 2.5 g) with compliant "shock absorbing" heels compared to barefoot reduced by a further 10% in athletic shoes compared to leather soles shoes (LaFortune & Henning. Clin Biomech 1992; 7: 181-184) Suggests that kinematic control & confinement of heel pad (leather soled shoes) as well as compliant shoes (athletic "shock absorbing" shoes) increase cushioning in vivo Impact Reduction: Shoes Athletic shoes are often promoted and purchased for their superior cushioning properties Do more expensive athletic shoes reduce injuries? 'The notion that impact can be moderated through the use of cushioning (reduction of the amplitude of impact forces) is invalid in humans' (Robbins et al. Do soft shoes improve running shoes. Biomechanics April 1998: 47-55. Impact (vertical GRF) remains constant when humans run/jump with cushioned midsoles of Shore A 30 or harder regardless of manufacturer, cost, model. Shoes with relatively soft midsoles (Shore A < 30) are associated with increased impact Effects of such shoes on heel strike transient (shock attenuation) is unclear Relationship between cushioning, stability, and impact forces (Robbins Hypothesis) Summary
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