Week 6 - Locomotion 3 BW PDF

Locomotion III DR JON SHEMMELL MEDI258: HUMAN NEUROMECHANICS Learning objectives: Locomotion III u The walk/run transition: phase transitions and hysteresis u Running mechanics: the control of leg stiffness u Dynamical systems model for locomotion The walk-to-run transition  When do...

Locomotion III DR JON SHEMMELL MEDI258: HUMAN NEUROMECHANICS Learning objectives: Locomotion III u The walk/run transition: phase transitions and hysteresis u Running mechanics: the control of leg stiffness u Dynamical systems model for locomotion The walk-to-run transition  When do you choose to run? u As our speed of locomotion increases, we make a spontaneous transition from walking to running u For most people, the Preferred Transition Speed (PTS) occurs at ~2 m/s or 7.2 km/h u Interestingly, our PTS does not occur at a speed where the Energy energetic cost becomes greater Optimal for walking than running Transition Speed u So, why do we start to run at 2 m/s? Changes in GRF during walking at increasing speeds u Increasing the speed of walking: u Increases GRF during loading phase u Decreases GRF during pre- swing/propulsion phase u Increases posterior GRF during loading phase (to a point) u Increases anterior GRF during propulsion phase (to a point) (Walking speed on graph shown as % of preferred transition speed) Do joint kinetics trigger our gait transition? u Walking requires produces greater plantarflexion torque than running across locomotion speeds u Greater ankle dorsiflexion, knee flexion and hip extension are required during the swing phase of walking above the PTS u Could the effort required to increase muscle activation around the ankle, hip and knee trigger the transition to running? Prilutsky & Gregor, 2001 Muscle mechanics may also play a role u Neptune and Sasaki, 2005 u Recorded EMG from 9 muscles u Modelled each muscle to estimate the force produced u Question: does the transition to running improve the force generating capacity of specific muscles?  Muscle mechanics may also play a role Increased walking speed: ⇧ deviation from optimal length (plantarflexors) ⇧ muscle activation (all muscles) ⇧ shortening velocity (all muscles) ⇩ plantarflexor force Muscle mechanics may also play a role u Transitioning to running: u Slightly increased plantarflexor and dorsiflexor activation u Slightly increased dorsiflexor force output u At least doubled plantarflexor force output u Conclusions: u Muscle mechanics make plantarflexor force production more difficult at high walking speeds u Running improves the mechanical situation for plantarflexors Running mechanics Jogging & running The running cycle: u stance phase u airborne phase (Hay, 1993, p 402) Jogging & running Stride length (SL) in running: § take off distance § flight distance § landing distance Take-off distance (Hay, 1993, p 399) Stride rate (SR) in running: § time in contact with ground § time in air Running vs walking: § airborne phase § increased flexion at joints § amount of energy expenditure § increased SL and SR Running kinematics Direction of force application u Foot contact in front of CoM will produce a horizontal force opposing the direction of motion (braking force) u Removing this force through altered foot placement and active hip extension increases CoM velocity Force application during sprinting Running and leg stiffness u Running can be modelled as a spring-mass system u Spring compression indicated by the vertical change in the CoM u Leg (spring) stiffness will largely dictate: u Duration of the stance phase u Vertical displacement of CoM u How rapidly force can be applied during stance phase u Stiffness u K = Force (Fg,y) / Displacement (y) Why is stiffness important? u To maintain speed running speed most efficiently, compression of the spring (leg) must be optimised u High leg stiffness allows u Ground reaction forces to be opposed rapidly u Vertical and horizontal u Force absorption to be minimised u Time of force application to be minimised (stance phase can be shortened) u BUT, also increases vertical CoM displacement (bad) Regulating leg stiffness u Leg stiffness is affected by: u The magnitude of joint torques → muscle forces → muscle activity → motor unit recruitment u Stiffness of elastic soft tissues u Muscle u Tendon u Connective tissue u Stiffness in vertical and anterior- posterior directions important u Leg configuration u Larger GRF moments (torques) at each joint will require greater muscle force to counteract Sprint mechanics u Goal: Cover a given distance in u ↑ stride frequency What do these the minimum possible time u ↓ time on ground things do to muscle u ↑ leg stiffness mechanics? u ↓ rate of joint rotation u ↑ stride length u ↑ flight distance u ↑ speed of release (take off) u ↓ angle of release u ↓ air resistance u Release height = landing height u Minimise landing distance u Minimise braking force  Muscle force considerations See if you can detect something unusual in Bolt’s gait pattern! What are the limits of sprinting? How do these concepts relate to distance running? What do the arms do? u Leg swing creates a torque around the longitudinal body axis u Movement of opposite arm counteracts this torque u Arm movement therefore cancels the longitudinal torques, avoiding u Trunk rotation u Medio-lateral force generation Main Points (Running mechanics) u Limit braking force u Foot strike u Muscle activation u High, but not maximal, leg stiffness is critical for running economy u Stiffness of elastic elements u Muscle activation u Joint configuration u Differences between sprint and distance running u Reliance on passive forces for distance running Models of movement control DYNAMICAL SYSTEMS Dynamical systems theory u The number of biomechanical degrees of freedom of the motor system is dramatically reduced through the development of coordinative structures or temporary assemblages of muscle complexes (Turvey, 1990). u The reduced dimensionality/complexity of the motor system encourages the development of functionally preferred coordination or "attractor" states to support goal-directed actions. u Within each attractor region (the “neighborhood” of an attractor) system dynamics are highly ordered and stable, leading to consistent movement patterns for specific tasks. u Transitions between multiple attractor regions, however, permits flexible and adaptive motor system behavior, encouraging free exploration of performance contexts by each individual. Glazier, Davids & Bartlett, 2003 Dynamical systems model u Describes human movement as u Views movement variability as self-organising based upon limb necessary and valuable for task dynamics performance and learning u Some coordination patterns are more ‘stable’ than others and our system gravitates toward these stable states u Predicts that movement can emerge without the need for specific motor commands from higher centres u Predicts that specific ‘control variables’ dictate transitions between stable coordination patterns (e.g. walking-running) Stable coordination patterns Switching between coordination patterns can The brain may also be voluntary or driven by have stable and change in a ‘control unstable patterns of variable’. activity, interacting with limb dynamics to produce stable movement. Switching is driven by preferred modes of Speed operation of the (sensorimotor) system or by a need to meet task goals most effectively. Run Walk Speed Main Points (Models of movement control) u Hierarchical models u Dynamical system models u Predict a critical role for the brain u Emphasises the mechanics of the in specifying muscle and human musculoskeletal system movement commands u Predicts that coordinated u Predicts that these commands movement can occur without interact with a modulate the descending control expression of lower reflexes Ideas from both of these models have been integrated in more recent models of movement control - We will discuss more recent models in Module 3