Neuromechanics of Striking & Throwing PDF

Neuromechanics of Striking & Throwing DR JON SHEMMELL MEDI258: HUMAN NEUROMECHANICS Learning objectives: Striking & Interception u Understand how ‘summation of speed’ underlies the kinematics of throwing and striking actions u Understand the role inertia plays in determining the velocit...

Neuromechanics of Striking & Throwing DR JON SHEMMELL MEDI258: HUMAN NEUROMECHANICS Learning objectives: Striking & Interception u Understand how ‘summation of speed’ underlies the kinematics of throwing and striking actions u Understand the role inertia plays in determining the velocity with which a struck ball will travel u Moment of inertia u Inertial interactions u Understand the structure and function of the major descending neural pathway that transmits the motor commands for throwing and striking actions Mechanical principles for throwing/striking There are two major principles designed to predict optimal movement mechanics during throwing and striking actions: u Optimal coordination of partial momenta (Van Gheluwe and Hebbelinck, 1985) u to achieve maximum speed at the distal end of an open-linked system, the angular speeds of all segments should reach a maximum value at the same time. u Summation of speed (Bunn, 1972) u to maximize the speed at the distal end of a linked system, the movement should start with the more proximal segments and progress to the more distal segments such that each segment starts its motion at the instant of greatest speed of the preceding segment and reaches a maximum speed greater than that of its predecessor. Excellent article: Putnam C (1993) Sequential motions of body segments in striking and throwing skills: Descriptions and explanations, J Biomech, 26(Supp 1) Maximising throwing/striking speed E.G. BASEBALL PITCHING Question: How can we maximise throwing/striking speed?  Wind up Leg lift u The front hip rotates closed to a u COG is raised and balanced over 90° angle or more, while keeping standing leg the weight back over a fairly straight firm posting leg to maintain balance u The knee should be angled back slightly over the rubber toward second base, which closes off the hips. u A good rule of thumb for most pitchers is that maximum knee height is somewhere between 60% and 70% of a pitcher's body height. http://www.youthpitching.com/mechanics.html  Stride u Point of maximum lead knee u Foot contact rapidly brakes height to the point of foot contact leading leg motion, inducing rapid rotation of hips and trunk u COG is lowered and accelerated towards home plate u Approximately 50% of ball velocity in the pitching motion is the result of forces accumulated in the stride and trunk rotation u Stride time determines force impulse, so long strides are preferred u Pelvis rotates at ~ 700°/s, producing spinal rotation http://www.youthpitching.com/mechanics.html  Arm cocking Late Cocking u The late cocking phase is initiated as the lead foot contacts the ground and ends with the maximum external rotation (MER) position of the throwing shoulder u Deceleration of trunk translation and trunk rotation combine with reduced biceps activation to induce rapid external shoulder rotation (~2300°/s) u Soft tissues around the shoulder and elbow joints are stretched (ulnar collateral ligament) http://www.youthpitching.com/mechanics.html  Arm acceleration u Arm acceleration phase of the pitching motion is initiated at the point of MER and ends at ball release u Large elbow extension with little/no triceps activity u Deceleration of the humeral horizontal adduction results in rapid internal rotation and of the shoulder (~9000°/s) u This motion is one of the fastest human physical movements in sports activity. http://www.youthpitching.com/mechanics.html  Deceleration (Follow through) u The deceleration phase begins at ball release and culminates with maximal dominant shoulder internal rotation u Has no bearing on ball trajectory, but motion may reflect any suboptimal mechanics http://www.youthpitching.com/mechanics.html Transferring momentum increases angular velocity L = I ! (angular momentum) u Muscle activation generates momentum in trunk u Transferring trunk momentum to smaller distal masses successively increases angular velocity u Further muscle activation can further increase momentum, but largely maintains joint integrity This is sometimes called ‘summation of speed’, ‘summation of force’ or ‘kinetic link principle’ Main Points (Summation of speed) u The ‘summation of speed’ theory describes effective throwing mechanics u To achieve maximum throwing/striking speed: u Angular velocity of each moving body segment should increase successively u The motion of proximal body segments with large relative mass is important for endpoint speed u Momentum should be transferred between moving body segments u Decreasing segment mass while maintaining momentum will increase angular velocity The role of inertia in striking mechanics E.G. BASEBALL HITTING Does momentum transfer apply equally to striking? Minimising moment of inertia during swing T = I! (torque) & I = mr2 u For a given amount of torque, angular acceleration will be greatest when the moment of inertia is minimised u During a swing, bat and body masses should be kept as close to the axis of rotation as possible u Maximises angular acceleration Maximising ball exit speed Conservation of momentum (bat + ball) >> m!i = m!f >> m!(bati) + m!(balli) = m!(batf) + m!(ballf) >> m!(ballf) = m!(balli) – m"!(bat) u Impulse/momentum relationship: >> m!(ballf) = m!(balli) - Ft(bat) u Ft = m"! >> m!(ballf) = m!(balli) - m#t(bat) So, maximum ball exit velocity depends on: 1) Ball velocity just before impact (slower generally better) 2) The deceleration of the bat during impact (less deceleration better) Minimising bat deceleration at impact T = I! (torque) u During a collision, bat deceleration will be minimised when the moment of inertia is maximised F = ma (force) u Force application (and therefore bat acceleration) during impact will counter deceleration due to ball force Main Points (The role of inertia in striking mechanics) u For maximum bat speed (and therefore momentum), moment of inertia of the bat and body mass should be minimised during the swing u Hitting does not follow the principle of summation of speed as closely as throwing, primarily due to the need to counteract the force of the ball during impact u The principle of “optimal coordination of partial momenta” may apply to hitting, but it does not appear to describe throwing mechanics well Cortical control of striking/throwing Organisation of the M1 Primary motor Cortex (M1) u Location of organisation of voluntary motor activity u Motor cortical areas are clearly defined u Motor cortical areas are somatotopically organised Organisation of the M1 u Cortical representation is not proportional to size of body part but to functional demands of body part u Monkey - fingers/feet larger than body u Human - hand/ fingers/ face larger representation Activation of M1 neurons: during movement u Edward Evarts unlocked one of the secrets of motor control u Correlated activity of single neurons with specific motor behaviours u What do cortical motor neurons do during natural movements? u Encode movement amplitude? u Encode movement direction? Activation of M1 neurons: during movement Evarts: 1) No load Lever movement Results Movement starts Activation of M1 neurons: during movement Evarts: 2) Load opposes flexionLever movement Results Movement starts Activation of M1 neurons: during movement Evarts: 3) Load assists flexion Lever movement Results Movement starts Activation of M1 neurons: during movement u Edward Evarts u Neuron activity associated with specific muscles and force magnitude, not displacement u Neuronal activity changes about 100 ms before movement onset (preparatory set) Activation of M1 neurons: during movement u A single cortical cell can specify a preferred movement direction u Increased firing nearest to the cells preferred direction Time 0 = Movement onset Each raster = 1 trial Increased firing (90 to 225 degrees) Georgopolous, et al, 1982 Activation of M1 neurons: during movement u A population of cells can specify many movement directions Population vector Movement direction Activation of M1 neurons: during movement u Cells change their level of firing according to the proximity of desired movement to their preferred direction u Populations of cortical neurons encode movement direction Activation of M1 neurons: during movement u Motor cortex neurons specify muscles to be activated and the magnitude of force u Populations of motor cortex neurons encode direction of multi- joint movements Summary u Primary motor cortex (as well as other parts of the motor system) is somatotopically organised u Individual neurons in the primary motor cortex appear to be associated with the activity of specific muscle groups or directions of force application u Populations of motor cortex cells fire according to the direction of movement, even in multi-joint tasks u May specify movement direction u May be linked to the activation of muscles required to move in different directions Pathways descending from the primary motor cortex The pathway from the cortex to limb muscles u The corticospinal tract represents the simplest (often monosynaptic) and fastest route of transmission of cortical commands to limb muscles u Pyramidal tract neurons project to both proximal (via anterior tract) and distal muscles (via lateral tract) u Major transmission pathway for dexterous actions requiring cortical planning, such as throwing and striking Activation of M1 neurons: during movement u The firing rate of corticospinal tract neurons is closely related to the muscle and joint torque produced u This demonstrates the direct involvement of the corticospinal tract in producing limb movement Probing motor system function in humans u Magnetic stimulation of the cortex can be used to assess integrity of corticospinal pathways u Activates pyramidal tract neurons (PTNs) indirectly u PTNs activated project to specific muscles, in which a twitch response can be recorded Probing motor system function in humans Main Points (Neural control of striking/throwing) u Pyramidal tract neurons in the motor cortex project directly (and indirectly) to alpha motor neurons in the spinal cord u Provide a mechanism for control of precise and dexterous movements u The function of this pathway can be examined in humans using magnetic brain stimulation u Indirectly activates pyramidal tract neurons