Force Platform Flashcards
What is a force platform? What are its specifications.
Instrument that measures ground reaction forces
Has a load cell in each of 4 corners, which are sensitive to forces along X Y Z axis (made of piezo electric or strain gauge components)
Bolted to concrete or portable baseplate
Measures forces 10-10,000N
Accurate to 2% of measuring range used
High natural frequency of >200hz
What are the variables measured on a force platform?
Fx, Fy, Fz (reaction force along co-ordinate axes)
My (friction torque about vertical axis)
Ax, Ay, (coordinates identifying point of force application or CoP)
What are the typical outcomes for force platform? Assuming Vy = 0 and Vy not = 0. Draw the graph.
When Vy = 0, W + Fg = 0 hence force platform is measuring weight
When Vy not = 0, e.g. Running
Fx (lateral) > extreme variability
Fy (horizontal) > biphasic
Fz (vertical) > double peak, especially for heel strikers
What is the outcomes measured when Vy = >0? Briefly describe the outcome measured.
Ground reaction force, usually expressed in BW to enable within subject (given moment) and between subject comparisons and within subjects over time
What are the mainstream GRF variables measured for running especially?
Impact Active Braking Propulsive Loading Rate Impulse
What are the applications for the research done with regards to GRF variables and running?
Tibial stress fractures > due to increase in dynamic loading related variables
What are the mainstream GRF variables measured for jumping especially?
Rate of force development (RFD) Fz Flight time CoM height Elasticity index Relative Strength Index
What does RFD measure? How is it calculated.
Measure of explosiveness, observed in Stretching-Shortening Cycles
Force / time
Two options: Force can be peak in 20ms windows or any time sampling interval windows 0-100, 0-250 etc.
What are the differences between the 3 types of jumps usually measured?
Squat jump (no SSC) Counter movement jump (long SSC, >250ms) - Elasticity Index Drop jump (short SSC, <250ms) - Reactivity Strength Index
Can be measured by iphone apps like My Jump 2
Briefly describe the Squat Jump and the variables it measures
Concentric muscle activation
1/2 squat position (90deg knees flex), trunk erect and hands at waist
No countermovement > prevent elastic energy storage
Complete extension of legs at takeoff and landing
Through elevation (flight time & formula) > get capability of inferior limbs to produce explosive force
Briefly describe the Counter Movement Jump and the variables it measures
Concentric contraction to the movement inversion
Start with legs extended, then go to 1/2 squad position. Trunk erect.
Jumping action performed with strengthening and shortening of elastic element of muscle (limited as extensors only activated during inversion)
How is Elasticity Index calculated
[(CMJ-SJ)/SJ] x 100
Briefly describe the Drop Jump and the variables it measures
Performing vertical jump after drop from predetermined heights at maximum speed after landing impact
Strong intervention of stretching reflex
How do we determine power from the GRF variables? What variables are needed and what can we calculate?
Power Peak and Power Mean can be estimated.
Variables needed: Vertical jump height (CoM Height in cm), Subject Height (cm), Subject Weight (kg),
Does vertical jump differ between athlete groups?
VJ was higher for sprinters and long jumpers compared to soccer, tennis & endurance athletes
Endurance athletes lowest VJ but RSI similar to soccer and tennis
What are the findings comparing between SJ and CMJ? What are the major factors differentiating?
Bottomline: Subjects able to jump higher in CMJ compared to SJ
- CMJ > storage and reutilisation of elastic energy > enhance performance
- CM > greater joint moments at push off
- CMJ > muscles higher level active state
- CMJ > as a result more work produced
What are the updated differentiating factors between SJ and CMJ performance differences?
Main:
1. Reduction of muscle slack in CMJ
Higher muscle stiffness > muscle fibers shorten at slower velocity > increase force producing capability
Stretching muscle-tendon unit > slack decreased
Higher GRF in CMJ > higher stiffness
2. Higher stimulation prior to shortening in CMJ (shown in EMG)
How should the main factors differentiating CMJ and SJ be trained?
Stiffness > movements under time pressure and applying unstable loads and surfaces
Muscle stimulation > coordination training
What are the 6 minor factors differentiating CMJ and SJ?
Fascicles passively lengthen during downward phase of CMJ
Potentiation during CMJ is greater
Stretching reflex activated during CMJ (firing rate or more MU recruited)
Minimal diff in kinematic patterns (during SJ, some do small CMJ)
Less lowering of CoM in SJ if not controlled for > smaller ROM over which force is produced
Elastic energy: stored during downward phase and reutilized during upward phase > increase force production
What are the findings for GRF and Running speed?
Vertical GRF (Fz) mean increases with running speed
What are the findings for GRF and Forefoot/Heelstrike running?
Heelstrike: Heel-ground collision > significant impact transient, nearly instantaneous large force > sent upwards to body through skeletal system
Forefoot: Forefoot-ground collision > minimal impact force > no impact transient
What are the implications for findings of Impact-deceleration mechanism?
Increased GRF on impact: greater limb velocities before foot contacts ground, exploit a motion-to-force mechanism during the impact portion of the contact time.
Lower time of contact on deceleration: stop limb more abruptly on contact
Results in rapid rising edge of the force-time relationship and a peak force that occurs well before the mid-point of the contact period
How is GRF related to musculoskeletal injuries?
GRF <2 BW = low
2< GRF <4 BW = moderate
>4 BW = high
How does GRF change in land and water?
For walking, GRF higher on land than in water
GRF decreases as depth increases
How to calculate GRF landing forces after CMJ jumping given landing acceleration in g.
How to calculate GRF for planche position given mass, distance hands-feet and CoM-feet.
Refer to slide 15
