Biomechanics of Exercise Training Flashcards
The Moment Arm Concept
T = F d (moment arm - derived from displacement)
- A moment arm determines the force to produce or prevent the rotation of an object around an axis.
- Perpendicular distance from line of action of force to joint of interest
Low bar position + high bar position = back squat
- To be able to hold this low bar position safely = person has to lean the trunk forwards more = shifts the weight of the bar
- In each the low + high bar case = the gravitational force vector = both acting vertically downwards = both have different perpendicular distances from the pivot points i.e. hip + knee joint
- High bar = hip moment arm less than low bar
- Conversely there is decreased knee moment arm in the high bar compared to the low
- Would assume in the low bar position = greater hip torque, greater muscle activity say in the glut max, less around the knee joint = opp is true for the high bar position
- Have changed joint loading, by changing the moment arm
External Moments in Gait: Knee Joint
- Knee Internal Moment
○ Acts actively to further promote knee flexion
○ Controls rate of flexion – eccentric muscle action - The external moment is determined by this GRF vector = external GRF, applies this external moment + it will tend to cause the knee to flex
Muscle Moment Arms about Knee
Forces in frontal plane = aren’t really there to produce motion but are there to resist motion
Clinical Example: Medial Compartment Loading
- If there are large external moment arms like in the image, knee adduction moments = they will cause compressive forces on the medial compartment of the knee = + that can lead to knee osteoarthritis on the medial side
- And anything that increases the GRFvector = i.e. a heavy gait, a high body mass = these to will increase the joint loading / joint torque on that medial side of the knee
Knee Osteoarthritis
- A degenerative joint disease that affects the knee
- Progressive loss of articular cartilage
○ Primary (unknown specific cause) - age, wear + tear, etc.
○ Secondary (known specific cause) – concentrated loading, trauma, abnormal articular cartilage - Knee OA is its own disease: cartilage degradation enzymes higher in OA
Knee Osteoarthritis - interventions
- Two major types of intervention:
○ Physiotherapy: Physical therapy includes muscle strengthening + increasing joint mobility in the hips
○ Biomechanics: Medial compartment off-loading
§ Gait modification – ipsilateral trunk lean, medial “knee-thrust” gait, toe-out gait, etc.
§ External intervention - valgus bracing, orthotic wedge, contralateral cane, etc - Can essentially remove the loading = reduce the GRF itself = so losing weight is an important consideration + also gait modification
- All those gait modifications = all based on the fact that you are actually changing the moment arm
Sports Example: ACL Injury
- Knees in valgus, especially a lot of valgus, can predispose people to a higher ACL injury risk = have to take special measures in terms of their landing technique + their strengthening in order to be able to minimise that risk
- Is an external knee abduction torque = what causes the problem = so having your segments aligned during high impact forces is the principle to take from that
- The other thing is: land on two legs = as half the force theoretically, at least not the full force, will be taken by both knee joints = so try to avoid one-legged lands
- Athletes also practice one-legged lands just in case they have to = so practice technique as well
LANDING TECHNIQUE
- Land balanced
- Land w/ both feet in control
- Keep anatomically neutral structures wherever possible
- Strengthen your muscles = get them used to dynamic loading
= WILL INCREASE YOUR SAFETY
Risk Factors: ACL
- Dynamic (during activity) risk factors:
○ More extended knee (extreme: knee hyperextension)
○ Excessive external GRF valgus moments
○ Stress on the ACL is the greatest w/ internal tibial torsion near full knee extension - Anatomical risk factors (traits):
○ Knee valgus
○ Knee Hyperextension
○ Intercondylar notch size
○ Integrity of the menisci
INTERNAL MOMENT ARM CHANGING
- THE INTERNAL MOMENT ARM = is the muscle moment arm
- The muscle moment arm is not constant, even external moment arm is not constant = varies as move through the motion
- Muscle moment arm can see in graph changes quite significantly = + that will effect the amount of force that, that muscle is going to produce, will effect joint torque about the hip joint
- Systems are linked = that will effect knee joint as well
- Somewhere around mid-point, not all the time, the muscle will get close the the max internal moment arm length
EXTERNAL MOMENT ARM CHANGING
- External moment arm changes as well
- When in anatomical position = your line of action of force is acting straight through the joint + hence there is no external joint reaction moment / no external moment arm = don’t require any muscle torque to hold that position - will need your linear force, the joint reaction force will be there but there is no actual muscle torque
- Get to next position, say 45-60° = the moment arm increases = the external load is much higher - the force will be the same, what does change is the external moment arm, muscle moment arm will also change but very small compared to the external moment arm = have to apply more shoulder torque to maintain this position
- When holding horizontally = 90° = then your external moment arm is the longest = harder because the external torque is much higher
Determine muscle group
- Consider torque, not the direction of movement
- Eg Knee action during a Squat
Why is the task so much easier when commencing at a knee angle of 90° instead of 120 °?
= LEG PRESS
Two reasons:
1. Looking at moment arms: in the 90° start there is a knee external flexion moment + also a hip flexion moment there, but when flex excessively (120° start) = ankle joint is basically level or below the level of the hips = start to push downward so GRF is upwards = external hip flexion moment is very large = outweigh the benefit of the knee extension moment and your hips actually come off the seat as it tries to rotate you anti-clockwise
Knee extension strength changes w/ joint angle
* Also other thing to realise w/ 120° start = don’t get ideal muscle fibre overlap = so get a slightly weaker force as well
* Also optimal fiber length for muscle force production
* Optimal fibre length can see is at an angle of around 30°
* Way past optimal fibre length at 120° = muscles very elongated, the overlap is not optimal, can’t get as much interaction b/w the fibres
HIGH BAR vs LOW BAR POSITION
- Will find in the low bar position = the posterior chain muscles of the leg i.e. Glut max, biceps fem, lumbar erector spinae = is consistently higher = what we would expect as the moment arm to the hip joint is longer
- In the concentric phase, your muscle activity is much higher (2-3x higher)
- Are some differences but not significantly greater
- Also not much difference in quadriceps activity = interesting
EMG: Results
* EMG bioelectrical activity during eccentric phase for all selected muscles were significantly higher during LBBS than in HBBS (60% 1RM and 65% 1RM).
○ During 70% 1RM squat test, those differences were also significant, except for RF and VLO
* During the concentric phase, a significantly greater muscle activity was observed during LBBS for LES, GM and BF for all tested loads.
* Posterior muscles of lower extremities - hip extensors, were considerably more activated during LBBS compared to HBBS.
* For knee extensors, such differences were negligible + during 60% 1RM insignificant
What determines the level of effort in a task?
- External force to be applied
- Moment arm of external force about a joint
○ Can decide where want to start the exercise depending on how you want to load that joint = important implication + applications for injury rehab = often don’t want to go through full ROM = especially where there is pain - Strength of muscles crossing that joint
- Moment arm of muscles crossing that joint (varies with joint angle)
- Length of muscles during task (affects muscle force)
- Velocity of contraction (R/s is complex)
