Biomechanics of Resistance Training

Concentric muscle action

muscle shortens because the contractile force is greater than the resistive force

the forces generated within the muscle and acting to shorten it are greater than the external forces acting at its tendons to stretch it

eccentric muscle action

muscle lengthens because the contractile force is less than the resistive force

forces generated within the muscle and acting to shorten it are less than external forces acting at its tendons to stretch it

isometric muscle action

muscle length does not change because the contractile force is equal to the resistive force

sagittal plane

left and right

frontal plane

front and back

transverse plane

upper and lower

agonist

most directly involved in bringing about movement

prime mover

antagonist

muscle that can slow down or stop movement

mechanical advantage

ratio of moment arm through which an applied force acts to that through which a resistive force acts

greater than 1 = applied muscle force less than resistive force

less than 1 = disadvantage

first class lever

muscle force and resistive force act on opposite sides of fulcrum

second-class lever

muscle force and resistive force act on the same side of fulcrum

muscle force acting through longer moment arm than resistive arm

req. muscle force is smaller than the resistive force

third-class lever

muscle force and resistive force on same side of fulcrum

muscle force moment is shorter than resistive force

mechanical disadvantage

mechanical advantage of the patella

patella maintains the quad tendon’s distance from the knee’s axis of rotation

without the patella, the tendon would be closer to the knee’s center of rotation —> shortening the moment arm through which the muscle force acts —> reduced mechanical advantage

mechanical advantage can change through ROM

elbow flexion:

• when the moment arm is shorter, there is less mechanical advantage

• at full extension and full flexion the perpendicular distance from the joint axis of rotation to tendon’s line of action is shorter/smaller

as weight is lifted, the weight moment arm (M) — that resistive torque acts through — changes with horizontal distance from weight to hinge

many injuries happen due to exceeding force capacity for a give _____ state

for a given lengthened state

Biomechanical factors in strength & power: Neural Control

affects max force output by determining number of motor units involved in contraction

Biomechanical factors in strength & power: Muscle cross sectional area

force a muscle can exert is related to its cross-sectional area (not volume)

this is because there is increased # of sarcomeres arranged in parallel

Biomechanical factors in strength & power: Arrangement of fibers

increases cross sectional area can change angle of pennation, increasing more force

Increased pennation angle → allows more fibers to be packed into the same muscle volume → increases physiological cross-sectional area → increases maximal force-generating capacity (despite the cosine reduction per fiber).

pennate muscle

muscle with fibers aligned obliquely with the tendon

unipennate: tibialis posterior

bipennate: rectus femoris

multipennate: deltoid

angle of pennation

angle between muscle fibers and an imaginary line between muscles origin and insertion

0 degrees means no pennation

Biomechanical factors in strength & power: Muscle length

at resting length: actin and myosin filaments next to each other, max # of cross bridges available

contracted: actin filaments overlap, reduced cross-bridge sites available, decreased force generation capacity

stretched: smaller proportion of the actin and myosin filaments next to each other, fewer cross-bridge sites available, can’t generate as much force

front squat vs backsquat: mechanical advantage

front squat: shifting weight/center of gravity to the anterior aspect lengthens the moment arm at the knee, forcing the knee extensors (quads) to generate more torque to control/produce the movement

back squat: shifting weight/center of gravity to the posterior aspect (more forward flex), mechanically advantages the hip muscles by lengthening the hip moment arm

• the knee moment arm is shortened, thus hip extensors (glutes/hamstrings) must produce more torque → more glute/hamstring activity

leg press mechanical advantage

seatback support removes the need for spinal stabilizer muscles to help stabilize the core/trunk

Biomechanical factors in strength & power: Contraction velocity

nonlinear — but in general, force capability of muscle decreases as contraction velo increases

Biomechanical factors in strength & power: joint angular velocity

muscle pulling on bone at a distance from axis of rotation

produces torque when attempting angular motion @ bones

produced by eccentric and concentric actions

strength

ability to generate the greatest force

power

force production * velocity = watts

force at a given speed

stretch shortening cycle

rapid stretch triggers neuromuscular response, muscle spindles help generate an elastic response

series elastic component + stretch reflex —> maximal increase in muscle recruitment over small amount of time

3 phases: eccentric, amortization, concentric

eccentric phase SSC

stretch of agonist muscle

elastic energy is stored in series elastic component

muscle spindles are stimulated

amortization phase ssc

pause phase

should be fast - must transition quickly to not lose energy

Type 1a afferent nerves synapse with alpha motor neurons

alpha motor neurons transmit signals to agonist muscle group

concentric phase ssc

shortening of agonist muscle fibers

elastic energy is released from the series elastic component

alpha motor neurons stimulate the agonist muscle group

relationship between force and velocity during eccentric muscle action

direct relationship

with a heavier weight, more force is needed to control

lighter weight requires less force and thus slower speed

relationship between force and velocity for concentric muscle action is

inverse

more weight, more force, slower speed

less weight, less force, faster speed

For concentric contraction: As velocity increases, the maximum force the muscle can produce _______

decreases

Eccentric muscle contraction: as velocity increases, eccentric force capability _____.

increases, eventually plateauing at/near 1RM

eccentrically, the muscle is acting as a brake — resisting and controlling a load moving faster than it's or lengthening

faster the load is moving, the more "braking force" is needed to control/decelerate it and transition back into the concentric phase

training + SAID principle

Specific Adaptations to Imposed Demands — force drops with speed concentrically; force rises with speed eccentrically

if you want to train force production at high velocities, you need to train specifically at that contraction type and speed — improving slow concentric strength won't automatically transfer to fast eccentric braking ability, and vice versa

the nervous system adapts to the speed and contraction type it is exposed to