stre&cond: chapter 2 - biomechanics of resistance exercise

terms

fulcrum: pivot point — ex: elbow joint

resistive force (Fr): weight/resistance you are trying to move — ex: dumbbell in hand

muscle force (Fr or Fa): internal effort/pull generated by your body to move against the resistance — ex: biceps muscle contracting and pulling on forearm bone

MFr: straight line distance from pivot point to the heavy weight

• longer distance = harder work

MFm: straight line distance from pivot point and the line of pull of working muscle — ex: from fulcrum to bicep tendon on forearm

• longer distance = easier work

torque: force x moment arm

first class

lever where Fm and Fr act on opposite sites with the fulcrum in the middle

• Fm — Fulcrum — Fr

• produces very high or very low forces

• moment arms depend on fulcrum placement

• force vs speed/ROM is balanced and depends on setup

• ex: tricep extension

  • fulcrum: elbow

  • Fm: tricep muscle pulls behind the joint

  • Fr: weight in hand

second class

lever where Fm is further from fulcrum than Fr on the same side

• Fulcrum — Fr — Fm

• always advantageous (> 1.0) — MAX force capability (less muscle force to lift heavy weights)

  • sacrifices speed and ROM

• ex: calf raise

  • fulcrum: toes

  • Fm: gastrocnemius pulling at the heel via achilles tendon

  • Fr: body weight transferring down thru leg bones in the middle

• ex: flip a tire end over and over

  • fulcrum: bottom edge of tire touches ground acts as fixed pivot

  • Fm: person lifts upward at opposite end

  • Fr: entire weight of tire acts downward between pivot and lifter

third class

lever where Fr is further from fulcrum than Fm on the same side

• Fulcrum — Fm — Fr

• always DISadvantageous (< 1.0) — MAX speed and ROM

  • sacrifices force

• most common in the human body

• ex: bicep curl

  • fulcrum: elbow joint

  • Fm: biceps tendon on radial tuberosity

  • Fr: dumbbell in hand further out

mechanical advantage

ratio of the moment arm of Fa to the moment arm of Fr

• Mfa / Mfr

• > 1.0 — Mfa is longer than Mfr → need less muscle force to lift a heavy weight (high strength/force efficiency)

• ex: patella — increases perpendicular distance (Mfa) by pushing quads tendon further from fulcrum

ascending-descending strength curve (curve)

shows how joint angle directly impacts force capability during movement

bicep curl from bottom to shoulder:

• ascending: lower joint angles (0-70 degrees) — Maf short bc parallel to bone → mechanical disadvantage

• maximal torque: between 70-120 degrees for elbow flexion — Maf maximized → mechanical advantage

• descending: beyond peak flexion (120 to 180 degrees) and joint angle closes — Maf short → mechanical disadvantage

variable resistance machines (VRM)

specialized mechanisms to change the machine’s leverage against you through an exercise’s ROM

• matches machine’s rotational resistance (resistive torque) to body’s strength curve (torque muscles can produce at diff joint angles)

• load gets heavier where you are strong and lighter where you are weak

VRM pros

more resistance at points in the ROM → muscles could exert greater torque

• muscles can work against max force throughout the whole ROM

VRM cons

slow, controlled angular velocity — moving too fast allows momentum to take over and ruin leverage

angles vary widely between athletes — standard cam rarely aligns perfectly with their actual strength curve

strength

capacity to exert force at any given speed of movement

• slower speeds, greater force output

• force = mass x acceleration

  • higher mass = less acceleration (change in velocity over time)

• “how much can you lift?”

power

the time rate of doing work

• = force x velocity

• = work/time

• not the explosive movement, but can be calculated during slow or fast movement

• work: product of force exerted on an object and the distance it moved — force x displacement

  • training is individualized: taller lifters perform more work

1 RM

max strength: 1 RM on heavy loads — moving near-max weight forces slow speed

• ex: power lifting, squat, deadlft — max force output

max power: lighter loads — bar can accelerate rapidly

• most athletic sports

• ex: weightlifting, snatch, clean and jerk — rapid force production

biomechanical factors

in human strength

• neural control: NS signals the muscle

  • recruitment: which and how many motor units involved

    rate coding: rate at which motor units fire

• muscle cross-sectional area: bigger muscle has more contractile units = more force

• joint angle: moment arms, optimal length for peak force, muscle insertions, lever

• muscle fiber arrangement

• muscle length

• muscle contraction velocity and joint angular velocity (force velocity curve)

muscle fiber arrangement (MFA)

relates to angle of pennation

• angle between muscle fibers and imaginary line running between muscle’s origin and insertion

greater pennation

greater angle: fibers packed diagonally side by side

• fits more muscle fibers in parallel to a smaller space

• = greater force production

lesser pennation

lesser angle: fibers run nearly parallel in a line directly between origin and insertion

• fibers pull together in series

• fits fewer fibers in parallel — lower force output

• = greater shortening speed and larger ROM

length tension relationship

max force production is achieved at a particular point in the ROM

• the POINT: optimal actin and myosin cross bridge / optimal Maf and Mrf (>1.0 mechanical advantage)

• ascending-descending curve

force velocity relationship

how force (torque) of a muscle can produce changes based on SPEED and DIRECTION

• torque (y): rotational force the muscle is producing around a joint

• joint angular velocity (x): how fast the joint is moving

• concentric zone

• isometric point

• eccentric zone

eccentric

muscle lengthening against a heavy load = PEAK torque

• Fr > Fm

• highest force

• negative angular velocity (low speed)

concentric

muscle shortening

• as joint angular velocity increases = less torque

  • slower speed: more time for cross-bridges = high torque

  • higher speed: less time for cross-bridges = low torque

• Fm > Fr

• positive angular velocity (higher speed)

isometric

muscle length stays the same

• machine controls and maintains constant movement speed while measuring torque

• 0 degrees/s angular velocity

• Fm = Fr