Chapter 2 Biomechanics of Strength

agonist muscle

the muscle most directly involved in bringing about a movement

• also called the prime mover

antagonist

a muscle that can slow down or stop movement

sagittal plane joint movements

flexion and extension

frontal plane joint movements

abduction and adduction

transverse plane

• pronation and supination

• internal and external rotation

fulcrum/axis

pivot point of a lever

torque

degree to which a force tends to rotate an object about a fulcrum

muscle force

force generated by biomechanical activity

resistive force

force generated by a source external to the body

moment arm

distance from line of action of the force to the fulcrum axil

lever mechanics help explain _______

why some exercises feel harder at certain positions and why body position or where the load is located changes force and RPE (rate of perceived exertion)

first class lever

• a lever for which the muscle force and resistive force act on opposite sides of the fulcrum

  • example: atlanto-occipital (AO) joint

• muscle force and resistive force act on opposite sides of tthe fulcrum

• muscle force — fulcrum/axis — resistive force

second-class lever

• resistance is between the axis and the force

  • example: plantar flexion against resistance (e.g. standing heel raise exercise)

• the required muscle is smaller than the resistive force

• fulcrum/axi — resistive force — muscle force

third class lever

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

  • example: biceps curl, leg extension

• muscle acting through a movement arm shorter than that through which the resistive force acts

• fulcrum/axis — muscle force — resistive force

what is the most common type of lever in the body?

third class lever

topics of biomechanical factors in strength

• neural control

• arrangement of muscle fibers

• joint angle

• muscle contraction velocity

• strength-to-mass ratio

• body size

With neural control, muscle force is greater when:

• more motor units are involved in a contraction

• the motor units are greater in size

• the rate of firing is faster

Much of the training improvements in strength during the first few weeks can be attributed to ______

neural adaptations

Why can someone get strong after 2 weeks without significant muscle growth/hypertrophy?

more efficient neural adaptations

types of muscle contractions

• concentric

• eccentric

• isometric

concentric muscle action

muscle shortens because contractile force greater than resistive force

eccentrci muscle action

• muscle lengthens because contractile force less than resistive force

• occurs in lowerinf phase of exercise, keeps the weight from being accelerated down by gravity

• ex: negative training, lifting with both legs and lowering with one during calf raises

isometric muscle action

• muscle length does not change because contractile and resistive forces are equal

• ex: wall sit, knee extension

• rehab: pain inhibition

proprioceptors

sensory receptors located within joints, muscles, and tendons

golgi tendon organ (GTO)

• proprioceptors that are located in the tendon adjacent to the myotendinous junction

• acts as a tension detector during stretch and contraction

• sensitive of tension in the tendon

muscle spindle

provide information regarding length and rate of change in length

stretch reflex

• contraction of muscle that occurs in response to its stretch

• it is not controlled by higher functioning centre i.e. the brain

• a monosynaptic response that is transmitted to the spinal cord

• exx: knee jerk patellar reflex

propriorceptrs, GTO, muscle spindle, and stretch reflex are all related to ______

tissue flexibility

sources of resistance

• gravity

• weight-stack machines

when the weight is horizontally closer to the joint, it exerts _____ resistive force

less

when the weight is horizontally farther from a joint, it exerts ____ resistive torque

more

advantages of free weights to machines

• whole body training

• increasing bone density

• simulation of real life activities

weight-stack machines

gravity is the source of resistance, but machines provide increased control over the direction and pattern of resistance

bone/spine

• 85 - 90% of all disk herniation at L4-L5 (lowest two lumbar)

• 20% of asymptomatic people under the age of 60

is lordotic or more “neutral spine” posure more effective than rounded back posture?

• historically taught to maintain lordosis or “neural spine”

• more important to avoid changing trunk position throughout lift

• current evidence suggests that load tolerance and movement consistency may be more important than mainiting a perfectly neutral spine

Why is bracing important?

it may matter biocmechanically because it increases trunk stiffness and helps transfer force between the upper and lower body

bracing sequence recommendation

• get in a position that you feel stable

• inhale through the belly/diaphragm

• stay locked in; try to maintain similar trunk/postural control throughout the lift

  • ex: do not start in a neautral trunk position and then flex foward mid lift during squat/deadlift

intra-abdominal pressure and lifting belts

• the “fluid ball” aids in supporting the vertebral column during resistance training

• weightlifting belts are “probably” effective in improving safety. follow conservative recommendations

• can weaken abdominals if over-used (conflicting evidence- likely no)

• avoid the valsalva maneuver (holding your breath)

the shoulder

• has the greatest range of motion of all joints in the human body

• less stable than the hip

stability of the shoulder depends on

• glenoid labrum, joint synovium, rotator cuff muscles (2), and pectorals

• the structures can rub/impinge on each other, which may influence tendonitis

critical zone of impingement

• anatomical area between the head of the humerus and the acromion process of the scapula

  • may be impinged/rubbed on with overhead movement

  • may influence wear and tear/degernation of supraspinatus tendon

    • is more likely related to poor blood supply and vascularity in combination with aginig, genetics, and varying anatomy

how to avoid shoulder pain in the area of impingement

maintain shoulder strength, genetics, and bone structure may play a role

the knee

• overuse injuries tend to be related to multiple factors

• strains of the hamstring

• plant/twist structural injuries

• knee wraps

overuse injuries of the knee tend to be related to multiple factors

• training related errors

• biomechanics

• load intolerance

strains of the hamstring

• related to eccentric or concentric component?

  • concentric is more common

plant/twist structural injuries

• ACL

• MCL

• meniscus

knee wraps

• commonly used in powerlifting

• no avaliable evidence that supports wraps protecting the knee against injury, however, they have been shown to improve performance by the spring effect, adding an additional 25lbs on average

• anecdotally, knee wraps may increase squat lifting force by 5-10%, more than 70 lbs in some cases