ch2 - biomechanics of resistance exercise Flashcards
equation for work?
Work = Force * Displacement
equation for power in terms of work?
Power = Work / Time
equation for power in terms of force?
Power = Force * Velocity
what is the equation for force during isometric exertion or constant-speed joint rotation?
FM * MM = FR * MR … (FM = muscle force, MM = muscle moment arm, FR = resistive force, MR = resistive force moment arm)
Angular velocity is what?
the object’s rotational speed, measured in radians per second (rad/s).
how is torque expressed?
in newton-meters (N*m), but should not be confused with work, which is also expressed in newton-meters.
how does torque differ when a weight is horizontally farther or closer to a joint?
when the weight is horizontally closer to the joint, it exerts less resistive torque; when it is horizontally farther from a joint, it exerts more resistive torque.
how is the angle of pennation defined?
the angle between the muscle fibers and an imaginary line between the muscle’s origin and insertion; 0° corresponds to no pennation, many human muscles are pennated, but few have angles of pennation in excess of 15°
net work performed when a weight is lifted is equal what?
the magnitude of the weight (F1) plus the force (F2) required for a desired acceleration rate multiplied by the displacement (D) in which the weight is lifted upward
what is the patella’s main function?
to hold the quadriceps tendon away from the knee axis of rotation, thereby increasing the moment arm of the quadriceps group and its mechanical advantage
why are humans at particular risk for back injury?
The advantage we gain from our upright posture and free use of the arms and hands is accompanied by the disadvantage of having our intervertebral disks under compressive force even when we are merely standing, sitting, walking, or running—and under even more compressive force when we are lifting and carrying. When we are in a standing position, any force we exert with the upper body must be transmitted through the back to the legs and ground. In addition, the back muscles act at a great mechanical disadvantage and must generate forces much greater than the weight of an object lifted.
muscle action
forces are generated within the muscle that pull the muscle’s ends toward each other if not prevented from doing so by external forces
concentric muscle action
forces generated within the muscle and acting to shorten it are greater than the external forces acting at its tendons to stretch it
swimming and cycling involve what muscle actions
concentric muscle action almost exclusively
eccentric muscle action
muscle lengthens because the contractile force is less than the resistive force
is the work done in eccentric actions negative?
no, the eccentric force exerted by the muscle keeps the weight from being accelerated downward by gravitational force
why is the knee is prone to injury?
because of its location between two long levers (the upper and lower leg)
in what plane do flexion and extension about the knee occur?
almost exclusively in the sagittal plane.
ligamentous and cartilaginous stabilizing structures prevent rotation in what plane(s)?
frontal and transverse
what does the stability of the shoulder depend on?
glenoid labrum, the joint synovium, and capsules, ligaments, muscles, tendons, and bursae. the rotator cuff muscles (supraspinatus, infraspinatus, subscapularis, and teres minor) and the pectorals are particularly instrumental in keeping the ball of the humerus in place.
when a football player is hit at midleg from the side while the foot is planted firmly on the ground – what kind of torque is this?
frontal plane torque on the knee
in what sports would strength-to-mass ratio be most important?
sprinting and jumping, and in sports involving weight classification.
why does strength-to-mass ratio not hold for larger athletes?
when body size increases, muscle volume (and concomitantly body weight) increases proportionately more than does muscle cross-sectional area (and concomitantly strength)
what is the classic formula?
load lifted is divided by body weight to the two-thirds power (accounts for relationship of cross-sectional area vs volume)
where do 85% to 90% of all intervertebral disk herniations occur?
at the disk between the lowest two lumbar vertebrae (L4 and L5) or between the lowest lumbar and the top sacral vertebra
two factors of neural control of muscles?
recruitment and rate coding. which and how many motor units are involved in a muscle contraction (recruitment) and the rate at which the motor units are fired (rate coding).
example of eccentric muscle action to create force?
cheating a bicep curl
isometric muscle action
the contractile force is equal to the resistive force
function of abdominal muscles during curl-up
the abdominal muscles act concentrically during raising phase of curl-up and eccentrically during the lowering phase, even though they act isometrically in a sit-up with the trunk held straight
the square (second power) of a linear body dimension is related to what?
muscle cross sectional area
the cube (third power) of a linear body dimension is related to what
muscle mass proportional to volume
the classic formula may favor what kind of athletes?
athletes of medium weight
why might favoring athletes of medium weight be unbiased anyway?
because of the bell-shaped curve describing the normal distribution of anthropometric characteristics among the population, the body weights of a majority of people are clustered close to the mean
most common sources of resistance for strength training exercises?
gravity, inertia, friction, fluid resistance, and elasticity
what kind of resistance do high-acceleration (explosive) exercises provide?
greater resistance to the muscles involved early in the range of motion and less resistance to the muscles involved toward the end of the range of motion
what are the implications for weight heaviness in explosive exercises?
because of the addition of inertia, heavier weights can be handled in accelerative exercises than in slow exercises, allowing near-maximal resistance to be attained for all muscles involved in the exercise
how does the inertia of heavy weight affect the attainable resistance in the power clean?
the strong leg, hip, and back muscles accelerate the bar vertically to a high enough velocity that, even though the weaker upper body muscles cannot exert vertical force equal to the bar’s weight, the bar continues to travel upward until the force of gravity decelerates it to zero velocity at the highest bar position
how do cam machines with variable radii work?
changes the length of the moment arm through which the weight stack acts, thereby providing more resistance at points in the range of motion where the muscles could exert greater torque, and less resistance where the muscles could apply less torque
pitfall of cam-based machines?
athlete has to move at a constant, slow angular velocity, which is difficult to do consistently, and cam-based machines frequently fail to match normal human torque capability patterns
what are the implications for agonist muscles in the acceleration patterns of exercises?
the agonist muscles receive resistance in excess of bar weight early in the range of motion, but resistance less than bar weight toward the end of the range of motion
biomechanically, how do athletes decelerate a bar?
by either (a) reducing upward force on the bar to less than bar weight to let some or all of the bar’s weight decelerate it or (b) pushing down against the bar using the antagonist muscles. In either case, the deceleration has the effect of providing less resistance to the agonist muscles late in the range of motion.
what are some examples of acceleration and deceleration in sport exercises?
sprinting requires the athlete’s arms and legs to go through repeated cycles of acceleration and deceleration; throwing a baseball, discus, shot, or javelin all involve sequences of body movements that accelerate the objects to high release speeds
how does the force-velocity relationship of muscle affect a shot-putter who trains with an extra-heavy shot?
develops greater forces during the accelerative movement than when using the normal shot because the inertia of the heavier implement forces the muscle to contract at relatively low speed, then when a relatively light shot is used, the lower inertia of the shot enables the putter to accelerate the shot more rapidly and to reach a higher speed of release
downsides of changing the implement loading to a heavy one in throwing or sprinting?
the body needs time to adjust the motor pattern for that particular movement with the new load
exercises using friction as the main source of resistance?
belt or brake pad-resisted cycle ergometers, and wrist curl devices.
what is the formula for exercise devices using friction, like cycle ergometers?
FR = k ∙ FN, where FR is the resistive force; k is the coefficient of friction for the two particular substances in contact; and FN is the normal force, which presses the objects against each other
what resistance does a weighted sled provide?
a device that is resisted by both friction and inertia. The resistance due to the sled’s inertia is directly proportional to both the sled’s mass and its acceleration. The resistance due to the friction between the sled’s runners and the ground is proportional to both the friction coefficient between surfaces in contact and the net force pressing the sled against the ground, which equals the gravitational force minus any upward force exerted by the individual pushing the sled.
despite not providing repeatable resistance, why are sleds valuable?
horizontal resistance, which cannot be directly provided by weights
describe the power output of a sled and how friction resistance relates to speed
It takes more force to get the sled moving than to keep it moving, because the coefficient of static friction is always greater than the coefficient of sliding friction. Once the sled is moving, the coefficient of sliding friction stays relatively constant. Therefore one should understand that friction resistance does not change as speed increases. However, in keeping with Power = Work / Time, power output increases with speed. Also, as expressed by F=M*A, during the transition from a lower to a higher speed there is added resistance due to acceleration.
biomechanically how does bad spinal form lead to greater likelihood of injury
When a weight is supported in the hands or on the shoulders and the trunk is inclined forward, there is great torque about the lower intervertebral disks due to the large horizontal distance between the lower back and the weight. The back muscles operate at an extremely low mechanical advantage because the perpendicular distance from the line of action of the spinal erector muscles to the intervertebral disks is much shorter (about 2 inches, or 5 cm) than the horizontal distance from the weight to the disks. As a result, the muscles must exert forces that frequently exceed 10 times the weight lifted. These forces act to squeeze the intervertebral disks between the adjacent vertebral bodies and can lead to injury.
high / middle / low injury risks for various sports?
high for team sports; intermediate for running and aerobics; and lowest for cycling, walking, and resistance training
injury rate of resistance training?
a study of collegiate American football players showed only 0.35 resistance training-related injuries per 100 players per season. injuries due to resistance training accounted for only 0.74% of the in-season injury-related time loss of the players
shape of vertebral disks when in S-shape?
flat
neutral back posture minimizes?
L5/S1 compressive forces and ligament strain
what factors can lordotic (neutral spine) reduce?
vertebrae, disks, facet joints, ligaments, and muscles of the back
how does abdominal cavity support vertebral column?
tensing surrounding muscle (deep abdominal muscles and diaphragm) which has been described as a “fluid ball” that aids in supporting the vertebral column during resistance training, this may significantly reduce both the forces required by the erector spinae muscles to perform an exercise and the associated compressive forces on the disks
what mechanical advantage do most muscles that rotate the limbs about body joints operate at?
less than 1.0, or mechanical disadvantage (also why internal muscle forces are much greater than the forces exerted by the body on external objects)
why can defining origin as “the more stationary muscle attachment” and insertion as “the more mobile structure” cause confusion?
during a straight-leg sit-up, the origin of the iliacus muscle is the femur, because of its relative immobility. the pelvis, being more mobile, is the insertion. however, during the leg raise exercise, the pelvis is relatively immobile and would therefore become the origin, while the more mobile femur would become the insertion.
how do the biceps/tricep function during throwing?
the triceps acts as an agonist, extending the elbow to accelerate the ball. as the elbow approaches full extension, the biceps acts as an antagonist to slow down elbow extension and bring it to a stop, thereby protecting elbow structures from internal impact.
when are synergists required to control body motion?
when the agonist is a muscle that crosses two joints. (for example, the rectus femoris muscle crosses the hip and knee, acting to flex the hip and extend the knee when contracting. rising from a low squat involves both hip and knee extension. if the rectus femoris is to act to extend the knee as a person rises without inclining the trunk forward, then hip extensor muscles such as the gluteus maximus must act synergistically to counteract the hip flexion that would otherwise result from tension in the rectus femoris)
what kind of forces experienced by muscles and tendons largely account for injury?
internal forces
how does the patella/kneecap help prevent large changes in mechanical advantage of the quadriceps muscle, during knee extension?
keeping the quadriceps tendon from falling in close to the axis of rotation
what kind of tendon insertion should enable an athlete to lift heavier weights?
a person whose tendons are inserted on the bone farther from the joint center (because muscle force acts through a longer moment arm and thus can produce greater torque around the joint)
what is the tradeoff of having tendons insert farther from the joint center?
loss of maximum speed because, with the tendon inserted farther from the joint center, the muscle has to contract more to make the joint move through a given range of motion. (a given amount of muscle shortening results in less rotation of body segments about a joint, which translates into a loss in movement speed)
when are tendon insertions farther from the joint advantageous and disadvantageous?
advantageous: powerlifting, disadvantageous: activities occurring at high speeds such as tennis serves
what plane does the barbell curl take place in?
sagittal
what plane does the standing lateral dumbbell raise take place in?
frontal
what plane does the dumbbell fly take place in?
transverse
throwing/tennis is an example of what kind of movement?
shoulder internal rotation
knee flexion is not usually incorporated into resistance training, but used in what sport?
sprinting
hip flexion is used in what sport movements?
kicking and sprinting
ankle dorsiflexion is used in what sport movement?
running
hip internal/external rotation is used in what sport movement?
pivoting
lateral cutting is an example of what kind of movement?
hip adduction and abduction
torso rotation is used in what sport movements?
throwing and batting
the various neck movements are used in what sport movements?
boxing/wrestling
Important sport movements not usually incorporated into standard resistance training programs include?
[1] shoulder internal and external rotation (throwing, tennis), [2] knee flexion (sprinting), [3] hip flexion (kicking, sprinting), [4] ankle dorsiflexion (running), [5] hip internal and external rotation (pivoting), [6] hip adduction and abduction (lateral cutting), [7] torso rotation (throwing, batting), [8] the various neck movements (boxing, wrestling)
why should athlete’s force capabilities be tested at various loads?
individual ability to exert force at different velocities, so strength scores obtained from isometric and low-speed resistance tests may vary in predictive ability when the force is required with concomitant high velocity
how does one calculate work for one rep of free weight exercise?
height of the bar relative to the floor at its low position is subtracted from the height of the bar at its high position
how does one calculate work for one rep of weight-stack exercises?
the vertical travel of the stack is measured
if it takes 40 seconds to perform the 10 repetitions, what is the average power output in watts for the set using [Power = Work / Time]?
power (positive) = 23,600 J / 40 seconds = 590 W
why is “negative work” a misnomer?
there is no such thing as negative work or power; really refers to work performed on, rather than by, a muscle (when a weight is lifted, muscles perform work on the weight; when the weight is lowered, its potential energy performs an equal amount of work on the athlete. the athlete and weight alternately perform work on each other, rather than the athlete’s alternately performing positive and negative work)
what determines the rate of power output in a set?
rate at which the repetitions are performed
The angle through which an object rotates is called what?
angular displacement, the SI unit for which is the radian (rad); 1 rad = 180° ÷ p = 57.3°, where p = 3.14.
what accounts for variation in force of muscle contraction in specific types of muscles?
variation in the arrangement and alignment of sarcomeres in relation to the long axis of the muscle, i.e. pennation angle.
how does pennation angle change?
[1] increases as the muscle shortens (any factor that affects angle of pennation would affect strength and velocity of shortening as long as cross-sectional area remains the same) [2] although angle of pennation may vary depending on hereditary factors, it is modifiable through training, which could account for some of the differences in strength and speed despite muscles of the same size
benefits in greater/lesser pennation?
greater pennation = more sarcomeres in parallel and fewer sarcomeres in series; therefore better force generation but lower maximal shortening velocity; lesser pennation = advantageous for producing high velocities due to the greater number of sarcomeres in a row, at the expense of number of sarcomeres in parallel
how does muscle at resting / contracted state affect force generation?
when a muscle is at its resting length, the actin and myosin filaments lie next to each other, so that a maximal number of potential crossbridge sites are available – this is why the muscle can generate the greatest force at its resting length; when it’s contracted below resting length, fewer crossbridge sites exist
why is the term muscle action is preferable to contraction?
contraction means “shortening,” which does not accurately describe two of the three muscle actions
during isokinetic (constant-speed) concentric exercise by human subjects, does torque capability decline or increase?
torque capability declines as angular velocity increases
why is it true that the greatest muscle force can be obtained during eccentric muscle action?
during eccentric exercise, as joint angular velocity increases, maximal torque capability increases until about 90°/s (1.57 rad/s), after which it declines gradually
proper way to weigh a weight?
on a calibrated spring or electronic scale; balance scale determines objects mass, so weight (Fg) must be calculated from [Fg = M * Ag] if this scale is not available. (acceleration due to gravity can vary by geographic location)
misconceptions about weight?
the pound is a unit of force, not mass; only the mass of a barbell plate stays constant, while its weight varies according to the local acceleration due to gravity
how to properly refer to a kilogram?
the kilogram designation on a weight plate refers to its mass. it is not correct to say that an object weighs a certain number of kilograms, since weight refers to force, not mass. instead, say “the mass of the barbell is 85 kg.”
how can exercise technique affect the resistive torque pattern during an exercise and shift stress among muscle groups?
in the back squat more forward inclination of the trunk brings the weight horizontally closer to the knees, thus reducing the resistive torque about the knees that the quadriceps must counteract. concurrently, the weight is horizontally farther from the hip, increasing the resistive torque about the hip that the gluteus and hamstring muscles must counteract
what exercises have no acceleration or deceleration?
all exercises involve some acceleration and deceleration (acceleration at the beginning to bring the bar from a zero to an upward velocity, deceleration at the top to bring the bar’s velocity back to zero so that it does not fly out of the lifter’s hands)
why is torque due to an object’s weight is the product of the weight and the horizontal distance from the weight to the pivot point (joint)?
(1) gravitational force on an object always acts downward (2) by definition, the moment arm by which a force produces torque is perpendicular to the line of action of the force, so the moment arm of a weight is always horizontal
why should resistance training exercises should generally be performed with the lower back in a moderately arched position?
when lower back is rounded = ventral edges of the vertebral bodies squeeze front portions of intervertebral disks; extreme arching of back = squeezing dorsal portions. therefore, uneven squeezing = increased likelihood of disk rupture.
can weightlifting belts improve safety?
due to increase in intra-abdominal pressure, probably
why should an athlete abstain from chronic belt usage?
if an athlete performs all of the exercises with a belt, the abdominal muscles that produce intra-abdominal pressure might not get enough training stimulus to develop optimally
why is a chronic belt user particularly at risk for back injury?
the abdominal musculature might not be capable of generating enough intra-abdominal pressure to significantly reduce erector spinae muscle forces. The resulting excessive compressive forces on the disks could increase the chance of back injury
guidelines for belt use?
(1) not needed for exercises that do not directly affect the lower back (2) refrain from wearing a belt during lighter sets but may wear one for near-maximal and maximal sets (3) may reasonably choose never to wear lifting belts if they build up the strength of their back muscles and the muscles that generate intra-abdominal pressure in a gradual and systematic manner and if they practice safe resistance training exercise techniques. (many world-class Olympic-style weightlifters never wear belts)
what joint has the greatest range of motion in the human body?
shoulder joint, but this mobility contributes to its vulnerability, as does the proximity of the bones, muscles, tendons, ligaments, and bursae in the shoulder
how can the range of motion of the shoulder work against it?
its various structures can easily impinge on one another, causing tendinitis as well as inflammation and degeneration of contiguous tissue
risk of repetitive high forces to the patellar tendon during resistance training (and high force activities such as running)?
can lead to tendinitis, which is characterized by tenderness and swelling. (no inherent risk of tendinitis, though; it’s a function of too much volume and intensity without appropriate progression)
side effects of knee wraps?
skin damage and chondromalacia patellae, the wearing down and roughening of the posterior surface of the patella
myths about knee wraps?
(1) they don’t prevent injury (2) they are performance-enhancing; they provide about 25lb of extra force through spring effect, and direct help extending the knee
what are the primary concerns with elbow and wrist injury?
overhead lifts, but the risk with overhead lifting is quite small in comparison to the common source of injury of these joints, which includes participation in overhead sports such as throwing events or the tennis serve
other sources of elbow injury?
elbow dislocation, sometimes observed in (gymnastics) and overuse-related injuries such as traction apophysitis (diving, wrestling, and hockey)
major concern of elbow or wrist injury?
epiphyseal growth plate damage or overuse either in the posterior aspect of the elbow or in the distal radius in young athletes
mechanics of valsalva
the glottis is closed, thus keeping air from escaping the lungs, and the muscles of the abdomen and rib cage contract, creating rigid compartments of liquid in the lower torso and air in the upper torso
advantage of valsalva
it increases the rigidity of the entire torso, making it easier to support heavy loads (15). For example, when lifting heavy loads in the back squat exercise, many athletes use the Valsalva maneuver, particularly when the trunk is most inclined forward, near the transition from the eccentric movement phase to the concentric movement phase
side effects of valsalva
exerts compressive force on the heart, making it more difficult for blood to return to the heart. Also, the Valsalva maneuver can transiently raise blood pressure to slightly elevated levels.
safer way to valsalva
The diaphragm and the abdominal muscles can contract without the glottis being closed, however, creating the fluid ball in the abdomen without pressurizing the chest compartment. This must be regarded as the safer way, of the two options, to add support to the lower spine without building up pressure in the chest, and is the technique that should be used for most resistance training. One can build up intra- abdominal pressure without building up chest pressure by consciously keeping the airway open. During a strenuous repetition, the abdominal muscles and diaphragm contract reflexively, even with the airway open. Athletes, particularly those who compete in Olympic lifting or powerlifting, may choose to use the Valsalva maneuver if they recognize and accept the risks involved and have the experience to avoid increasing pressure to the point of blackout.
acceleration
change in velocity per unit time
angle of pennation
the angle between the muscle fibers and an imaginary line between the muscle’s origin and insertion; 0° corresponds to no pennation.
angular displacement
the angle through which an object rotates
angular velocity
the object’s rotational speed, measured in radians per second (rad/s).
bracketing technique
the athlete performs the sport movement with less than normal and greater than normal resistance
classic formula
the load lifted is divided by body weight to the 2/3 power
concentric muscle action
contractile force is greater than the resistive force, forces generated within the muscle and acting to shorten it are greater than the external forces acting at its tendons to stretch it
dorsal
toward the posterior
eccentric muscle action
contractile force is less than the resistive force. the forces generated within the muscle and acting to shorten it are less than the external forces acting at its tendons to stretch it. (this occurs during the lowering phase of any resistance exercise. during standard resistance training, the eccentric force exerted by the muscle keeps the weight from being accelerated downward by gravitational force)
fibrous attachments
blend into and are continuous with both the muscle sheaths and the connective tissue surrounding the bone. They have additional fibers that extend into the bone itself, making for a very strong union.
first-class lever
lever for which the muscle force and resistive force act on opposite sides of the fulcrum
fleshy attachments
most often found at the proximal end of a muscle, muscle fibers are directly affixed to the bone, usually over a wide area so that force is distributed rather than localized
fluid resistance
resistive force encountered by an object moving through a fluid (liquid or gas), or by a fluid moving past or around an object or through an opening. sources of resistance are surface drag and form drag.
form drag
the way in which a fluid presses against the front or rear of an object passing through it. Cross-sectional (frontal) area has a major effect on form drag
insertion
distal attachment, sometimes defined as the more mobile structure
isometric muscle action
muscle length does not change, because the contractile force is equal to the resistive force. The forces generated within the muscle and acting to shorten it are equal to the external forces acting at its tendons to stretch it. (example: during a sit-up with the trunk held straight, the abdominal muscles act isometrically to maintain the rigidity of the trunk, while the hip flexors carry out the sit-up movement. In contrast, the abdominal muscles act concentrically and eccentrically during the raising and lowering phases of the curl-up exercise, respectively)
kyphotic
slightly rounded
lever
rigid or semirigid body that, when subjected to a force whose line of action does not pass through its pivot point, exerts force on any object impeding its tendency to rotate
line of action
line of action of a force is an infinitely long line passing through the point of application of the force, oriented in the direction in which the force is exerted
mechanical advantage
ratio of the moment arm through which an applied force acts to that through which a resistive force acts. For there to be a state of equilibrium between the applied and resistive torques, the product of the muscle force and the moment arm through which it acts must equal the product of the resistive force and the moment arm through which it acts.
moment arm
The perpendicular distance from the line of action of the force to the fulcrum. The line of action of a force is an infinitely long line passing through the point of application of the force, oriented in the direction in which the force is exerted.
muscle force
Force generated by biochemical activity, or the stretching of noncontractile tissue, that tends to draw the opposite ends of a muscle toward each other.
origin
proximal attachment (toward the center of the body), sometimes defined as the more stationary structure to which the muscle is attached but this can cause confusion
pennate muscle
fibers that align obliquely with the tendon, creating a featherlike arrangement
power and work
work = force * displacement, power = work / time
proximal
toward the center of the body
rate coding
rate at which the motor units are fired
recruitment
how many motor units are involved in a muscle contraction
resistive force
Force generated by a source external to the body (e.g., gravity, inertia, friction) that acts contrary to muscle force.
second-class lever
A lever for which the muscle force and resistive force act on the same side of the fulcrum, with the muscle force acting through a moment arm longer than that through which the resistive force acts, as when the calf muscles work to raise the body onto the balls of the feet. Due to its mechanical advantage (i.e., relatively long moment arm), the required muscle force is smaller than the resistive force (body weight).
surface drag
the friction of a fluid passing along the surface of an object
synergist
indirect assistance in a movement (inessential, but that do help – stabilizer muscles for example)
third-class lever
A lever for which the muscle force and resistive force act on the same side of the fulcrum, with the muscle force acting through a moment arm shorter than that through which the resistive force acts. The mechanical advantage is thus less than 1.0, so the muscle force has to be greater than the resistive force to produce torque equal to that produced by the resistive force.
torque formula
magnitude of a force times the length of its moment arm
