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.
