Chapter 2 - Biomechanics of Resistance Exercise Flashcards
Origin and Insertion
O: proximal (doesn’t move; closer to body).
I: distal (moves; further from body)
Agonist
muscle most directly involved
aka Prime Mover
Antagonist
muscle that can slow down or stop a movement (opposite)
Synergist
muscle that assists indirectly in a movement.
Mechanical Advantage
Ratio of moment arm through which an applied muscle force acts to that through which a resistive force acts.
- 0+ = Fm < Fr to produce equal torque; advantage.
- 1.0= Fm > Fr to produce equal torque; disadvantage.
First Class Lever
MF and resistive force act on opposite sides of the fulcrum.
Ex. triceps ext.
Second Class Lever
MF and Fr act on same side of fulcrum, with MF acting through a moment arm longer than that of Fr.
Mechanical Advantage.
Ex. Calf Raise
Third Class Lever
MF and Fr act on same side of fulcrum, with MF acting through a moment arm shorter than that of Fr. Mechanical Disadvantage.
Ex. Bicep Curls
Patella and Mechanical Advantage
Patella increases MA of quads by maintain the quads tendon’s distance from the knee’s axis of rotation.
Absence of patella allows tendon to fall closer to knee’s center of rotation, shorting the moment arm
Moment Arm and Mechanical Advantage
Longer moment arm = more MA
Shorter moment arm = less MA
Ex. Biceps curl; moment arm length changes during the ROM. Shorter at full ext., longest at middle, and shortest at full curl.
Variations in Tendon Insertion
Tendons attach to bones.
Insertion further from joint = ability to lift heavier weights, but loss of max speed and reduced MF capability during faster movements.
Anatomical Planes
Sagittal (L,R)
Frontal (F,B,
Transverse (U,L)
Strength
Capacity to exert force
Positive Power
Precisely defined as the time rate of doing work.
Power (watts) = Work/Time (seconds)
Positive Work
Product of the force exerted on an object and the distance the object moves in the direction in which the force is exerted.
Work (Joules) = Force x Displacement (meters)
Negative Work and Power
All such “negative” power and work occur during eccentric muscle action (i.e. lowering weight or decelerating at the end of a rapid movement.
Angular Work and Power
Required for objects rotating about an axis or to change rotational velocity.
Angular Displacement
Angle in which an object rotates on a circular path.
Angle between starting ands final position.
Measured in radian (rad).
Angular Velocity
Object’s rotational speed. Measured in radians per second (rad/s).
Torque
Degree to which a force tends to rotate an object about a specified fulcrum, aka moment. Measured in newton-meters (N-m)
Rotational (Angular) Work and Power
Rotational Work = Torque x Angular Displacement
Rotation Power is same as linear power.
Strength v. Power
Although STR is associated wit slow and PWR with high velocities of movement, both reflect ability to exert force at a given velocity. (i.e. weightlifting has higher power component than powerlifting b/c of higher movement velocities with heavy weights.
Power is direct mathematical function of force and velocity.
Biomechanical Factors in Human Strength (8)
Neural Control Muscle Cross-Sectional Area Arrangement of Muscle Fibers Muscle Length Joint Angle Muscle Contraction Velocity Joint Angular Velocity Strength-to-Mass Ratio Body Size
Neural Control
MF is greater when: (a) more motor units are involved ini a contraction (recruitment), (b) the motor units are greater in size, or (c) the rate of firing is faster (rate coding).
Muscle Cross-Sectional Area
Force a muscle can exert is related to its cross-sectional area rather than to its volume.
Arrangement of Muscle fibers
Varation exists in the arrangement and alignment of sarcomeres in relation to the long axis of the muscle.
Pennate Muscle and Angle of Pennation
Pennate Muscle: muscle with fibers that align obliquely with the tendon, creating a featherlike arrangement.
Angle of Pennation: angle between the muscle fibers and an imaginary line between muscle’s origin and insertion; 0 degrees corresponds to no pinnation.
Muscle Length
Resting: actin and myosin lie next to each other; max number of potential cross-bridge sites are available; muscle can generate greatest force.
Stretched: smaller proportion of actin and myosin lie next to each other; fewer cross bridge sites, can’t generate as much force.
Contracted: actin overlaps myosin; number of cross-bridge sets reduced; decreased force generation capability.
Joint Angle
Amount of torque that can be exerted about a given body joint varies throughout the joint’s ROM, largely bc of its relationship of force v. muscle length, as well as the ever-changing leverage of brought about by the dynamic geometry of muscles, tendons, and internal joint structures.
Additional factors are: the body joint in question, the muscles used at that joint, and speed of contraction.
Muscle Contraction Velocity
Nonlinear, but in general, the force capability of muscle declines as the velocity of contraction increases.
i.e. vert jump: arms swing up causing downward force on body at shoulders, slowing the upward movement and forcing hip and knee extensors to contract slower, enabling them to generate higher force for longer times.
Joint Angular Velocity
Concentric action: shortens; contractile force > resistive force (weight).
Eccentric action: lengthens; contractile force < resistive force.
Isometric: stays the same; contractile force = resistive force.
During isokinetic concentric exercise, torque capability decreases as joint angular velocity increase.
In contrast, during eccentric exercise, as joint angular velocity increases, maximal torque capability increases until about 90 degrees/s
Strength-to-Mass Ratio
Directly reflects athlete’s ability to accelerate body when sprinting or jumping.
Helps to determine when strength is highest relative to that of other athletes in same weight class.
Body Size
As size increases, body mass increases more rapidly than muscle strength.
Given constant body proportions, the smaller athlete has a higher strength-to-mass ratio than the larger athlete.
Sources of Resistance to Muscle Contraction
Gravity Inertia Friction Fluid Resistance Elasticity
Gravity
When weight is horizontally closer to the joint, it exerts less resistive torque. When further from joints, t exerts more resistive torque.
Ex. front squat produces more torque than high or low back squats bc the distance from bar to knee joint is greater.
Inertia
When a weight is held in a static position or when it is moved at a constant velocity, it exerts constant resistance only in the downward direction.
However, upward or lateral acceleration of the weight requires additional force.
Ex. back squat; accelerating the bar upward.
Friction
the resistive force encountered when one attempts to move an object while it is pressed against another object.
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 orifice.
ex. water and air resistance; swimming/rowing (water), sprinting/pitching (air).
Elasticity
the more an elastic component is stretched, the greater the resistance. Begins with low resistance and ends with high resistance. Limited adjustability.
Back Injuries
Low back is particularly vulnerable.
Resistance training exercises should generally be performed with low back in a moderately arched position.
Intra-Abdominal Pressure and Lifting Belts
Abdominal “fluid ball” and tissues aid in supporting vertebral common during resistance training.
Valsalva maneuver: glottis is closed, thus keeping air from escaping the lungs, and the muscles of the abdomen and rib cage.
Weightlifting belts: improve safety. Not needed for non-low back exercises. Refrain from using during lighter sets.
Shoulder Injuries
Prone to injury during weight training b/c of its structure and forces to which it is subjected. Warm up with light weights.
Knee Injuries
Prone to injury b/c located between two long levers. Minimize use of wraps.
Elbow and Wrist Injuries
Primarily concern is during overhead lifts. Lower risk of injury during resistance training than during actually sport competition. In young athletes, primary concern is epiphyseal growth plate damage or overuse in posture aspect of elbow or in the distal radius.
Reducing Risk of Resistance Training Injuries
Warm up sets with using relatively light weights, especially for knee and shoulder exercises.
Perform exercises through full ROM.
use lighter weight for new exercises or resuming training after layoff of 2+ weeks.
Do not ignore pain around joints.
Never attempt max loads without proper prep.
Performing several variations of an exercise = more complete muscle development and joint stability.
Safely integrate plyometrics into a training program.
Acceleration
Change in velocity per unit of time.
Force (Newtons) = Mass x Acceleration
