Chapter 2: Biomechanics of Resistance Exercise Flashcards
Biomechanics
Study of the mechanisms through which musculoskeletal components interact to create movement
Origin
Proximal muscle attachment
Insertion
Distal muscle attachment
Proximal
Toward the center of the body
Distal
Away from the center of the body
Forms of Muscle Attachments
- Fleshy attachments
- Fibrous attachments
Fleshy Attachments
- Most often found at the proximal end of the muscle
- Muscle fibers are directly affixed to the bone
- Usually distributed over a wide area instead of focused on a single spot
Fibrous Attachments
- Blend into and are continuous with muscle sheaths and connective tissue surrounding the bone
- Ex: tendons
Tendons
A flexible but inelastic cord of strong fibrous collagen tissue attaching a muscle to a bone
Agonist
- The muscle most directly involved in movement
- Prime mover
Antagonist
A muscle that can slow down or stop the movement
Synergist
A muscle that assists directly in a movement
Lever
A rigid/semi-rigid body that exerts a force on any object impeding its tendency to rotate
Fulcrum
The pivot point of a lever
Muscle Force
Force generated by muscle activity; tends to draw the opposite ends of a muscle toward each other
Resistive Force
Force generated by a source external to the body which acts contrary to muscle force
First-Class Lever
- A lever for which the muscle force and resistive force act on opposite sides of the fulcrum
- FAR
Second-Class Lever
- A lever for which the muscle force and resistive force act on the same side of the fulcrum
- FRA
Third-Class Lever
- A lever for which the muscle force and resistive force act on the same side of the fulcrum
- RFA
Mechanical Advantage
- The ratio of the applied force and the resistive force
- Mechanical advantage often changes continuously during human movement
- MA > 1.0 → applied force can be less than the resistive force to produce an equal amount of torque
- MA < 1.0 → applied force has to be greater than the resistive force to produce an equal amount of torque
Moment Arm
- The perpendicular distance from the line of action of the force to the fulcrum
- AKA force arm, lever arm, or torque arm
Torque
- AKA moment;
- Force times the moment arm, causes a rotation
Why are muscle forces greater than the forces exerted by the body on objects?
Most muscles operate at a mechanical disadvantage
The trade-off in tendon insertion
The mechanical advantage gained by having tendons insert farther from the joint center is accompanied by a loss of maximum speed
What causes the tendon insertion trade-off?
When the tendon is farther from the joint center, the muscle has to contract more to make the joint move through a given range of motion
Anatomical Position
The body is erect, the arms are down at the sides, and the palms face forward
Sagittal Plane
Plane which divides the body into equal left-and-right portions
Frontal Plane
Plane which divides the body into equal front-and-back portions
Transverse Plane
Plane which divides the body into equal upper-lower portions
Sagittal Axis
Axis perpendicular to the frontal plane, about which all movements in the frontal plane occur
Frontal Axis
Axis perpendicular to the sagittal plane, about which all movements in the sagittal plane occur
Longitudinal Axis
Axis perpendicular to the transverse plane, about which all movements in the transverse plane occur
Strength
The ability to produce force
Force
- A push or pull
- F = ma
Acceleration
Change in velocity over unit time
Work
W = force x distance
Power
- Time rate of doing work
- Force x velocity
Force Conversion Factors
- LBS x 4.448 = N
- KG (mass ) x g = N
- KG (force) x 9.807 = N
Distance Conversion Factors
- FT x 0.3048 = m
- IN x 0.0254 = m
Radians to Degrees Conversion Factor
DEGREES x 0.01745 = RADIANS
Weight
- F = mg
- g = acceleration due to gravity
Positive Work/Power
- Positive work: when the force exerted on a weight is in the same direction as the weight’s movement
- Positive power: the rate at which positive work is done
Negative Work/Power
- Negative work: when the force exerted on a weight is in the opposite direction as the weight’s movement
- Negative power: the rate at which negative work is done
- Negative work actually refers to work done by the weight on the muscle
- Weight lifted → muscle work increases weight’s potential energy
- Weight lowered → weight work decreases weight’s potential energy
Angular Displacement
The angle through which an object rotates
Angular Velocity
The object’s rotational speed
Rotational Work
Torque x angular displacement
Rotational Power
Rotational work/time
Strength vs Power
Strength is the capacity to exert force at any given velocity, and power is the mathematical product of force and velocity at whatever speed
Biomechanical Factors in Human Strength
- 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
Affects the maximal force output of a muscle by determining which and how many motor units are involved in a muscle contraction and the rate at which the motor units are fired
Recruitment
How many motor units are involved in a muscle contraction
Rate Coding
The rate at which the motor units are fired
Neural Factors increasing force production
- More motor units
- Motor units are bigger
- Faster firing rate
Muscle Cross-Sectional Area
Greater cross-sectional area → greater strength (all other things equal)
Arrangement of Muscle Fibers
- Maximally contracting muscles have been found capable of generating forces of 23 to 145 psi
- This wide range can be accounted for by the variation in fiber arrangement
Pennate Muscle
Fibers align obliquely with the tendon
Angle of Pennation
- The angle between the muscle fibers and an imaginary line between the muscle’s origin and insertion
- 0 = no pennation
Radiate Pennation
Gluteus medius
Longitudinal Pennation
Rectus abdominis
Fusiform Pennation
Biceps brachii
Multipennate
Deltoid
Bipennate
Rectus femoris
Unipennate
Tibialis posterior
Pennation vs Nonpennation Strength and Velocity
Greater pennation results in: - More sarcomeres in parallel, rather than series - Better able to generate force - Lower max shortening velocity Lesser pennation results in: - Higher contraction velocity
Role of Muscle Length in Force Production
There’s an ideal amount of tension which maximally aligns actin and myosin, which allows for the greatest amount of force generation
Muscle Contraction Velocity
Force capability decreases as velocity increases
Concentric Muscle Action
- Contraction → muscle shortens
- Torque capability decreases as angular velocity increases
Eccentric Muscle Action
- Contraction → muscle lengthens
- Torque capability increases with angular velocity until ~90 degrees/second, then it declines
- Greatest force capability is seen in eccentric muscle action
Isometric Muscle Action
Contraction → muscle maintains length
Strength-to-Mass Ratio
If mass is equal, the stronger athlete has an advantage
Body Size
- All other things being equal, a smaller athlete will be stronger pound-for-pound
- As body size increases, mass increases faster than strength
Classic Formula
- Use to compare loads lifted between lifters
- The load lifted is divided by body weight to the 2/3 power, accounting for the relationship of cross-sectional area vs volume
Sources of Resistance to Muscle Contraction
- Gravity
- Inertia
- Friction
- Fluid Resistance
- Elasticity
“Advantages” of Stack Machines
- Safety
- Design flexibility
- East of use
Advantages of Free Weights
- Whole-body training
- Simulation of real-life activities
Inertial Force
Due to inertial forces, resistance is greater than bar weight in the beginning of a movement, less than at the end
Bracketing Technique
The athlete performs the sport movement with less than normal and greater than normal resistance as a form of acceleration training
Friction
- The resistive force encountered when one attempts to move an object while it is pressed against another object
- FR = k x FN
FR = resistive force (friction)
k = coefficient of friction
FN = normal force
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
- Plays a large role in swimming, rowing, golf, sprinting, discus throwing, and baseball pitching
Surface Drag
Results from the friction of a lfuid passing along the surface of an object
Form Drag
- Results from the way in which a fluid presses against the front or rear of an object passing through it
- Cross-section area has a major effect on this type of drag
Elasticity
- Resistance provided by an elastic component is proportional to the distance it is stretched
- Problem with elastic devices → muscles are stronger at beginning of ROM, elastic devices are most difficult at end ROM
- FR = k * x
FR = resistive force
k = constant reflecting physical characteristics of elastic component
x = distance stretched
Back Injury and Resistance Training
85-90% of all disk herniations occur b/t L4 and L5
Lordotic Spine
- Lordotic lumbar spine refers to the curvature of the lower back, normally slightly arched
- Lordotic lumbar spine is superior to rounded back for avoiding injury
Kyphotic Spine
Kyphotic thoracic spine refers to the curvature of the upper back, normally slightly rounded
Vertebral Column
Vertebral column has a natural S-curve
Ventral
Toward the anterior
Dorsal
Toward the posterior
What is the fluid ball?
The abdominal fluids and tissue kept under pressure by tensing surrounding muscle (deep abdominal muscles and diaphragm)
Valsalva Maneuver
- Glottis is closed → Air cannot escape → abs and rib muscles contract → creates rigid components of liquid and air in torso and chest
- Makes it easier to support heavy loads
Effect of weight belts
- Increase intra-abdominal pressure
- Belts are helpful, but should be used wisely
What makes the shoulder joint so susceptible to injury?
Structures in the shoulder can easily impinge on one another, causing inflammation and degeneration of tissue
Rotator Cuff Muscles
- Supraspinatus
- Infraspinatus
- Subscapularis
- Teres minor
Knees and risk of injury
- Rotary forces make the knee susceptible to injury
- Patella is most susceptible to injury from resistance training
Knee Wraps
Only use knee wraps if absolutely necessary, and only then on the heaviest sets
Primary Concerns for the Elbow and Wrists
Most common source of injury is throwing style movements
