Chapter 2
Biomechanics of Force
Production
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Introduction
• Biomechanics
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Science of applying principles of mechanics to biological systems
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Applies to:
• All motor skills performed in sports
• All training modalities
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Kinetics: deals with forces that cause motion
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Kinematics: description of motion (displacement, position,
velocity, acceleration)
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Muscle Actions (Not Contractions!)
• Concentric (CON): muscle shortening
• Eccentric (ECC): muscle lengthening (40% stronger,
DOMS—strength & hypertrophy)
• Isometric (ISOM): no change in muscle length (joint
stabilization)
• Isokinetic: velocity-controlled CON & ECC muscle
actions
• Greatest force (in order): ECC—ISOM—CON
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The Influence of Muscle Length
• Skeletal muscle plays a vital role in force production—this is
the Length-Tension Relationship.
• Greatest tension is produced slightly past resting muscle
length.
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Active Muscle Length-Tension
Relationship
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Passive Muscle Length-Tension
Relationship
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Stretch-Shortening Cycle
• An ECC muscle action that precedes a CON action results in a more
forceful CON action. This phenomenon is known as the StretchShortening Cycle (SSC). This allows the athlete to develop large force
and power outputs. Depth Jumps…
• This consists of the Stretch Reflex (SR), allowing the muscle to store
elastic energy within its series and parallel elastic components. The
SR is initiated by a specific sensory receptor the Muscle Spindle which
responds to both the magnitude and rate of muscle length change.
The resulting affect is that the SSC can enhance performance by an
average of 15-20%.
• It is most prominent in Type II muscle fibers.
• Critical to SSC performance is that the CON action follows quickly…as
stored elastic energy can be lost as heat energy (which offers little to
no effects). Stretching beforehand can reduce this and is not advised.
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Force-Velocity Relationship
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Muscle Architecture
• Nonpennate: fibers parallel to muscle’s line of pull
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Longitudinal (strap): sartorius
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Quadrate (quadrilateral): rhomboids
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Fan-shaped (radiate, triangular): pectoralis major
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Fusiform: biceps brachii
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Designed for ROM and contraction velocity
• Pennate: fibers oblique to line of pull
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Unipennate: tibialis posterior
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Bipennate: rectus femoris
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Multipennate: deltoid
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Designed for strength & power
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Figure 2.11
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Muscle Architecture (cont’d)
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Muscle Architecture (cont’d)
• Muscle Fiber Arrangement
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Angle of pennation
• Angle between fibers & central tendon
• Low (≤5°)
• High (>30°)
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Muscle fascicle length: It is thought that greater fascicle
lengths favor high contraction velocity (greater running speed)—
and may be a mechanism contributing to hypertrophy.
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Muscle Fascicle
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Torque and Leverage
• Linear Motion
• Angular Motion
• Torque
(aka ‘moment’)
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Rotation caused by a force about a specific axis
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Product of force & moment arm length
• Lever
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Used to overcome large resistance & enhance speed & ROM
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Components: fulcrum (pivot point), resistance, & force
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First-, second-, & third-class levers
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Figure 4.2
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Key Term
• Mechanical Advantage: The ratio of the moment arm
through which an applied force acts to that through
which a resistive force acts. A mechanical advantage
greater than 1.0 allows the applied (muscle) force to be
less than the resistive force to produce an equal amount
of torque. A mechanical advantage of less than 1.0 is a
disadvantage in the common sense of the term.
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Torque Generation at Two Angles of Force
Application
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Three Classes of Levers
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• Based on lever systems, it appears that the human body
was designed to produce motion at higher speeds at the
expense of the large force application. In other words,
people are made more for speed of movement than for
strength.
• Most single-joint movements in the human body operate
via third-class levers.
• Bones act as levers, joints act as fulcrums and skeletal
muscles act as the force.
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Tendon Insertion
• Can favor speed (tendon close to axis) or force (tendon
farther from axis).
• This is a GENETIC factor which does NOT change with
training.
• Moment arms and bodily proportions.
• Limb length—longer for pitching is advantageous (longer
resistance arm—for higher velocity, stroke length but
disadvantageous for strength in weightlifting).
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Effort Arm Changes During Elbow Flexion
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Ascending-Descending Strength (Torque)
Curve
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Ascending Strength (Force) Curve
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Descending Strength (Force) Curve
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Action/Reaction Forces and Friction
• Action Force
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Force applied to an object with the intent to accelerate, decelerate, stop,
maintain, or change direction
• Reaction Force
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Equal & opposite force in response to action force (Newton’s 3rd law of
motion)
• Friction
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Force parallel to action & reaction forces that acts to oppose relative
motion of these two surfaces
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Static friction-between 2 objects not moving relative to each other and
Dynamic or sliding friction-between 2 surfaces moving relative to
each other resulting in sliding.
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• The first law says that an object at rest tends to stay at rest, and an object in
motion tends to stay in motion, with the same direction and speed. Motion
(or lack of motion) cannot change without an unbalanced force acting.
• The second law says that the acceleration of an object produced by a net
(total) applied force is directly related to the magnitude of the force, the
same direction as the force, and inversely related to the mass of the object
(inverse is a value that is one over another number... the inverse of 2 is 1/2).
The second law shows that if you exert the same force on two objects of
different mass, you will get different accelerations (changes in motion). The
effect (acceleration) on the smaller mass will be greater (more noticeable).
• The third law says that for every action (force) there is an equal and opposite
reaction (force). Forces are found in pairs. Think about the time you sit in a
chair. Your body exerts a force downward and that chair needs to exert an
equal force upward or the chair will collapse.
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Ground-Reaction Force Curves
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• Friction is the force resisting the relative motion of solid
surfaces, fluid layers, and material elements sliding
against each other. There are several types of friction:
Dry friction resists relative lateral motion of two solid
surfaces
• Static friction-between 2 objects not moving relative to
each other where there is sufficient stability between the
contact surfaces. (weightlifting)
• Dynamic (sliding) friction acts between 2 surfaces
moving relative to each other, resulting in sliding.
(skating & skiing)
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Stability
• Ability of an object to resist changes in equilibrium
• Principles of stability
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Greater stability is seen when:
• Center of gravity (COG) is lower
• Line of gravity is aligned equidistantly within base support
• Base support is wide
• Objects with larger mass
• Level of friction is greater
–
Stability decreases when external loading is applied to upper
body
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Mass and Inertia
• Mass
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The amount of matter an object takes up
• Inertia
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Resistance of an object to changing its motion
• In Linear Motion:
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Greater mass & greater inertia = greater stability
• In Angular Motion:
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Distribution of mass is critical
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Moment of inertia: property of an object to resist changes in
angular motion; a product of object’s mass & mass distribution
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Moment of Inertia
• Changing an object’s center of mass changes the moment of
inertia.
• Decreasing the moment of inertia--Choking up the bat in
baseball increases bat velocity (but hurts power hitting).
• Increasing the moment of inertia—performing a torso rotation
with the arms straight rather than bent puts greater tension on
the muscles—becomes more difficult to start and stop.
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Momentum and Impulse
• Linear impulse (F × T) = linear momentum (m ×
∆v)
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So, F × T = m × ∆v
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Increasing force and/or time increases impulse
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Increasing mass and/or velocity increases momentum
• Angular impulse = torque × time
• Angular momentum = joint angular velocity ×
moment of inertia
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Maximizing angular momentum necessitates optimal combination
of angular velocity & moment of inertia
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Body Size
• Larger the body size, the larger the force potential
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Relative to muscle mass
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Positive relationship between muscle mass & absolute force
production
• As body size increases, body mass increases to a greater
extent than muscle strength.
• A football player who increases mass and maintains or
increases velocity can generate more linear momentum
at the point of contact with the opponent.
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Other Kinetic Factors in S&C
• Intra-Abdominal Pressure (IAP)
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Pressure developed within abdominal cavity during contraction
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Pushes against spine & helps keep torso upright
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Prevents lower-back injuries
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Increased by:
• Abdominal contraction & subsequent trunk muscle training
• Breath holding (Valsalva maneuver)
• Lifting belts
• Wraps
• Bench press shirts, Lifting suits
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Intra-Abdominal Pressure
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Other Kinetic Factors in S&C (cont’d)
• Lifting Accessories
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Lifting belts
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Wraps
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Bench press shirts
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Lifting suits
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