chapter

2Biomechanicsof ResistanceExercise

Chapter Objectives • Understand and apply biomechanical terms to

resistance training• Understand basic concepts of the musculoskeletal

system & differentiate among different types of levers. • Describe factors contributing to human strength and

power. • Evaluate resistive force and power patterns of exercise

devices. • Recommend ways to minimize injury risk during

resistance training. • Analyze sport movements and design movement-

oriented exercise prescriptions.

Biomechanics Defined

• Application of Mechanical Principles to Biological Structures– Engineering and Physics

Mechanics Integrated with Anatomy and Physiology

(m)

(a, α)

perpendicular distance from line of action of force to axis of rotation

I = mr2 (r = distance from m to axis of rotation)

Linear and Angular Kinematic and Kinetic Terms Defined

(v, ω)(d, θ)

(F)

(T) 1 N ~ ¼ lb (0.22)

Weight = mg,g = 9.8 m·s-2

Internal Muscle Force and its Rotary and Stabilizing

Force Components

Internal Muscle Torque Resulting From Internal Muscle Force and Moment Arm

For Example, if Fm = 100 N and d = 0.05 m,Tm = (100 N)(0.05 m) → Tm = 5 N·m

Figure 4.7Changes in Muscle Moment Arm & Mechanical

Advantage (MA) Through a Range of Motion (ROM)

• During elbow flexion, the biceps’ brachii moment arm varies throughout the joint ROM, being greatest at 90º.

• A greater moment arm (M) increases the MA, while a shorter M decreases the MA.

Figure 4.8• As a weight is lifted, the

moment arm (MA)through which the weight acts, and thus the resistive torque (Fw)(MA), both changes as the elbows moves through a ROM.

• The biceps curl exercise is hardest at 90º elbow flexion because both MA and resistive torque are greatest at this position.

MA4

MA5

MA1

MA3

Fw

Fw

Fw

Fw

Fw

Changes in Resistance Moment Arm (MA) and Mechanical Advantage Through a ROM

MA2

External Force and Torque Generated from

Weight of Arm (i.e., Force Due to Gravity

acting at Center of Mass of Arm) During

Shoulder Abduction

Note: The moment arm is the perpendicular distance from the line of force to the axis of rotation. Therefore, “c” is the moment arm for the top arm position when the shoulder is abducted 90 deg, but NOT the moment arm for arm positions “B” and “C”

C

Effects of Rotating Cam on External

Torque Generated by Weight Stack

and Internal Torque Generated by Muscle Force, Making it Easier (B) or Harder (C)

to Perform the Biceps Curl

Fm

Fm

Newton’s Laws of Motion

ΣF = ma ΣT = Iα

Note: A torque (T) is a turning or twisting force

Note: In 2-D analyses, ΣFx = max & ΣFy = may

Inertial Force (ma) in Lifting• F - W = ma → F = W + ma• If a lifter accelerates a 100 kg mass

(about 1000 N, or 220 lbs) barbell upwards at a peak rate of 1 m/s2, then the force at the hands is transmitted down the arm to the shoulder, and is calculated as follows:

– F = 1000N + (100kg)(1m/s2) = 1100 N, or 10% more than the weight being bench pressed.

• Performing an exercise slowly decreases inertial forces, as well as joint and muscle stresses.

• A solid strength base is needed prior to performing explosive acceleration training, such as plyometrics

F1F2

W

Lever Systems in the Body: 1st Class Lever

(Muscle force & resistive force act on opposite sides of fulcrum)

Moment Arm = perpendicular distance from line of action of force to axis of rotation

Lever Systems in the Body: 2nd Class Lever

Note: Muscle force & resistive force act on same side of fulcrum, with muscle force moment arm longer than resistive force moment arm. Mechanical advantage always > 1 (resistive force > muscle force).

Lever Systems in the Body: 3rd Class Lever

Note: Muscle force & resistive force act on same side of fulcrum, with resistive force moment arm longer than muscle force moment arm. Mechanical advantage always < 1 (resistive force < muscle force). Most levers in the body are 3rd class. which implies they are better suited for ROM and speeds of movement, but not as effective in producing muscular torque.

Figure 4.6

Reprinted, by permission, from Gowitzke and Milner, 1988.

(b) Absence of the patella allows the tendon to fall closer to the knee’s center of rotation, shortening the moment arm through which the muscle force acts and thereby reducing the muscle’s mechanical advantage.

(a) The patella increases the mechanical advantage of the quadriceps muscle group by maintaining the quadriceps tendon’s distance from the knee’s axis of rotation.

The Patella and Mechanical Advantage

Key Point

• Most of the skeletal muscles operate as 3rd

class levels and thus at a considerable mechanical disadvantage. Thus, during sports and other physical activities, internal forces in the muscles, tendons, and joints are much higher than the external resistive forces acting on the body (such as barbell or dumbbell weights). But with this level system you gain ROM and speeds of movement

Figure 4.10

Planes of the Human Body

Figure 4.16

Reprinted, by permission, from Harman, Johnson, and Frykman, 1992.

– Planes of movement are relative to the body in the anatomical position unless otherwise stated.

– Common exercises that provide resistance to the movements and related sport activities are listed.

Major Body Movements

Figure 4.16 (continued)

Reprinted, by permission, from Harman, Johnson, and Frykman, 1992.

Major Body Movements(continued)

Key Point

• Specificity is a major consideration when one is designing an exercise program to improve performance in a particular sport activity. The sport movement must be analyzed qualitatively or quantitatively to determine the specific joint movements that contribute to the whole-body movement. Exercises that use similar joint movements are then emphasized in the resistance training program.

Skeletal Musculature

• Skeletal musculature – A system of muscles enables the skeleton to move. – Origin = proximal (toward the center of the body)

attachment.– Insertion = distal (away from the center of the body)

attachment.

Muscle Actions• Isometric: A muscle remains a

constant length as it contracts because the resistive torque is equal and opposite to the muscle torque

• Concentric: A muscle shortens as it contracts because the muscle torque is greater than the resistive torque

• Eccentric: A muscle lengthens as it contracts because the resistive torque is greater than the muscle torque

Key Terms for Muscles that Produce Movement

• Agonist: The muscle most directly involved in bringing about a movement; also called the prime mover (eg, quadriceps are prime movers during the seated knee extension exercise).

• Antagonist: A muscle that can slow down or stop the movement, or typically has an opposite function of the agonist (eg, during the seated knee extension, the hamstrings are the antagonists)

• Synergist: A muscle that assists the agonist in a movement.

Human Strength and Power • Basic Definitions

– Strength: capacity to exert force at any given speed.

– Acceleration: The change in velocity per unit of time. This is associated with resistive force by Newton’s second law (Force = Mass ×Acceleration).• Speeding up or slowing down

Human Strength and Power • Basic Definitions

– Work: product of force & linear displacement or torque and angular displacement (angle through which an object rotates.

·Negative work: work performed on a muscle; muscle torque & angular displacement in opposite direction, such as lowering dumbbell during biceps curl

·Positive work: work performed by a muscle; muscle torque & angular displacement in same direction, such as raising dumbbell during biceps curl

– Power: product of force & linear velocity or torque & angular velocity; work per unit time or the rate of which work is done. Outside of the scientific realm, power is loosely defined as “explosive strength.”

·The rate at which repetitions are performed determines power output.

Key Points

• Although the word strength is often associated with slow speeds and the word power with high velocities of movement, both variables reflect the ability to exert force at a given velocity.

• The sport of weightlifting (Olympic lifting) has a much higher power component than the sport of powerlifting, due to the higher movement velocities with heavy weights of the weightlifting movements.

Human Strength and Power • Biomechanical Factors in Human Strength

– Neural Control• Muscle force is greater when: (a) more motor units are

involved in a contraction, (b) the motor units are greater in size, or (c) the rate of firing is faster.

– Muscle Cross-Sectional Area• The force a muscle can exert is related to its cross-

sectional area rather than to its volume (area x length).– Arrangement of Muscle Fibers

• Pennate muscles (oblique orientation of fibers) can generally produce more force than fusiform muscles (parallel orientation of fibers)

Figure 4.11Examples of Muscle Fiber Arrangements

Human Strength and Power • Biomechanical Factors in Human Strength

– Joint Angle• Amount of torque depends on force versus muscle length,

leverage, moment arm, type of exercise, the body joint in question, the muscles used at that joint, and contraction speed.

– Muscle Contraction Velocity• Force-Velocity Curve (force decreases as concentric

velocity increases and increases as eccentric velocity increases)

– Muscle Length• Force-Length Curve (force decreases as muscle

length decreases or increases relative to resting length)

Figure 4.13Force-Velocity Curve

Note: Maximal eccentric strength is estimated to be approx 1.5-2 times stronger than maximal concentric strength.

Force-Length Relationship

Note: Resting length (or slightly above resting length) optimizes the arrangement of actin and myosin filaments

Force-Length RelationshipNote: Sarcomere lengths less than resting length results in decrease in force due to overlapping of actin & abutting of myosin against Z-discs.

Force-Length Relationship

Note: Sarcomere lengths greater than resting length results in a decrease in force due to less overlapping of actin and myosin.

Human Strength and Power

• Biomechanical Factors in Human Strength– Strength-to-Mass Ratio

• In sprinting and jumping, the ratio directly reflects an athlete’s ability to accelerate his or her body.

• In sports involving weight classification, the ratio helps determine when strength is highest relative to that of other athletes in the weight class.

Human Strength and Power

• Biomechanical Factors in Human Strength– Body Size

• As body size increases, body mass increases more rapidly than does muscle strength.

• Given constant body proportions, the smaller athlete has a higher strength-to-mass ratio than does the larger athlete.

Key Point

• In sport activities such as sprinting and jumping, the ratio of the strength of the muscles involved in the movement to the mass of the body parts being accelerated is critical. Thus, the strength-to-mass ratio directly reflects an athlete’s ability to accelerate his or her body.

Sources of Resistanceto Muscle Contraction

• Gravity – Applications to resistance training

• When the weight is horizontally closer to the joint, it exerts less resistive torque.

• When the weight is horizontally farther from a joint, it exerts more resistive torque.

Key Point

• Exercise technique can affect the resistive torque pattern during an exercise and can shift stress among muscle groups.

Sources of Resistanceto Muscle Contraction

• Gravity – Weight-stack machines

• Gravity is the source of resistance, but machines provide increased control over the direction and pattern of resistance.

• Inertia– Things at rest tend to stay at rest and things in motion

tend to stay in motion unless compelled to change that state through the action of an external net force.

Sources of Resistance to Muscle Contraction

• Friction– Friction is the resistive force encountered when one

attempts to move an object while it is pressed against another object – it is a force that opposes motion

• Fluid Resistance – Fluid resistance is the resistive force encountered by an

object moving through a fluid (liquid or gas), or by a fluid moving past or around an object (eg, aquatic exercise).

• Elasticity – The more an elastic component is stretched, the greater

the resistance (eg, using theraband during exercise).

Joint Biomechanics:Concerns in Resistance Training

• Back – Back Injury

• The lower back is particularly vulnerable. • Resistance training exercises should generally be

performed with the lower back in a moderately arched position.

– Intra-Abdominal Pressure and Lifting Belts• The “fluid ball” aids in supporting the vertebral column

during resistance training. • Weightlifting belts are probably effective in improving

safety. Follow conservative recommendations.

Figure 4.15“Fluid Ball”

• The “fluid ball” resulting from contraction of the deep abdominal muscles and the diaphragm – helps “unload” the lumbar spine.

Key Term

• Valsalva maneuver: 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. Increases intra-abdominal and intra-thoracic pressures and “unloads the spine, but also increases blood pressure and impedes blood flow back to heart.

Joint Biomechanics: Concerns in Resistance Training• Shoulders

– The shoulder is prone to injury during weight training because of its structure and the forces to which it is subjected.

– Warm up with relatively light weights.– Follow a program that exercises shoulders in a balanced way.– Exercise at a controlled speed.

• Knees – The knee is prone to injury because of its location between two

long levers.– Prevent the knees from translating beyond the toes during the

squat, lunge, leg press, etc…– Minimize the use of knee wraps.

• Elbows and wrists– The primary concern involves overhead lifts. However, the most

common source of injury to these areas is from overhead sports such as throwing events or the tennis serve.

Joint Biomechanics:Concerns in Resistance Training

• How Can Athletes Reduce the Risk of Resistance Training Injuries?– Perform one or more warm-up sets with relatively

light weights, particularly for exercises that involve extensive use of the shoulder or knee.

– Perform basic exercises through a full ROM.– Use relatively light weights when introducing new

exercises or resuming training after a layoff of two or more weeks.

– Do not ignore pain in or around the joints.(continued)

Joint Biomechanics: Concerns in Resistance Training

• How Can Athletes Reduce the Risk of Resistance Training Injuries? (continued)– Never attempt lifting maximal loads without proper

preparation, which includes technique instruction in the exercise movement and practice with lighter weights.

– Performing several variations of an exercise results in more complete muscle development and joint stability.

– Take care when incorporating plyometric drills into a training program – 1st build solid strength base and employ a periodization regimen.