Mechanics of Third Class Levers

Third Class Lever

• Force is applied between the axis and the resistance
• Example: Hitting a baseball with a bat
• Body example: Biceps curl

Levers

• Most body levers are third class
• Third class levers have advantages in terms of movement speed and range of motion.
• Third class levers have disadvantages in terms of requiring larger forces to move the joint
• Mechanical advantage is reduced if the distance between the resistance and the axis is long (e.g. long arms or legs)
• Mechanical advantage is reduced if the distance between the force and the axis is short (e.g. tendon insertion is close to the axis)

Mechanical Advantage

• Mechanical Advantage (MA) = Effort Arm ÷ Resistance Arm
• MA = 2.4 in the example provided in the text

Strength Curve

• Depends on the exercise, angle of pull, and other factors
• Force generating capacity increases when moving from the bottom to the top position
• Force generating capacity decreases when moving from the bottom to the top position

Factors Impacting Torque Curve Shape & Amplitude

• Muscle cross-sectional area
• Length of the force and resistance arm
• Angle of pull of the muscle on the bone
• Muscle micro and macro architecture
• Fiber type
• Neural stimulus
• Force-velocity relationship
• Length-tension relationship

Factors Influencing Strength

• Central nervous system (CNS) and neural control
• Muscle cross-sectional area
• Muscle fiber arrangement
• Muscle length
• Joint angle
• Contraction velocity
• Muscle action
• Strength-to-mass ratio

Neural Control

• Affects the number and types of motor units (MU’s) recruited
• Greater force is achieved by:
  • Increasing the number of MU’s recruited
  • Recruiting more high threshold MU’s
  • Increasing the rate coding

Muscle Cross-Sectional Area (CSA)

• Moderate correlation between muscle CSA and strength
• More contractile apparatus is available for force generation

Arrangement of the Fibres

• Arrangement of sarcomeres in relation to the long axis of the muscle
• Pennation angle = angle at which the sarcomeres are in relation to the line down the middle of the muscle
• Higher pennation, greater force generated
• Non-pennate muscles (e.g. Rectus Abdominis) primarily stabilize the body

Muscle Length

• Muscle length influences the overlap of actin and myosin filaments
• Optimal muscle length = maximal force production

Joint Angle

• Torque must be generated to produce movements
• Joint angle influences the amount of torque that can be generated:
  • Muscle length
  • Distance from the pivot

Muscle Action

• Concentric muscle action: muscle shortens under tension
• Eccentric muscle action: muscle lengthens under tension, resisting gravity
• Isometric muscle action: muscle gains tension but does not change length

Strength-to-Mass Ratio

• Ratio is important in many sports.
• Sprinters need large amounts of muscle mass but too much will compromise sprint velocity
• Athletes in weight category sports need to be as strong as possible at a comfortable weight

Sources of Resistance

• Power: Force x Velocity
• Velocity is the rate of change in position
• Power is specific to athletic movements
• Ground contact time for sprinting is short, requiring rapid force exertion

Athletic Movements

• Require different degrees of:
• Force
• Rate of force development
• Speed
• Power
• Endurance

The importance of muscular strength in power development

• Strength is the foundation of power, meaning that stronger individuals can generate more force and therefore more power.

Strength & Power Trade-off

• Strength = the amount of force developed
• Power = The amount of force exerted at a given velocity

Strength & Power Relationships

• Stronger people can produce more power since power = force x velocity
• Strength training without power training can improve only one end of the force-velocity curve
• Strength and power training together can improve both ends of the force-velocity curve
• Strong people tend to have larger and faster adaptations to power training

Influence of Strength on Power - Conclusions

• Power can be improved via strength training, particularly in weaker individuals
• Specific power training is required to maximise power potential
• Efficacy of power training increases with strength
• Strength = foundation of power

Levers

• Types of levers:
  • First class lever: Axis between force and resistance (e.g. scissors, triceps extending the elbow)
  • Second class lever: Resistance between axis and force (e.g. wheelbarrow, push-up)
  • Third class lever: Force between axis and resistance (e.g. hitting a baseball, bicep curl)

Other Important Factors

• Gravity: acts downwards
• Inertia: resistance to movement, can act in any direction
• Friction: resistive force between two objects, stationary objects require more force to move
• Elasticity: greater the stretch in elastic objects, the greater the resistance

Safety

• Proper exercise execution is imperative to reduce the risk of injury.
• Load should never compromise technique.

Injury Prevalence

• Some sports have a higher prevalence of injuries than others.

Important Factors for Increasing Strength

• Training intensity (load lifted)
• Progressive overload
• Law of diminishing returns
• Type of muscle action
• Type of exercise
• Contraction speed

Machine Weights

• Pulley systems move adjustable weights against gravity
• Movement in a fixed plane
• Weight stack and plate loaded machines are available

Machine Weights - Benefits

• Hypertrophy
• Strength improvements
• Controlled movement
• Safety
• Injury rehab

Variable Resistance Machines

• Many machines use a cam design to alter lever length and resistance.
• Aims to mimic human torque curve, increasing resistance during mechanical advantage and decreasing resistance during mechanical disadvantage.