Conservation of Momentum and Forces in Biomechanics

Introduction to Gait Mechanics

  • Understanding body mechanics and movement during walking and running.

Walking Mechanics

  • Body Mass Movement:

    • When walking, the body mass shifts over a fixed point, typically represented by a line of gravity.
    • Once the body mass passes this line, it needs to redistribute to avoid falling, which is akin to a controlled fall.

  • Role of Legs:

    • After passing the line of gravity, the opposite leg must be brought forward to assist in recovery and maintain momentum.

  • Reciprocal Motions:

    • Reciprocal motions between the legs and body are crucial for efficient walking.
    • It is stated that the total work done in opposing angles must equal zero due to the conservation of momentum.
    • Work done while shifting body mass is equal in amount but opposite in direction.

  • Conservation of Momentum:

    • Defined by the principle that mass and velocity in one system should total zero as they interact with forces.
    • This principle is a fundamental part of mechanics studied in physics.

Running Mechanics

  • Distinction from Walking:

    • In running, the mechanics differ as it involves no fixed point; thus, it includes flight phases where body mass is airborne.
    • This adds complexity to controlling momentum and movement.

  • Key Angles in Movement:

    • Success in running is dependent on strategically opposing angles to balance forces and conserve momentum.
    • Studies show the backswing of the leg plays a significant role in momentum conservation. The coordination of back and forward swings enhances the efficiency of movement.

  • Kinetic Chain Segments:

    • Kinetic chain segments can start movement at different times, which is key in sports like broad jumping.
    • The timing of segments affects the efficiency of the jump and can impact performance.

Example Exercise: Counterbalance Jump

  • Instructions:
    • Participants are guided to simulate a counterbalance jump to observe bodily mechanics.
    • Observations:
    • Attention is drawn to which body segments move first (head, shoulders, trunk).
    • Purpose: Understanding how to optimize movement efficiencies based on body mechanics.

Key Factors in Performance

  • Initiation of Movement:

    • Movement begins with the head to manage balance and generate forward momentum.
    • The trunk follows to allow the legs to push against the ground optimally.

  • Importance of Arms:

    • Arms are last to move as they provide balance and rotational energy. Their lightness allows them to accelerate quickly, contributing to overall efficiency in movement.

  • Synchronizing Components:

    • All segments of the leg must stop at the same time during a jump or run to maintain momentum and force efficiency.

Energy and Biomechanics of Walking

  • Walking Speed Dynamics:

    • Walking occurs at various speeds which aim to minimize energy expenditure, with one foot always on the ground, impacting efficiency.
    • Graphs of power or watts versus speed indicate an optimal point where work performed is equal to energy cost.

  • Physiological vs. Biomechanical Cost:

    • Physiological costs involve metabolic energy required for movement.
    • Biomechanical costs relate to physical factors reducing speed as we try to walk faster, including potential and rotational energy.

Optimal Walking and Running Energetics

  • Kinetic and Potential Energy:

    • Increasing leg height during running boosts potential energy, increasing resultant kinetic energy.
    • The point of optimal performance reflects the balance of energy expenditure and mechanical work.

  • Transition from Walking to Running:

    • Graphical illustrations indicate the cost of walking rises significantly beyond certain speeds, complicating physical strain during running (increased anaerobic engagement).
    • Transitioning from a predominantly aerobic state to anaerobic as speed increases.

Kinetics vs. Kinematics

  • Definitions and Principles:
    • Kinetics is the study of forces acting on a body, considering mass and acceleration as described by Newton's second law: F = ma (Force = mass × acceleration).
    • Kinematics describes motion without addressing mass but focuses on positional change.

Forces in Motion

  • Force Definition:

    • A force is an interaction that changes the motion of an object, defined as the action of one object on another.

  • Types of Forces:

    • Non-conservative Forces:
    • Gravity exemplified as a consistent force acting downwards through the body’s center of mass.
    • Normal Force:
    • A force opposing gravity, acting at 90 degrees to the surface during movement.
    • Changes based on directional slopes affecting how weight is distributed.
    • Weight Calculation:
    • Defined by W = mg, where W is weight, m is mass, and g is the acceleration due to gravity (9.8 ext{ m/s}^2).

Conclusion

  • Further Exploration:

    • Future lessons will explore contact vs. non-contact forces, emphasizing their role in biomechanics.
    • An assignment is due on Friday focusing on graphing and analyzing differences in athletic performance metrics.

  • Final Remarks:

    • The nuances of gait mechanics, energy optimization, and the role of forces will continue to be elaborated in subsequent classes.