Dynamics II: energy and power

Dynamics II: Energy and Power in Biomechanics

1. Introduction

Energy and power are essential concepts in biomechanics, determining how the body stores, transfers, and utilizes mechanical energy during movement. These principles help in analyzing sports performance, injury prevention, rehabilitation, and prosthetic design.

• Energy is the capacity to do work.

• Power is the rate at which energy is transferred or work is done.

Understanding energy and power in biomechanics helps in improving movement efficiency, reducing fatigue, and optimizing athletic performance.

2. Mechanical Energy in Human Movement

a. Types of Mechanical Energy

Type	Definition	Example in Human Motion
Kinetic Energy (KE)	Energy due to motion KE=12mv2KE = \frac{1}{2} mv^2KE=21​mv2	Sprinting, jumping
Potential Energy (PE)	Stored energy due to position PE=mghPE = mghPE=mgh	Standing at the top of stairs
Elastic Energy	Energy stored in stretched tissues	Achilles tendon during running

• Kinetic Energy increases with speed and mass (important in sprinting and jumping).

• Potential Energy is highest when the body is elevated (e.g., high jump).

• Elastic Energy is crucial in sports like running, where tendons store and release energy for efficiency.

3. Work-Energy Principle in Biomechanics

• Work is the transfer of energy through force application: Work=Force×DistanceWork = Force \times DistanceWork=Force×Distance

• The work-energy principle states that the work done on a body results in a change in its energy.

• In biomechanics, work occurs when muscles generate force to accelerate, decelerate, or lift the body.

Example: Work Done in Vertical Jumping

• Muscles do work against gravity to store potential energy at takeoff.

• Upon landing, energy is absorbed through eccentric muscle contraction.

4. Power in Human Movement

a. Definition of Power

• Power is the rate at which work is done: Power=WorkTime=Force×VelocityPower = \frac{Work}{Time} = Force \times VelocityPower=TimeWork​=Force×Velocity

• Higher power output enables faster movements and explosive actions in sports.

b. Types of Power in Biomechanics

Type	Description	Example
Positive Power	Energy generation (concentric contraction)	Pushing off the ground in a jump
Negative Power	Energy absorption (eccentric contraction)	Landing from a jump, decelerating

• Explosive movements (e.g., sprinting, weightlifting) require high power production.

• Controlled movements (e.g., landing, braking) involve power absorption.

5. Energy Efficiency & Economy in Human Motion

• Energy efficiency refers to how effectively the body uses energy.

• Energy economy is crucial in endurance sports (e.g., marathon running).

• Stretch-shortening cycle (SSC):

  • Tendons store and release elastic energy, reducing metabolic cost.

  • Example: Achilles tendon recoil during running improves efficiency.

6. Applications in Biomechanics

Field	Application
Sports Performance	Maximizing power output in sprinting, weightlifting
Injury Prevention	Managing impact forces and energy absorption
Rehabilitation	Restoring energy efficiency in movement patterns
Prosthetics & Exoskeletons	Designing energy-efficient assistive devices
Ergonomics	Reducing energy expenditure in workplace tasks

7. Conclusion

Energy and power are critical in understanding human movement efficiency, athletic performance, and injury mechanisms. By optimizing energy transfer and power output, biomechanics helps in sports training, rehabilitation, prosthetic development, and ergonomic design, leading to better performance and reduced injury risks.