Force-Velocity & Power-Velocity Relationships

Force-Velocity and Power-Velocity Relationships

• Different activities need different amounts of strength (force) and power.
• Absolute or peak force a muscle generates depends on the speed of muscle lengthening or shortening.
• Muscles lengthen and shorten at different velocities based on the load.
• At the highest speed of muscle shortening (concentric movement), the muscle generates the lowest force.
• As the speed of muscle shortening decreases, force output increases to isometric contraction (no muscle movement).
• At the highest speed of muscle lengthening (eccentric movement), the muscle generates the highest force.
• Fast-twitch muscle fibers produce greater force at higher speeds compared to slow-twitch fibers.
• Greater peak power occurs in fast-twitch fibres.
• Athletes in sports with high power bursts have a higher percentage of fast-twitch fibers in their muscles.

Purpose

• To determine the effects of velocity on peak torque (force) and power of maximal concentric and eccentric contractions in the right knee extensor muscles.
• To determine general classification of slow and fast muscle groups using various velocity settings using an isokinetic dynamometer.

Equipment

• Cybex isokinetic dynamometer is used to measure the force and power.

Methods - Cybex

• Subject is set up on the Cybex in the proper position for right knee extensor muscle testing.
• Peak torque of the right knee extensor muscles is measured at selected concentric and eccentric angular velocities.
• Concentric velocities are 60, 90, 150, 300 degrees/second (deg/s).
• Eccentric velocities are 60, 90, 150, 300 degrees/second (deg/s).
• A rest period between each velocity testing session will be included in the Cybex programming.
• Lower velocities measure strength and higher velocities measure power.
• Peak torque values (ft-lbs) from the data sheets are converted to N-m by multiplying the value by 1.3558 (1 ft-lb = 1.3558 N-m).
• The 'Original Peak force output data (N-m)' in Table 1 is modified so that the Force-Velocity graph will look similar to the graph in Figure 1.
• Degrees/second (deg/s) are converted into radians/second (rad/s) by dividing by 57.3.

Calculations

• Peak power in Watts is calculated by multiplying the peak force (N-m) by velocity (rad/s).
• Watts and rad/s are calculated from the data in Table 1 using the formula for calculating power.
• Power = force x velocity
• Angular velocity needs to be expressed in radians/second (rad/s).
• To convert degrees/second to rad/s, divide by 57.3 (1 rad/s = 57.3 degrees/s).

Graphing Guidelines

• Graphs are made using EXCEL.
• The graph should have a vertical middle line indicating “0” velocity.
• Eccentric velocities should be placed to the left of the middle line (in the EXCEL spreadsheet, indicate eccentric velocities as negative numbers, otherwise, the graph will be incorrect.
• Concentric velocities should be placed to the right of the vertical middle line.
• For the force-velocity graph:
  • x-axis is Angular Velocity (degree/second).
  • y-axis is Peak Torque (N-m).
  • Use a Scatterplot with curved lines
• For the power-velocity graph:
  • x-axis is Angular Velocity (radians/second).
  • y-axis is Power (Watts).
  • Use a Scatterplot, dots only, add in a polynomial trendline

Force-Velocity Relationship

• Explain the impact of velocity on force production during eccentric and concentric muscle contractions.
• Explain the difference in force production between eccentric and concentric muscle contractions.
• Explain the shape of the force-velocity curve for both eccentric and concentric velocities.
• Use reference to cross-bridge mechanism of muscle contraction (e.g., cross-bridge sliding, attachment, breaking action, # of attached cross-bridges).

Power-Velocity Relationship

• Describe the relationship between peak power and velocity.
• Explain the shape of the power-velocity curve for both eccentric and concentric velocities.