The force velocity relationship illustrates that fiber level force output increases as contraction velocity goes down. Since we are talking specifically about fiber level force production, this relationship exists due to the greater degree of active cross-bridging occurring at a given time as a result of the slower contraction velocities.
Fiber Level vs Whole Muscle force production
- It’s important to mention that fiber level force production should not be confused with the total force produced by a whole muscle when shortening, since whole muscle force is influenced by multiple (usually hundreds) of motor units and the fibers they control.
Force velocity relationship of repetitions within an exercise
- The same force-velocity relationship that’s observed at the fiber level does hold true for isolated single repetitions of a movement when performed with maximal intent to accelerate (i.e., a 1 RM attempt). However, this same force-velocity relationship does not apply to the force output observed when performing multiple dynamic contractions due to the effects of fatigue. For example, if you were to squat your 10 RM, the 10th rep would produce considerably less force than the reps early in the set, assuming each rep was being performed with maximal intent to accelerate. In that example, force would go down with velocity.