Before you begin reading about the force-velocity relationship and its role in strength training, you may want to read about force production in strength training as an understanding of force will help you understand what follows.
Force-velocity Relationship: A property of skeletal muscle contraction in which the force capability of a given muscle contraction is dependent on the velocity of shortening of the muscle.
There are three primary characteristics which affect the force potential of human skeletal muscle: velocity, length, and time. For those engaged in the pursuit of maximal strength, the force-velocity relationship may be the most important mechanical characteristic of human skeletal muscle. It is also the characteristic that is least known and understood by strength trainees.
It is especially problematic because of the relationship between most athletic endeavors and power. That is, most athletic pursuits require high power outputs rather than high total force outputs. Since most information on strength training concerns, this end goal rather than training for maximal strength, the student of maximal strength is left with mixed and confusing information. Even so, it is important for everyone to know that up to a certain point in strength advancement, increases in absolute strength always correspond to a measurable increase in power.
Specifically, the force-velocity relationship means that as the velocity of shortening increases the force capability decreases. In other words, high velocity or “fast” movements correspond to low force output from the muscles and low-velocity movements correspond to high force output from the muscles. The reverse, therefore, is also true, in that the force resisting the muscle (e.g. the weight of an object being lifted) dictates the velocity of muscle shortening. In this way, high force movements (e.g. from lifting max loads) correspond with low velocity of muscle shortening and low force movements (e.g. lifting lighter loads) correspond with a high velocity of shortening.
Of all the mechanical characteristics of muscle, the force-velocity relationship may the most important for strength trainees as it shows that the force of an active muscle is variable in all muscle actions, eccentric, isometric, and concentric) and completely dependent on the speed of movement, and therefore the relative weight of the load being lifted. Take a light weight and attempt to exert maximal force against it. What happens? You move the weight very quickly. The muscles are shortening very quickly. The quicker this happens the less total force the muscles can apply. When many muscle force experiments are plotted on a graph the “force-velocity curve” emerges. The following image is an idealized version of this curve for illustration purposes only.
The best way to imagine how this image applies is to view it left to right. The red line is total force and the axis at the bottom is the velocity of shortening. Think of the far right of the bottom axis as maximum velocity. As you can see the red line bottoms out. Virtually no force at the maximum velocity of shortening. Notice that as we move from right to left and velocity of shortening slows the red line goes up steadily. So as the velocity of shortening slows down, force production goes up. As we approach further to the left and velocity reaches zero we see an abrupt rise in force production. Think of zero velocity as an isometric action, then. The muscle can produce its maximum force. But no actual work is being performed.
The green line represents power. Power is the “sweet spot” between velocity and somewhere around 40% of maximum all the way to 80%, depending on the person and the specifics of the movement. This is the area the Olympic lifter is concerned with and any athlete that needs to express strength as power. This is commonly referred to as “explosive strength”. We, however, are concerned with the area to the left of this intersection where velocity is decreasing but is not nil. This is the area where maximum force can be developed.
Explosive strength may well be on our menu from time to time but it is not our goal and for advanced lifters increases in explosive strength will have little to no bearing on absolute force production against near-maximal loads.
The force velocity relationship has a physiological basis. Or at least it has a theoretical physiological basis. Muscles contract by forming cross-bridges between actin and myosin (these are defined below) filaments in the myofibrils. These cross-bridges take a finite amount of time to occur. As the velocity of shortening increases the filaments slide past one another faster and faster causing less cross bridges to be able to form.
Here is an animation of the actin-myosin cross-bridge. Watch closely to see the steps involved. If you imagine the animation speeding up but the steps involved in each cross-bridge not speeding up you will see how the force-velocity relationship might occur.
What is Actin and Myosin?
Actin is a protein in a muscle fiber sarcomere that, with myosin, is responsible for contraction and relaxation. During contraction, actin filaments are pulled toward the center of the sarcomere by the action of the myosin filaments, causing the sarcomere to shorten.
The actin proteins in the sarcomere make up the thin I-band of the myofilament, together with troponin and tropomyosin, which form the core of the thick filament made up the myosin proteins. Each actin molecule has a myosin binding site which the head of the myosin filament attaches, which is referred to as a cross-bridge.
Actin is a major component of the cytoskeleton of cells and is present in all eukaryotic cells.
Myosin is a protein in a muscle fiber that, with actin, is responsible for contraction and relaxation. These proteins comprise about 65% of total muscle protein.
Myosin molecules make up the thick A-band of the muscle sarcomere and are comprised of long chains with numerous tails coming off them. Each tail of the myosin filaments has a head which is able to attach to a binding site on an actin molecule.