• Introduction
  • The basics of the force-velocity relationship
  • Practical applications of the force-velocity relationship
  • Final thoughts
  • Sources
  • Actin: an abundant intracellular protein found in all eukaryotic cells. These thin filaments play a crucial role in muscle contraction and in cell movements.
  • Cross-bridge: the attachment of myosin with actin within the muscle cell.
  • Myosin: a type of molecular motor and converts energy from the hydrolysis of ATP into mechanical energy.
  • Sarcomere: the smallest contractile unit of a muscle fiber.
  • Tetanic contraction: repeated and overlapping stimuli at short intervals in which the muscle fiber does not fully relax before a new contraction.
  • Twitch contraction: a rapid contraction where the muscle fiber does not produce peak force output.


Skeletal muscle contractions begin when an action potential from a motor neuron reaches the muscle fiber and signals it to contract. This isolated contraction is called a twitch, which can last from a few milliseconds to 100 milliseconds depending on the muscle. Each twitch undergoes three distinct phases; 

  1. The latent period: action potential propagates along the sarcolemma and calcium ions (Ca2+are released from the sarcoplasmic reticulum.
  2. The contraction phase: Ca2+ ions bind to troponin, causing tropomyosin to move and expose the actin binding sites. This allows actin and myosin form cross-bridges and shorten the sarcomere.
  3. The relaxation phase: Ca2+ ions are pumped out of the sarcoplasm into the sarcoplasmic reticulum. Cross-bridge cycling stops and the muscles return to their resting length (see: muscle contraction).

A single twitch produces little tension because the muscle relaxes before reaching maximal force potential. Therefore, muscles need a series of action potentials to produce movement. If an additional action potential were to stimulate a muscle contraction prior to the full relaxation of a twitch, the successive twitch would sum onto the previous one. Thus, a progressively higher frequency of action potentials increases the force produced by the muscle. This phenomenon is known as wave summation, which allows muscles to produce varying levels of force (graded muscle response). When the frequency of action potentials is high enough to prevent muscles from relaxing between two successive stimuli, the muscle will become fully contracted. This is known as tetanic contraction, or tetanus. 

The force generated by a muscle is dependent on its length and shortening velocity. Together, these two factors have an impact on the muscle’s biomechanical properties, and thus, physical performance (strength, power, etc.). The force-velocity relationship demonstrates that muscles generate the greatest amount of force when at their resting length, whereas the least amount of force is produced when muscles are shortened or stretched. Force production is also controlled by 1) the velocity and 2) the force of muscle contraction. The optimal power output takes place at approximately 1/3 of maximum velocity.

This post explains the basics of the force-velocity relationship, and its implications in physical performance and training.

The basics of the force-velocity relationship

The force-velocity relationship describes the speed at which a muscle changes length in relation to the force of the contraction and the resulting power output (force x velocity = power). This is often displayed in a force-velocity curve, where the x-axis (i.e horizontal axis) indicates velocity of movement (measured in meters per second), and the y-axis (i.e. vertical axis) indicates the amount of force produced (measured in Newtons). The curve shows an inverse relationship between force and velocity – an increase in force results in a decrease in velocity and vice versa. For example, a countermovement jump (CMJ) produces low levels of force at a high movement velocity, whereas a squat generates high levels of force at a slow movement velocity. This tradeoff between force and velocity is thought to be a result of reduced time available for cross-bridge formation between actin (thin) and myosin (thick) filaments. The more time is available, the more cross-bridges are formed, and thus, more contractile force is produced. 

At maximum velocity, actin and myosin filaments are unable to form cross-bridges. Thus, no force is generated, and no power is produced (right edge of graph). Interestingly, the opposite is true when the muscle is stretched. Even though the muscle’s ability to produce force is increased, there is no velocity of contraction. Therefore, no power is generated (left edge of graph). The maximum amount of power is generated at roughly one-third of maximum shortening velocity.

Because alternating the movement velocity produces varying levels of force, different exercises can be placed in various sections of the force-velocity curve. This allows for a deeper analysis of each exercise and the training adaptations they offer. The information can later be used when creating a periodized training program.

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The Force-Velocity Curve

Twitch contractions are short and do not reach peak force.Tetanic contractions are long, reach peak force, and then plateau.The shortening velocity describes the speed at which a muscle changes length during a contraction.As velocity increases, the force of a muscle contraction decreases.Maximum power is generated at ~1/3 of maximum shortening velocity.

Practical applications of the force-velocity relationship

Power is the product of force multiplied by velocity (Power = Force x Velocity). Improving either of these components can offer significant benefits for physical performance (explosiveness, agility, etc.). The purpose of strength and power training is to improve the rate of force development (how fast force can be produced) and shift the force-velocity curve to the right. In practice, this means that larger loads can be moved at higher velocities. To achieve this, all parts of the force-velocity curve should be trained in order to maximize rate of force development.

Training programs that combine strength (high load, low velocity) and power (low load, high velocity) training have repeatedly shown greater improvements in rate of force development. This is due to the fact that by only training one component of the power equation, the person is likely to see improvements in that specific section of the paradigm. For example, maximal strength training can improve force production, but also reduce contraction velocity.

In addition to combining the aforementioned components, a well-thought-out training program should consider the individual’s 1) fitness level, 2) training goals, 3) training background, 4) main sport and position, and 5) the stage of the yearly macrocycle.

Final thoughts

The force-velocity relationship suggests that muscles are able to regulate their energy output depending on the load imposed on them. This is demonstrated in the force-velocity curve, which shows an inverse relationship between force and velocity. Thus, an increase in one results in a decrease in another.

Understanding the importance of the force-velocity curve is integral for physical training and should therefore be considered when planning a periodized seasonal program. For example, if a person is strong but lacks in power, more time should be spent on high-velocity movements. This may be crucial for athletic development since most sports benefit from power and explosiveness. Therefore, most training programs aim to increase the rate of force development and shift the force-velocity curve to the right. 

Did you learn anything new about the force-velocity relationship? Let us know in the comments.


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