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The force–velocity relationship in skeletal muscle is a well-studied area, with the velocity of muscle contraction being a function of the load lifted. This relationship is represented by the force-velocity curve, which shows an inverse relationship between force and velocity. The force–velocity relationship is influenced by the type of muscle fibres present, which can be slow oxidative (SO), fast oxidative (FO), or fast glycolytic (FG). FG fibres produce quick and powerful movements but fatigue rapidly, whereas SO fibres are slower but more resistant to fatigue. The speed of contraction is also influenced by the rate of ATP regeneration, which can occur through creatine phosphate, anaerobic glycolysis, or aerobic metabolism. Velocity-based training has been shown to be an effective method for improving strength and conditioning, as it allows for optimal load dictation during training.
| Characteristics | Values |
|---|---|
| Muscle contraction velocity | Depends on the muscle type, e.g., vascular smooth muscle shortens 50 times slower than fast skeletal muscle |
| Muscle contractile force | Depends on the muscle type, e.g., fast glycolytic (FG) fibers produce high levels of tension |
| Load lifted | Velocity decreases as the load increases |
| Muscle length | Velocity is a function of the load lifted and muscle length |
| Stimulus frequency | As the stimulus frequency is increased, the force increases until a maximum is reached, after which it decreases |
| Training type | Training on different parts of the force-velocity curve will impact performance differently |
| Training intensity | Velocity-based training can be used to define and manipulate training intensity |
| Muscle growth | Velocity-based training can be used to optimise hypertrophy |
| Muscle fatigue | Muscle fatigue occurs when a muscle can no longer contract due to an oxygen debt |
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What You'll Learn
The relationship between force and velocity
This relationship demonstrates that an increase in force results in a decrease in velocity, and vice versa. For example, a 1RM back squat produces high levels of force but is lifted at a slow velocity, whereas a counter-movement jump produces high movement velocity but low levels of force. This indicates a trade-off between force and velocity.
The force-velocity curve is particularly important for strength and conditioning coaches when prescribing exercises for athletes. For instance, a coach should understand the differences between a one-repetition maximum (1RM) deadlift and five-repetition maximum (5RM) jump squats, as one will produce higher forces and lower velocities than the other. Training programmes that combine strength and power training have been shown to improve athletic performance more than strength or speed training alone. Therefore, coaches often take an all-rounded approach, training athletes at each section along the force-velocity curve to maximise their explosiveness.
The force-velocity relationship has been studied in the context of skeletal muscle and muscle fibres. Muscle contraction can be characterised by the velocity of shortening and the magnitude of isometric force (tension). When the magnitude of isometric force reaches a value equal to the load imposed on the muscle, the muscle starts shortening with a constant velocity. This relationship between force and velocity of shortening can be measured simultaneously in experimental setups.
Furthermore, the force-velocity relationship has been observed in the mechanical properties of smooth versus striated muscle. Smooth muscle, found in the walls of many hollow organs, shortens 50 times slower than fast skeletal muscle but generates comparable force using 300 times less chemical energy. Vascular smooth muscle contraction results from the cyclic interaction of the contractile protein myosin. Similarly, striated muscle contraction involves the interaction of myosin and actin filaments, with changes in muscle length altering the active force by varying the degree of overlap of these filaments.
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Muscle contraction velocity
The velocity of muscle contraction is inversely related to the force exerted, as described by the force-velocity curve. This means that as the force generated by a muscle increases, the contraction velocity decreases, and vice versa. For example, a back squat produces high levels of force but at a slow velocity, while a counter-movement jump produces low levels of force and high velocity. This relationship is important for strength and conditioning coaches when designing training programmes to optimise athletic performance.
The type of muscle fibre also plays a role in contraction velocity. Human skeletal muscle contains three types of fibres: slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG). FG fibres produce rapid and forceful contractions, making them suitable for quick and powerful movements, while SO fibres contract more slowly. The speed of contraction depends on how quickly myosin's ATPase hydrolyzes ATP to produce cross-bridge action. Fast fibres hydrolyze ATP more rapidly, resulting in quicker cross-bridge cycling.
Additionally, the load lifted during a contraction affects velocity. As the load increases, the velocity of contraction decreases. This relationship has been studied using experimental setups with weights attached to a lever arm, where the muscle shortens under a constant load.
Velocity-based training has been explored in muscle hypertrophy. Research suggests that dictating training intensity based on movement velocity can lead to greater strength adaptations compared to traditional loading approaches. A larger velocity drop-off facilitates a greater hypertrophic response, indicating that velocity plays a significant role in muscle growth and adaptation.
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Muscle fibre types
There are three main types of skeletal muscle fibres: slow oxidative (Type I), fast oxidative (Type IIa), and fast glycolytic (Type IIx). Slow oxidative fibres contract slowly and use aerobic respiration (oxygen and glucose) to produce ATP. In contrast, fast oxidative fibres contract quickly and also use aerobic respiration to generate ATP. Fast glycolytic fibres, on the other hand, contract rapidly and rely primarily on anaerobic glycolysis for energy production. This means they don't require oxygen, instead breaking down glucose directly to generate ATP.
The speed of muscle contraction is influenced by the rate at which myosin's ATPase hydrolyzes ATP to produce cross-bridge action. Smooth muscle, found in the walls of hollow organs like the cardiovascular and respiratory systems, shortens 50 times slower than fast skeletal muscle. However, it can generate comparable force using 300 times less chemical energy.
The force-velocity relationship in skeletal muscle is often studied using frog sartorius muscle, which exhibits a rectangular hyperbolic relationship between force (P) and velocity (V). This relationship is described by the Hill equation, which indicates the energy output rate during muscle contraction. The maximum velocity of shortening under zero load (Vmax) is one of the critical values in this equation.
Training programmes that focus on a single aspect of the force-velocity curve, such as maximum strength, may enhance force production but can reduce muscle contractile velocity. Therefore, a comprehensive training programme should incorporate various sections of the force-velocity curve to maximise athletic performance and explosiveness.
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Muscle length
The force-velocity relationship was first explored by Hill in 1922, who stated, "the force exerted is greater the less the rate of movement, and vice versa." Hill's work laid the foundation for subsequent studies on muscle contraction, specifically the relationship between force, velocity, and muscle length.
The force-velocity relationship is further influenced by the preload and inotropy of cardiac muscle fibres. Preload refers to the initial length of the muscle fibre, and an increase in preload results in a greater velocity of shortening at a given afterload. This is due to the increased tension-generating capability at longer preloads, allowing the muscle to contract faster. Inotropy, unique to cardiac muscle, also impacts the force-velocity relationship. An increase in the inotropic state of cardiac fibres leads to a parallel shift in the force-velocity curve, resulting in increased Vmax (maximum velocity) and Fmax (maximum force).
Additionally, the type of muscle fibre plays a role in the force-velocity relationship. For example, vascular smooth muscle shortens 50 times slower than fast skeletal muscle but generates comparable force using significantly less chemical energy. This highlights the distinct mechanical properties of different muscle types and their unique contributions to the force-velocity relationship.
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Muscular hypertrophy
There are two types of muscle hypertrophy: myofibrillar and sarcoplasmic. Myofibrillar hypertrophy focuses on increasing the size of myofibrils, which are chains of sarcomeres within a muscle cell. This type of hypertrophy leads to increased muscular strength and speed. On the other hand, sarcoplasmic hypertrophy involves increased muscle glycogen storage, resulting in larger muscles without a significant increase in strength. This type of hypertrophy is more common in bodybuilders, as it leads to a greater increase in muscle size.
To achieve muscular hypertrophy, progressive overload is often used as a strategy. This involves gradually increasing the resistance or repetitions over successive bouts of exercise to maintain a high level of effort. Additionally, mechanical tension and metabolic fatigue are important factors. Exercises that involve concentric (shortening) movements at fast-to-moderate speeds and eccentric (elongating) movements at slower speeds have been found to be highly effective. For example, lifting weights at a moderate speed and lowering them back down slowly. It's also important to continuously challenge the muscles by varying exercises and increasing resistance over time to promote growth.
Training variables such as frequency, intensity, and total volume also play a role in muscular hypertrophy. Incorporating a full range of motion (ROM) at elongated muscle lengths, such as deep squats and full-ROM deadlifts, increases mechanical tension on muscle fibers and may stimulate greater muscle growth. Additionally, proper rest between sets, adequate sleep, and a balanced diet rich in macronutrients, especially protein, are crucial for optimizing muscle gain.
It's worth noting that there is a rare genetic condition called myostatin-related muscular hypertrophy, which results in reduced body fat and increased muscular size and strength. This condition is caused by mutations in the MSTN gene and is usually non-debilitating, with most individuals experiencing no serious medical complications.
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Frequently asked questions
There is an inverse relationship between force and velocity in muscles, meaning that an increase in force causes a decrease in velocity, and vice versa.
The force-velocity curve is a representation of the relationship between force and velocity. It is important for strength and conditioning coaches to understand this relationship to prescribe optimal training.
Muscle contraction velocity is dictated by the rate at which myosin's ATPase hydrolyzes ATP to produce cross-bridge action. Fast-twitch fibres hydrolyze ATP twice as fast as slow-twitch fibres, resulting in quicker cross-bridge cycling.
Velocity-based training has been shown to achieve greater strength adaptations with less training volume. A larger velocity drop-off will facilitate a greater hypertrophy response.
Slow stretch-shortening plyometric drills such as counter movement jumps and single-leg high hurdle jumps train the lower velocity section of the curve. Fast stretch-shortening plyometric drills such as hopping, bounding, sprinting and assisted sprinting train the higher velocity section.
