This article explains how the contractile force of leg muscles changes with exercise intensity, covering the properties and roles of the muscle fibers of the ground and fast-twitch fibers. It also analyzes how the order and size of motor unit mobilization affects performance, providing a physiological basis for improving athletic performance.
Suppose a student goes from a brisk walk to a fast trot. What exercise physiology principle is at work here? When performing an exercise, the force exerted by a muscle, or muscle contraction force, increases in proportion to the intensity of the exercise. So, as the athlete increases the speed at which they run, the muscle contraction force in their leg muscles increases accordingly.
The type of muscle fiber and the mobilization pattern of the motor unit play a large role in the increase in muscle contraction force. Skeletal muscles, including leg muscles, are composed of many different muscle fibers. Skeletal muscles are the muscles that move the skeleton under the control of central nerves, and myofibrils are the contractile fibrous cells that make up muscle tissue. These myofibrils are contracted by the stimulation of motor nerves, and a motor nerve and the myofibrils it controls are called a “motor unit”.
Motor units are mobilized in an order that depends on the intensity of the exercise, which can be explained by the principle of size. The principle of magnitude refers to the sequential mobilization of smaller to larger motor units, which means that muscle contractile force increases through progressive activation of motor units when transitioning from low- to high-intensity exercise. This results in smaller motor units (i.e., the vastus lateralis muscle fibers) being primarily used during low-intensity activities, while larger motor units (i.e., the fast-twitch muscle fibers) are additionally mobilized as the activity progresses to higher intensities.
The muscle fibers that make up the muscles of the leg can be divided into two main types: long and short fibers. The latter are called red muscle fibers because of their red color due to the high content of myoglobin, which is involved in the storage and transport of oxygen in the muscles, while the former are white due to their relatively low content of myoglobin. On a per-motor unit basis, there are between 10 and 180 muscle fibers connected to a single motor nerve, and between 300 and 800 muscle fibers connected to a single motor nerve. The more muscle fibers that are connected to a single motor nerve, the greater the contractile force of the muscle. For this reason, motor units composed of fast-twitch fibers generate a much stronger contraction force.
The distribution and characteristics of muscle fibers have an important impact on athletic performance. Different types of muscle fibers have different contractile forces, contraction speeds, and resistance to fatigue. The muscle fibers of the deep muscle have relatively low contractile force, slow contraction speed, and high fatigue resistance. Fast-twitch fibers are further divided into type A and type B fibers based on their specific physiological characteristics. Type B muscle fibers fatigue more quickly than their counterparts, but they also produce fast and explosive contraction forces. On the other hand, type A muscle fibers have properties that are intermediate between those of type B and type A. They contract faster than type B muscle fibers, but at the same time, they have a higher fatigue resistance than type B muscle fibers. Therefore, people with a high proportion of type A muscle fibers in their muscles have greater endurance, which makes them suitable for long-distance sports such as marathons. On the other hand, people with a higher percentage of fast-twitch fibers are better suited for short-distance sports, such as running 100 meters.
These muscle fiber properties and the mobilization of motor units are the basis for shaping adaptations to specific exercise types. For example, if repetitive high-intensity training promotes the activation and development of fast-twitch muscle fibers, the muscles will have an increased ability to exert greater force in a shorter period of time. Conversely, if you increase the utilization of your extensor fibers through prolonged low-intensity exercise, the muscles will become more resistant to fatigue, making them better suited for longer distances.
As the intensity of the exercise increases progressively, the contractile force of the muscles increases proportionally. One of the principles that applies here is the principle of magnitude. According to this principle, when performing exercises of progressively increasing intensity, the motor units are mobilized sequentially according to their size. In low-intensity exercises, the smallest motor units are recruited. Then, as the intensity of the exercise increases, additional motor units are recruited from the fast-twitch fibers, which have larger motor units. So, in low-intensity walking, most of the leg strength is recruited from the vastus lateralis fibers, while in moderate-intensity running, the vastus lateralis fibers are joined by the type A fast-twitch fibers. In addition, during high-intensity sprinting, the vastus lateralis and type B muscle fibers are activated in addition to the vastus lateralis and type A muscle fibers. This illustrates how when a student transitions from walking to running, which muscles are mobilized and in what way.
Ultimately, the selective mobilization of muscle fibers based on exercise intensity is an important factor in determining athletic performance. As the body adapts to exercise intensity, the direction of muscle fiber development changes, leading to long-term training effects. The distribution and utilization of muscle fibers depends on the type and intensity of the exercise, and understanding this is essential for effective training planning.
