Motor Units – What is it?

The contraction of a muscle occurs when a nerve or multiple nerves stimulate a specific group of muscle fibers to shorten. This group of muscle fibers, along with the nerve that activates them, is referred to as a motor unit. Muscles are composed of numerous such motor units, each playing a crucial role in muscle function. When a motor unit is activated, all the muscle fibers within that unit contract simultaneously. It is important to note that it is not possible to partially contract a motor unit; this phenomenon is known as the all-or-nothing principle of motor unit activation. This principle ensures that when a signal is sent, the entire motor unit responds fully, contributing to the overall strength and precision of muscle contractions.

Size of Motor Units

The size of a motor unit is determined by the number of muscle fibers it contains. Generally, the greater the number of fibers, the larger the motor unit. The number of muscle fibers within a single motor unit can vary significantly, ranging from as few as 5 to 10 fibers to more than 100 fibers. As a general rule, smaller muscles that perform precision tasks, such as the muscles in the hand, are composed of motor units with fewer muscle fibers. In contrast, larger muscles found in the trunk and limbs contain motor units with a greater number of muscle fibers. The force output of a motor unit is directly related to its size; that is, the larger the motor unit, the greater the force it can produce.

The size of a motor unit is also closely related to the type of muscle fibers it contains. In human muscles, differentiating motor units based on their physical properties can be challenging. Most studies utilize a classification system based on histochemical staining to identify different types of muscle fibers.

When slices of human muscles are preincubated in acid, ATPase staining reveals two major categories of muscle fibers: slow-twitch (type I) and fast-twitch (type II), with further subgroups of fast-twitch muscle fibers. Understanding these classifications is essential for grasping how motor units function during various activities, especially in relation to the anaerobic threshold and aerobic threshold.

Type I (ST): Slow-twitch fibers are characterized by their fatigue resistance, low glycogen content, and high mitochondrial content. These fibers are primarily used for long-lasting, low-level force production. Their energy supply is facilitated by dense capillarization found in red muscles. Type I fibers play a significant role in maintaining posture and supporting extremely long endurance activities. Additionally, due to their ability to oxidize lactate, these fibers are crucial for recovery between repeated bursts of high-intensity activity. This recovery is particularly important when considering the anaerobic threshold, which refers to the point during exercise at which the body transitions from primarily aerobic energy production to anaerobic energy production.

Type IIA (FOG): Fast-twitch fibers, known as fatigue-resistant fibers, possess a high content of both glycolytic and oxidative enzymes. These fibers are designed for activities that require quick bursts of energy and strength. They are particularly effective in recruitment during high-intensity exercises. When engaging in such activities, the question arises: which motor units are activated first? Typically, smaller motor units, which are often composed of slow-twitch fibers, are recruited first. As the intensity of the activity increases, larger motor units with fast-twitch fibers are activated to meet the higher demands for force production.

Understanding the dynamics of motor units and their classifications is essential for athletes and fitness enthusiasts alike. By recognizing how different types of muscle fibers contribute to performance, individuals can tailor their training programs to optimize their strength and endurance. This knowledge also aids in understanding the physiological responses during exercise, particularly in relation to the anaerobic threshold and aerobic threshold.

In summary, motor units are fundamental to muscle contraction and function. Their size, composition, and recruitment patterns play a vital role in determining how muscles respond to various physical demands. By studying these aspects, we can gain insights into improving athletic performance and enhancing overall physical fitness.

Aerobic Threshold

The aerobic threshold has been defined as the point just below the level of energy metabolism where blood lactate concentration increases distinctly from its resting level. It is also the exercise level below which the great majority of the muscle fibers are working aerobically. The aerobic threshold occurs because of a change in the type of muscle fiber recruited during the activity. During lower intensity exercise, the slow-twitch muscle fibers are recruited. As the intensity of exercise increases, more muscle fibers are activated. When slow-twitch fibers can no longer handle the required workload, fast-twitch fibers are activated. It has been proposed that the aerobic threshold is the point where Type IIa fibers are first recruited, resulting in an increase in blood lactate.

Anaerobic Threshold

Anaerobic threshold has been given many definitions.

It may be easiest to think of anaerobic threshold as the specific point during exercise when the athlete begins to produce lactic acid at a rate that exceeds their body’s ability to eliminate it. This imbalance leads to an accumulation of lactic acid in the bloodstream, which can result in fatigue and decreased performance.

As we have already suggested, there is no single cause of anaerobic threshold. Several mechanisms have been proposed to explain this phenomenon.  The most common explanation is that anaerobic threshold represents the point where Type IIc fibers are first recruited. These fibers are capable of producing large amounts of lactate but have limited capacity for lactate removal, resulting in a net influx of lactate into the bloodstream.

VO2 max (maximal aerobic power; MAP)

Maximal aerobic power is one of the most commonly measured physiological variables in sports science. It serves as the best measure of the functional limits of the cardiovascular system and is often used as a key indicator of physical fitness. VO2 max refers to the maximum amount of oxygen that the body can take in and utilize during exercise. Specifically, it is the amount of oxygen that can be consumed per unit of time during activities involving large muscle groups, which progressively increase in intensity until the subject reaches exhaustion.

VO2 max is dependent upon the integrated function of several physiological systems, including pulmonary ventilation, diffusion of oxygen from the lungs to the blood, cardiac output, redistribution of blood flow, and the extraction and utilization of oxygen in the blood, as outlined by the ACSM (1991).

It has been suggested that VO2 max is related to endurance performance in various sports. When comparing athletes of different levels, those with higher VO2 max scores are likely to be better performers. However, a poor relationship between VO2 max and endurance performance is observed when individuals with similar VO2 max values are compared. In such cases, anaerobic threshold tends to be a more reliable indicator of performance in endurance athletes who possess similar abilities.

Understanding the relationship between anaerobic threshold and aerobic threshold is essential for athletes aiming to improve their performance. The anaerobic threshold marks the transition point where the body shifts from primarily aerobic energy production to anaerobic energy production. This shift is crucial for athletes, as it determines their ability to sustain high-intensity efforts over time.

In summary, the anaerobic threshold is a critical concept in exercise physiology, representing the point at which lactic acid begins to accumulate in the body due to the recruitment of specific motor units. The interplay between various physiological mechanisms, including oxygen supply, lactate removal, and the activation of different muscle fiber types, all contribute to this important threshold. By understanding these factors, athletes can better tailor their training programs to enhance their performance and delay the onset of fatigue during high-intensity activities.

Aerobic training can be performed across a broad spectrum of intensities, allowing for flexibility in workout design. To better understand this range, training Zone systems have been established. These systems are grounded in three key physiological points: aerobic threshold, anaerobic threshold, and VO2 max. They enable coaches and athletes to create specific training adaptations and effectively target selected muscle fiber types during workouts. By accurately determining each of the five aerobic zones, much of the guesswork in training program design is eliminated. This precision helps athletes understand which motor units are activated first and how they can optimize their training to improve performance. Ultimately, knowing what anaerobic threshold is and how it relates to aerobic threshold is vital for enhancing athletic capabilities.