2.4 Nervous system and motor unit recruitment
4 min read•august 14, 2024
The nervous system plays a crucial role in strength and conditioning. It controls muscle contractions through motor neurons and motor units. Understanding how the nervous system recruits and coordinates muscle fibers is key to optimizing training and performance.
Neural adaptations are a major factor in early strength gains from training. These include increased motor unit recruitment, improved synchronization, and faster firing rates. Over time, these changes lead to more efficient and powerful muscle contractions.
Nervous System Structure and Function
Central and Peripheral Nervous System
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- The nervous system is divided into two main parts: the (CNS) and the (PNS)
- The CNS consists of the brain and spinal cord, which process and integrate sensory information, make decisions, and initiate motor responses
- The PNS consists of all the nerves that connect the CNS to the rest of the body, including sensory receptors, motor neurons, and autonomic nerves
- Sensory (afferent) neurons carry information from sensory receptors (mechanoreceptors, chemoreceptors, photoreceptors) to the CNS for processing
- Motor (efferent) neurons carry signals from the CNS to effector organs (muscles, glands) to initiate a response
- The autonomic nervous system (ANS) is part of the PNS and regulates involuntary functions (heart rate, digestion, respiratory rate). It is divided into the sympathetic and parasympathetic divisions
Neuron Structure and Communication
- The basic functional unit of the nervous system is the neuron, which consists of a cell body, dendrites, and an axon
- Neurons communicate with each other through electrical and chemical synapses
- Electrical synapses allow direct and rapid transmission of signals between neurons through gap junctions
- Chemical synapses involve the release of neurotransmitters (glutamate, GABA, dopamine) from the presynaptic neuron, which bind to receptors on the postsynaptic neuron, triggering a response
Motor Neurons in Muscle Contraction
Motor Neuron Structure and Function
- Motor neurons are specialized nerve cells that carry signals from the CNS to skeletal muscles, causing them to contract and generate movement
- The cell body of a motor neuron is located in the spinal cord or brainstem, and its axon extends out to the muscle fibers it innervates
- At the neuromuscular junction (NMJ), the axon terminal of the motor neuron releases the neurotransmitter acetylcholine (ACh), which binds to receptors on the muscle fiber membrane, initiating a series of events that lead to muscle contraction
Motor Neuron Size and Innervation
- The size of the motor neuron and the number of muscle fibers it innervates determine the precision and force of the resulting movement
- Smaller motor neurons innervate fewer muscle fibers and are recruited for fine, precise movements (hand muscles)
- Larger motor neurons innervate more muscle fibers and are recruited for gross, powerful movements (leg muscles)
- The activation of motor neurons is regulated by descending signals from the motor cortex and other brain regions, as well as by sensory feedback from muscle spindles and Golgi tendon organs
Motor Units and Recruitment Patterns
Motor Unit Structure and Function
- A motor unit consists of a single motor neuron and all the muscle fibers it innervates. It is the smallest functional unit of the neuromuscular system
- The number of muscle fibers innervated by a single motor neuron varies depending on the muscle's function. Muscles involved in fine, precise movements have smaller motor units (extraocular muscles), while those involved in gross, powerful movements have larger motor units (quadriceps)
Motor Unit Recruitment and Synchronization
- Motor units are recruited in a specific order during muscle contraction, known as the . Smaller motor units, which generate less force, are recruited first, followed by progressively larger motor units as the force requirement increases
- The recruitment of motor units is regulated by the nervous system to match the force output to the task demands. This is achieved through the modulation of motor neuron firing rates and the number of motor units recruited
- During sustained muscle contractions, motor units are recruited and de-recruited in a cyclical manner to prevent fatigue, a process known as motor unit rotation
- The synchronization of motor unit firing rates can also contribute to increased force output during high-intensity muscle contractions
Neural Adaptation in Strength Training
Neural Adaptations and Strength Gains
- Neural adaptation refers to the changes in the nervous system that occur in response to strength training, leading to improved muscle force production and coordination
- In the early stages of strength training, neural adaptations are the primary contributors to strength gains, while muscle hypertrophy becomes more significant in later stages
- Neural adaptations include increased motor unit recruitment, improved , and enhanced firing rates of motor neurons
- Strength training can increase the number of motor units recruited during a muscle contraction, allowing for greater force production
- Synchronization of motor unit firing rates can also improve, leading to more efficient and coordinated muscle contractions
- The firing rates of individual motor neurons can increase, allowing for more rapid and forceful muscle contractions
Motor Learning and Coordination
- Neural adaptations also involve changes in the organization and efficiency of the motor cortex and other brain regions involved in motor control
- Improved intermuscular coordination, or the coordination between different muscle groups (agonists, antagonists, synergists), is another neural adaptation that contributes to enhanced performance in complex movements
- The extent and rate of neural adaptations vary depending on factors such as training intensity, volume, frequency, and the individual's training status
Key Terms to Review (18)
A.V. Hill: A.V. Hill was a British physiologist known for his pioneering work in exercise physiology and muscle energetics, particularly regarding how muscles generate force and consume energy during physical activity. His research laid the groundwork for understanding the relationship between muscle contraction, oxygen consumption, and lactic acid production during intense exercise, which connects directly to how the nervous system controls motor unit recruitment to meet the demands of physical activity.
Central Nervous System: The central nervous system (CNS) is a crucial part of the body that includes the brain and spinal cord, serving as the main control center for processing information and coordinating responses. It plays a vital role in motor unit recruitment, which is the activation of muscle fibers to produce movement, highlighting its importance in strength training and physical performance.
Exercise Intensity: Exercise intensity refers to the level of effort or exertion put into physical activity, which can be measured in various ways such as heart rate, perceived exertion, or metabolic equivalents (METs). Understanding exercise intensity is essential for tailoring training programs and achieving specific fitness goals, as it influences the physiological responses of the body, including energy expenditure and muscle activation.
Fast-twitch muscle fibers: Fast-twitch muscle fibers, also known as Type II fibers, are specialized muscle fibers that are designed for quick bursts of strength and speed. These fibers contract rapidly and generate more force than slow-twitch fibers, making them ideal for explosive movements like sprinting and weightlifting. Fast-twitch fibers rely primarily on anaerobic metabolism for energy, which allows for rapid but short-lived muscular contractions.
Heavy resistance training: Heavy resistance training refers to a form of strength training where individuals lift weights that are challenging enough to induce muscular fatigue and stimulate muscle growth. This type of training typically involves performing exercises with higher loads, often around 75% to 90% of an individual's one-repetition maximum (1RM), which requires significant engagement of the nervous system for effective motor unit recruitment. Heavy resistance training is essential for developing strength, power, and muscular endurance.
Kinesthetic Awareness: Kinesthetic awareness is the ability to sense the position and movement of one's body in space. It involves the brain's understanding of where the body parts are located, how they move, and the forces acting on them. This awareness is crucial for effective movement patterns and plays a key role in motor control and coordination, enabling individuals to perform physical activities with precision and efficiency.
Mark H. McNitt: Mark H. McNitt is a recognized figure in the field of exercise science, specifically known for his contributions to understanding the nervous system and motor unit recruitment in strength and conditioning. His work emphasizes how the nervous system plays a critical role in muscle activation and coordination during physical activities, which ultimately influences performance outcomes.
Motor unit synchronization: Motor unit synchronization refers to the coordinated activation of motor units, which are made up of a motor neuron and the muscle fibers it innervates, during muscle contractions. This synchronization enhances the efficiency and force of muscle contractions, leading to improved strength and power output in various physical activities. The ability of the nervous system to recruit and synchronize multiple motor units is crucial for maximizing performance in strength training and athletic endeavors.
Muscle fatigue: Muscle fatigue is the decline in the ability of a muscle to generate force or power, often resulting from prolonged or intense physical activity. This phenomenon is influenced by several factors, including metabolic changes, depletion of energy substrates, and neural adaptations that affect motor unit recruitment. Understanding muscle fatigue helps in optimizing training methods, exercise selection, and endurance testing strategies.
Muscle Fiber Composition: Muscle fiber composition refers to the specific types and proportions of muscle fibers present in skeletal muscles, primarily categorized into Type I (slow-twitch) and Type II (fast-twitch) fibers. This composition plays a crucial role in determining an individual's strength, endurance, and overall athletic performance, influencing how muscles respond to different types of training stimuli and fatigue.
Neural Efficiency: Neural efficiency refers to the ability of the nervous system to effectively recruit motor units with minimal energy expenditure and effort. It highlights how well the nervous system can activate the appropriate muscles during physical activities, allowing for improved performance and reduced fatigue. Higher neural efficiency is often associated with better skill execution and increased strength gains, as the body becomes more adept at using its existing muscle fibers optimally.
Peripheral Nervous System: The peripheral nervous system (PNS) is the part of the nervous system that connects the central nervous system (CNS) to the limbs and organs. It plays a critical role in motor control and sensory information processing, facilitating communication between the brain, spinal cord, and the rest of the body. The PNS is essential for enabling voluntary movements and reflexes through its complex network of nerves.
Plyometrics: Plyometrics are explosive exercises that involve rapid stretching and contracting of muscles, designed to increase power and speed. This training method leverages the stretch-shortening cycle, where muscles are pre-stretched before a powerful contraction, enhancing athletic performance across various activities.
Proprioception: Proprioception is the body's ability to sense its position, movement, and action in space through internal signals from muscles, tendons, and joints. This sensory feedback is crucial for maintaining balance, coordination, and body awareness during movement. It connects closely with how the nervous system recruits motor units to perform tasks and plays an important role in rehabilitation and injury prevention alongside sports medicine professionals.
Rate Coding: Rate coding refers to the process by which the frequency of action potentials in motor neurons encodes the strength of muscle contraction. When a motor neuron fires action potentials at a higher frequency, it recruits more muscle fibers and increases the force output, allowing for precise control of movement and power generation in various physical activities.
Recovery Time: Recovery time refers to the duration needed for the body to repair and return to its baseline state after physical exertion. This process involves physiological adaptations and restoration of energy stores, and it is influenced by factors like the intensity and duration of the exercise, as well as the individual’s fitness level and recovery strategies employed.
Size Principle: The size principle is a physiological concept that describes how motor units are recruited during muscle contractions, based on the size of the motor neuron. Smaller motor units, which are made up of slow-twitch muscle fibers, are recruited first when a low level of force is needed, followed by larger motor units that contain fast-twitch fibers as the demand for force increases. This recruitment pattern helps optimize muscle performance and energy efficiency during various activities.
Slow-twitch muscle fibers: Slow-twitch muscle fibers, also known as Type I fibers, are a type of muscle fiber that is more efficient in using oxygen to generate energy for prolonged, endurance activities. These fibers contract slowly and are highly resistant to fatigue, making them ideal for activities that require sustained effort over longer periods, such as long-distance running or cycling. Their ability to maintain performance over time is due in part to their rich supply of mitochondria and myoglobin, which facilitate aerobic metabolism.