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The Neuromuscular Response to Resistance Training is the adaptation of the neuromuscular system caused by human movement against some form of resistance. The neuromuscular system is the system of muscles and nerves that work together in the body to produce that movement. Training in fitness and athletics utilizes the neuromuscular system, causing adaptation to occur. Adaptations from training vary by type, frequency, intensity, and duration of exercise. Additionally, factors such as fitness level and genetics play a large role in the body’s adaptation process.

Resistance Training
There are various forms of training, each focused on improving a specific aspect of the individual’s performance. These forms of training include power, resistance, stability/mobility, and endurance. Resistance training aims to increase strength output, muscle size (hypertrophy), and endurance, depending on the repetitions, intensity, and various other training factors. In order for the body to grow from resistance training, it must undergo neuromuscular adaptation. These adaptations allow the individual to perform at greater capacities both physically and mentally.

Neuromuscular Adaptations
Neural adaptations are often found in strength gains where there is no hypertrophy or increases in evoked tension, meaning neural adaptations play a large role in development during resistance training. Neural adaptations are pre-dominant in strength gains associated with early stages of resistance training (8-12 weeks) with gradual greater contribution of hypertrophic factors as training proceeds. When new lifters go to the gym and quickly gain strength, the sudden improvement is often attributed to neural adaptations, commonly referred to as “muscle-mind connection” in the fitness community. This phenomenon is why gym-goers typically hit a plateau in their strength progress around 8-12 weeks of exercise.

There are several adaptations that can be analyzed to understand how resistance training affects the neuromuscular system: EMG activity, reflex potentiation, and co-contraction of antagonist muscles. Unique training methods such as vascular occlusion and hypoxia training may also be analyzed to understand training adaptations in unique training conditions.

EMG Activity
Electromyography (EMG) is a technique for evaluating and recording the electrical activity produced by skeletal muscles with some visual or audible signal or display using electrodes attached to the skin. Increased EMG activity corresponds proportionally with increased neural drive which is proportional to applied torque (or muscular tension). EMG activity can be compared with other factors such as muscle activation, rate coding, and synchronization of motor units to understand how these neuromuscular factors adapt with resistance training.

Muscle activation is the relative amount of muscle fiber units recruited during muscular contraction. Muscle activation was measured in trained and untrained individuals to analyze if the trained individuals have a greater EMG activity. Using Interpolated Twitch Technique (ITT), a method to determine muscle activation during voluntary contraction by sending an electrical stimulus to the muscle during contraction, shows full activation of tibialis anterior, elbow flexors (biceps), abductor digiti minimi, quadriceps, adductor pollicis, plantar flexors, and soleus in untrained individuals. These results show that trained individuals and untrained individuals can both fully activate their muscles during resistance training, indicating muscular activation is not a neuromusclar adaptation largely effected by resistance training.

Rate coding is the rate at which motor units discharge action potentials (create a change in resting membrane potential). Researchers suggest an S curve of firing rate to tension; this relationship means graduations in contractile force are controlled mainly by recruitment instead of rate coding except at high and low levels of strength. It is also found that Standard firing frequencies is around 25-60Hz but ballistic contractions may achieve 60-120Hz. Based on these findings, rate coding does not seem to have much effect on max tension/power in most cases but rather on rate of force development when it comes to neuromuscular adaptations.

Synchronization of motor units is the coincident timing of impulses from two or more motor units. Studies have shown force output is greater and smoother with asynchronous stimulation at submaximal stimulation frequencies. Rate of force development in brief maximal contractions is faster in voluntary synchronous contractions than in evoked synchronous contractions. Voluntary asynchronous contractions are optimal but do not show great correlation to increased strength, meaning that optimal synchronization of motor units is only a minor adaptation from resistance training.

Reflex Potentiation
Reflex potentiation is a measure of the ability of motor units to fully activate during maximal contraction. Studies of primates and humans show that we can voluntarily modulate the size of the H-reflex, the reaction of muscles after electrical stimulation of sensory fibers in their nerves. Studies of explosive athletes (volleyball, sprinters, etc) show H-reflex much smaller than that of control subjects while in another study, enhanced reflex potentiation was found in sprinters. These various study results make the role of reflex potentiation in strength gains from resistance training unclear.

Co-contraction of antagonist muscles
Co-Contractions of Antagonist muscles is the contraction of antagonist muscles that provides stability for a joint when the agonist muscle is contracting. There are various patterns of co-activation of antagonists depending on the skill to be mastered. Increased co-contractions are shown in high speed and alternating direction resistance movements (tennis, speed skating, etc) and vice versa. In a study comparing isometric quad contractions of trained and untrained individuals prior to and following fatigue, trained individuals showed insignificant lesser coactivation prior to fatigue while also showing significantly greater coactivation following fatigue. Synergistic muscle activity may help promote greater force output for trained individuals with activation and coordination of synergist muscles being less than optimal for untrained individuals, meaning that co-activation is a neuromuscular adaptation that occurs with resistance training.

Vascular Occlusion
A unique method for observing the effects of resistance training on the neuromuscular system is through vascular occlusion training. Vascular Occlusion is the blockage or closing of a blood vessel. It is thought that Muscular fatigue may be hastened during low intensity muscular contractions by occluding arterial blood flow. There has been shown to be an increase in EMG activity during occlusion, indicating that a greater number of what would normally be inactive muscle fibres are required to lift relatively light loads; occlusion allows more muscle fibers to be recruited. This recruitment allows for increasing muscle strength and hypertrophy in trained and untrained individuals despite the utilization of relatively low (40–50% 1 RM) exercise intensities. However, increased strength could also result from neural adaptations, such as increased agonist muscle activation through increased motoneuron firing frequency and/or motoneuron excitability.

Hypoxic Training
Hypoxic Training is a form of training also known as altitude training where the individual has restricted oxygen, typically by breathing in low-oxygen content air. This type of training is known for allowing athletes to increase VO2 max, lactate threshold, and reduce recovery time. Various studies show only minor Neuromuscular adaptations from hypoxia training. One study that tested hypoxia resistance training over a period of 5 weeks showed little differences in maxinal isokinetic strength between hypoxia resistance trainers and standard resistance trainers, most improvements of which were found only in leg strength. Muscular strength capacity, cardiopulmonary capacity, blood volume, fat free mass, and other factors in young people and older people all remained about the same between hypoxic and non-hypoxic groups. Many studies on hypoxia resistance training have come back inconclusive due to insignificant differences in training type. It seems that most positive effects of hypoxia training are relevant in cardiovascular training rather than muscle power development and hypertrophic resistance type training.