Neuromuscular Adaptation to Resistance Training

Acute Resistance Training (Weeks):
- Strength adaptations primarily due to neural changes.
- No significant changes in muscle size observed.
Chronic Resistance Training (Years):
- Further adaptations in the nervous system.
- Significant muscle hypertrophy (increase in muscle cross-sectional area).
- Increased force generating capacity is due to both neural adaptations and muscle hypertrophy.

Most training studies are approximately twelve weeks in duration due to logistical and financial reasons.
Neural adaptations likely dominate during the early training phase.
Long-term adaptations are more related to muscle hypertrophy.

Strength increases rapidly, then plateaus after years of training.
Varying training stimulus is needed to bring about smaller but continued improvements in strength.
Muscle hypertrophy contributes significantly after the initial weeks of training.
Neural adaptations occur rapidly initially, then were thought to plateau.
Varying the training stimulus may evoke ongoing positive neural adaptations.

The more complex the movement, the longer the time required to stimulate muscle hypertrophy due to a longer neural adaptation time.
The more complex the motor pattern, the greater the long-term potential for hypertrophy.

Arm Curl:
- Increase in strength observed at both mid-training (ten weeks) and post-training (twenty weeks).
- Lean arm muscle mass significantly improved at the mid-training point.
Bench Press & Leg Press:
- Increases in one-repetition maximum (1RM) strength at both mid and post-training points.
- Lean muscle mass improvements primarily observed at the post-training point due to the longer neural adaptation time required for complex exercises.

Strength training leads to:
- Increased agonist muscle activation.
- Appropriate muscle synergist activation.
- Potential decrease in antagonist activation.
These adaptations result in:
- Increasing force.
- Increased rate of force development.
- Overall improved strength performance.

Panel A: Higher threshold motor units can be activated after strength training.
Panel B: Increase in motor unit firing rate occurs, allowing the muscle fiber to operate further up on its force-frequency relationship.
Panel C: Increased motor unit firing rate increases the speed at which force is generated.

Small increases in peak force (especially in untrained individuals).
Large increases in rate of force development (e.g., 24%).
Increase in EMG activity during peak force development (e.g., 8%).
Increase in EMG activity during the early phase of contraction (rate of force development) (e.g., 38%).

Untrained Subjects: May exhibit a phase of inhibition of EMG activity during the high stretch-load component of a task (e.g., drop jump).
Trained Subjects: May have a period of facilitation of EMG activity during the eccentric phase of high stretch-load, enabling better performance.

RFD might increase by as much as 80% with ballistic/explosive type training.
The rate of motor unit firing also increased from somewhere between $58$ to $98$ hertz before training to as high as $182$ hertz after training.
Increased firing rates contribute to increased rate of force development.

Meta-analysis found:
- Increase in corticospinal excitability.
- Intracortical facilitation.
- Reduction in the corticospinal silent period (decreased inhibition).
No effect on short interval and long interval intracortical inhibition.
Strength training increased the excitability of corticospinal axons.

12 sessions of high force isometric contractions of the elbow flexors.
- Strength training group (7 females, 3 males).
- Control group (2 females, 9 males).

Peak torque increased by approximately 13% after 4 weeks in the strength training group (no change in control).
Biceps EMG increased by approximately 28% after the training period (no change in control).
No change in triceps EMG (antagonist muscle).
Improvement in voluntary activation from 88% to approximately 93% after training (no change in control).
No change in cervical medullary evoked potential (indicating no adaptations at the spinal level).
- Concluded that improvements were due to better voluntary output from the motor cortex.

Demonstrated a shift in the time to peak activation after training (shorter time).
Shift upwards in the relationship between force and recruitment threshold.

Four weeks of strength training in ankle dorsiflexor muscles with isometric contractions.

Greater motor unit firing rates during steady contraction periods after training.
No change in discharge rate during recruitment or de-recruitment phases.
Lower recruitment threshold after the training period.
- For example, at 35% contraction strength, recruitment threshold went from $311$ to $271$ . (Intervention group).
No change in force per motor unit firing after training.
- Suggests that adaptive changes are due to neural mechanisms, not muscular mechanisms.

Persistent inward currents are strongly upregulated by serotonin and noradrenaline and help motor neurons fire more easily.
Study by Esato et al. showed increased persistent in recurrent activity after 6 weeks of power-oriented resistance training.
Demonstrated in older adults where persistent inward current activity is initially reduced.

String training produces significant alterations in the neuromuscular junction, increased total area dispersion, stain perimeter and total perimeter of the neuromuscular junction.
Conducted on rats

Increase in the length of neuromuscular junction branching.
Increased complexity of neuromuscular junction branching.
At the postsynaptic level:
- Decrease in length of neuromuscular junction branching.
- Decreased complexity of neuromuscular junction branching.

Presynaptically: no effect on presynaptic parameters measured.
Postsynaptically:
- Decrease in length of neuromuscular junction branching.
- Decrease in complexity.

Lower cross-sectional area after low-intensity training.
Significant increase in citrate synthase activity for both high and low intensity training groups.
Exercise interventions for high-intensity training: 5 days per week for 12 weeks (treadmill running at 24 meters per minute).
Low-intensity group training: 5 days per week for 12 weeks (treadmill running at 24 meters per minute).