Biological Basis of Increasing Muscular Strength

Question 1

Brief Historical Overview

Answer 1

• Strong relationship has been demonstrated between absolute strength and total muscle cross- sectional area (Rodahl, 1962; Close, 1972)
• Increase in muscular strength in the absence of measurable hypertrophy (Bowers, 1966; Coleman, 1969; deVries, 1968)
• Sometimes after several weeks of high intensity strength training (deVries 1968; Komi, 1978)
• Kawakami, 1955; Carcraft, 1977: Suggested that firing patterns of motor units change in a manner that is specific to how the muscle is trained (static or dynamic; eccentric or concentric)

Question 2

Framework for ‘neural factors’ - early work

Answer 2

• Theoretical model for whether neural or muscle factors would explain increases in strength
• Hypertrophy gains likely understated
• Transfer effect evident but largely overstated

Research alluded to neural factors which contribute to strength

Increases in strength early on is due to neural factors while hypertrophy of the muscle is low. Hypertrophy increases throughout time

Question 3

Framework for ‘neural factors’ - Later work
HINT: Individuals with more training experience…

Answer 3

• Individuals with more training experience seem to be stronger
• Individuals with more training experience show lower activation of antagonist muscles
• Can take up to 20 days of training to see considerable change to muscle cross sectional area. But takes 10 days for architectural changes

Question 4

Neural Factors- How might we probe the source of adaptation (look further into)

Answer 4

• Probe different parts of the circuitry to identify what may be contributing to the “neural” adaptations we are seeing.
• Yes, the research does involve simple movements but it does not discount from the potential insights we gain.

Question 5

Neural Factors - increases voluntary activation?
HINT: Send shock at the…

Answer 5

Send a shock at the TMS by which the pulse travels down to the alpha motor neuron . If the pulse increases the amount of force that is being produced suggest that during the contraction, not all motor units were recruited.

The smaller the peak produced from the shock, the more motor units that the individuals are able to recruit

Resistance training does not change the peripheral stimulation, however, data suggest increased ability to recruit available motor units

Question 6

Neural pathway - Recap
HINT: Stimulate at the cortex…

Answer 6

• Stimulate at the cortex = force response
• Training + Stimulate at the cortex = force response (bigger force response)
• Peripheral twitch responses (from nerve only stimulation) the same, suggesting the increase in muscle force is contributed to by some change in the nervous system circuitry BEFORE the muscle (proximal to neuromuscular junction)
• Something is happening to AMPLIFY the signal at the level of the motoneuron

Question 7

Neural factor - Support
HINT: resistance training improves our ability to…

Answer 7

• Resistance training improves our ability to recruit more motor units earlier
• Motor unit recruitment threshold decreases
• Discharge rates - motor units fire earlier with resistance training
• With resistance training we are able to start recruiting earlier which decreases the time

Question 8

Neural factors - Is initial adaptation just motor learning?

Answer 8

• Study shows that practicing of a skill increases the representation of the body part that is being trained in the brain
• Some of the adaptation are coming from practicing the skill but also because of the added resistance from the resistance training

Question 9

Neural Factors - Spinal Chord?
- H reflex

Answer 9

The Hoffmann (H) reflex
- Assesses the excitability of spinal motoneurons and the synaptic efficacy of Ia afferents -
- Spinal excitability refers to the ‘input-output’ response of the motoneuron (think the ‘amplifier’ analogy)

As you increase the volage of the stimulator, the H reflex gets larger initially and then starts to decrease at really high levels of impulse because of collision of action potentials that start to cancel themselves out. Therefore, we start losing the h reflex because it gets cancelled out by the high stimulation frequency

Question 10

Neural Factors - Spinal cord?
- V wave

Answer 10

• Although actually constituted by a volley of H-reflex impulses, this evoked potential is denoted a V-wave to indicate its presence during volitional contraction but absence during rest.
• Changes in V-wave consistently observed following RT protocols
• This change in V wave amplitude has been suggested to indicate an increase in “neural drive” e.g. (increase activation of a-motor neuron pool)
• Neural drive => increased activation of motor neuron pool -> enhanced excitatory input to motor neurons from descending pathways (corticospinal) and reduced inhibition from reflex and interneuron circuits.

Question 11

Neural Factors - Spinal Cord?
- H and V reflex

Answer 11

• Benefits do not appear at rest.
• Early work suggested some evidence for H reflex adaption but continued work shows mixed results.
• Consistent results observed for improvement in Vwave effects. Suggested of “supraspinal” adaptations to RT.
  
  • Resistance training increases our ability to send a stronger stimulus down to the spine

Question 12

Neural Factors - Specific adaptation
SAID principle: Specific to Adaptation to Imposed Demands

Determining the site of neural adaptations to resistance training

Answer 12

• *Not much adaptation during rest but more during active contraction (Short-interval intracortical inhibition)
• Surprisingly not a whole heap of work done in this area. E.g. (Transfer effect)
• *Adaption to RT seems to be quite specific to the exercise and movement pattern performed. -
• Early work may overstate the differences observed between some exercises via not allowing for appropriate learning periods.
• Neural adaptions using current techniques may not fully explain the discrepancies observed in magnitude of differences across exercises.