1. Neuromuscular Adaptation
Muscle Physiology
420:289

2. Agenda
Introduction
Morphological
Neural
Histochemical

3. Introduction
The neuromuscular system readily adapts to various forms of training:
Resistance trainin
Plyometric training
Endurance training
Adaptations vary depending on type of training
Skeletal muscle adapts in many different ways
Morphological
Neural
Histochemical

4. Agenda
Introduction
Morphological
Neural
Histochemical

5. Morphological Adaptations
Morphology: The study of the configuration of structure of animals and plants
Most obvious morphological adaptation is increase in cross-sectional area (CSA) and/or muscle mass
Hypertrophy vs. Hyperplasia

6. Hypertrophy and Myofibrillar Proliferation
Two mechanisms in which protein is accumulated  muscle growth
Increased rate of protein synthesis
-Myosin and actin added to periphery of myofibrils
Decreased rate of protein degradation
-Proteins constantly being degraded
-Contractile protein ½ life = 7-15 days
-Regular and rapid overturn  adaptability

7. Hypertrophy and Myofibrillar Proliferation
Mechanism of action:
Myofibrils increase in mass and CSA due to addition of actin/myosin to periphery
Myofibrils reach critical mass where forceful actions tear Z-lines longitudinally
Myofibril splits

8. Figure 8.3 b, Komi, 1996

9. Figure 8.3 a, Komi, 1996

10. Hypertrophy and Myofibrillar Proliferation
Hypertrophy of different fiber types:
Fast twitch:
-Mechanism: Mainly increased rate of synthesis
-Potential for hypertrophy: High
-Stimulation: Forceful/high intensity actions
Slow twitch:
-Mechanism: Mainly decreased rate of degradation
-Potential of hypertrophy: Low
-Stimulation: Low intensity repetitive actions
-FT may atropy as ST hypertrophy

11. FT ST FOG
Figure 8.5, Komi, 1996

12. Hypertrophy and Myofibrillar Proliferation
Role of satellite cells
History:
First identified in 1961 – Thought to be non-functioning
Adult myoblasts
Believed to be myoblasts that did not fuse into muscle fiber
Called satellite cells due to ability to migrate

13. Brooks, et al., Fig 17.2, 2000

14. Brooks et al., Fig 17.3, 2000

15. Hypetrophy and Myofibrillar Proliferation
Satellite cell activation due to injury:
Dormant satellite cells become activated when homeostasis disrupted
Satellite cells proliferate via mitotic division
Divided cells align themselves along the injured/necrotic muscle fiber
Aligned cells fuse into myotube, mature into new fiber and replace old fiber

16. Figure 5.7, McIntosh et al. 2005

17. Hypertrophy and Myofibrillar Proliferation
Satellite cell activation due to resistance training:
Resistance training causes satellite cell activation as well
Interpretation:
-Satellite cells repair injured fibers as a result of eccentric actions
-Hyperplasia

18. Hyperplasia
Muscle fiber proliferation during development – 4th week of gestation  several months postnatal
Millions of mononucleated myoblasts (via mitotic division) align themselves
Fusion via respective plasmalellae (Ca2+ mediated)
Myotube is formed
Cell consituents are formed  myofilaments, SR, t-tubules, sarcolemma . . .

19. Evidence of Hyperplasia
Animal studies:
Cats: 9% increase in fiber number after heavy resistance training (Gonyea et al, 1986)
Quail: 52% in latissimus dorsi fiber number after 30 days of weight suspended to wing (Alway et al, 1989)

21. Evidence of Hyperplasia
Human study: MacDougall et al. (1986)
Method of estimation:
Fiber number Fn of total muscle area (CT scan) and fiber diameter (biopsy)
Compared biceps of elite BB, intermediate BB and untrained controls
Results: Range:
172,000 – 419,000 muscle fibers
Means between groups not significant
Conclusion:
Large variation between individuals
Variation due to genetics

23. Other Morphological Adaptations
Angle of pennation
In general  as degree of pennation increases, so does force production
Why?
More muscle fibers/unit of muscle volume
More cross-bridges
More sarcomeres in parallel

25. Sarcomeres in series  displacement and velocity
Sarcomeres in parallel  force
Figure 17.20, Brooks et al., 2000

26. Figure 17.22, Brooks et al., 2000
Muscle length (ML) to fiber length (FL) ratio also an indicator of force and velocity properties of muscle

27. Training?

28. Other Morphological Adaptations
Capillary density:
High intensity resistance training: Decrease in capillary density
Endurance training: Increase in capillary density (body building)
Mitochondrial density:
High intensity resistance training: Decrease in mitochondrial density
Endurance training: Increase in mitochondrial density

29. Agenda
Introduction
Morphological
Neural
Histochemical

30. Neural Adaptations Recall:
Motor unit: Neuron and muscle fibers innervated
Increasing force via recruitment of additional motor units  Number coding

31. Figure 9.6, Komi, 1996

32. Neural Adaptations Recall:
Increasing force via greater neural discharge frequency  Rate coding
Maximum force of any agonist muscle requires:
Activation of all motor units
Maximal rate coding

33. Neural Adaptations
Timeline

34. Fig 20.8, Brooks et al. 2000

35. Neural Adaptations Increased activation of agonist motor units:
Untrained subjects are not able to activate all potential motor units
Resistance training may:
Increase ability to recruit highest threshold motor units
Increase rate coding of all motor units

36. Neural Adaptations Neural facilitation
Facilitation = opposite of inhibition
Enhancement of reflex response to rapid eccentric actions

37. Fig 20.10, Brooks et al., 2000

38. Neural Adaptations Co-contraction of antagonists
Enhancement of agonist/antagonist control during rapid movements
Joint protection
Evidence: Sprinters greater hamstring EMG during knee extension compared to distance runners

40. Neural Adaptations Neural disinhibition: Golti tendon organs (GTO):
Location: Tendons
Role: Inhibition of agonist during forceful movements
Examples:
Muscle weakness during rehabilitation
Arm wrestling
1RM

41. 1. High muscle tension
GOLGI TENDON REFLEX
3. GTO activation
4. Inhibition of agonist
2. High tendon tension
Figure 4.16, Knutzen & Hamill (2004)

43. Neural Adaptations Progressive resistance training may inhibit GTO
Anecdotal evidence:
Car accidents
Hypnosis

44. Neural Adaptations Resistance training vs. plyometric training Load:
RT: Heavy
PT: Light
Velocity of movement:
RT: Low
PT: High
Stretch shortening cycle (SSC):
RT: Minimal
PT: Yes

46. Agenda
Introduction
Morphological
Neural
Histochemical

47. Histochemical Adaptations
Histochemistry: Identification of tissues via staining techniques
Recall

48. Table 12.8, McIntosh et al., 2005

49. Histochemical Adaptations
Muscle fiber distribution shifts
Generally believed that ST do not change to FT and vice-versa
Several studies have observed IIB  IIA in humans
Fiber shifts from ST to FT and vice-versa have been observed in animals under extreme conditions

50. Histochemical Adaptations
Chronic long term low frequency (10 Hz) stimulation of rabbit tibialis anterior
3 hours: Swelling of SR
4 days: Increased size/# of mitochondria, increased oxidative [enzyme], increased capillarization
14 days: Increased width of Z-line, decreased SERCA activity
28 days: ST isoforms of myosin and troponin, decreased muscle mass and CSA

51. Rapid bursts of stimulation?
Figure 18.2, McIntosh et al., 2005