Strength Training

1. Strength Training Patricia A. Deuster, PhD, MPHUniformed Services University

2. Outline of Presentation • Define strength training; • Factors affecting force generation; • Development of muscle strength; • Muscular power and endurance; • Approaches to strength training; • Benefits of strength training; • Designing a strength training program.

3. Objectives • Identify strength training terms; • Discuss trends in the prevalence of strength training; • Discuss factors that determine muscle force development; • Identify and differentiate skeletal muscle fiber types; • Discuss strength training terms and how to develop a strength training program; • Describe benefits of strength training.

4. Adaptation Muscular Strength Muscular Hypertrophy Muscular Power Muscular Endurance Motor Performance Progressive Overload Specificity and Variation Periodization Loading Training Volume, Impulse Exercise Selection and Order Rest Periods and Frequency Muscle Action and Velocity of muscle action Strength Training Terms Kraemer et al; American College of Sports Medicine position stand. Progression models in resistance training for healthy adults. Med Sci Sports Exerc. 2002 Feb;34(2):364-80.

5. Healthy People 2010 Objective and Strength Training • Increase to 30% the proportion of adults who perform physical activities that enhance and maintain muscular strength and endurance on > 2 days per week; • Also recommended by the American College of Sports Medicine.

6. Prevalence of Strength Training by Gender

7. Prevalence of Strength Training by Ethnicity

8. Factors Affecting Muscular Force Generation • Muscle Architecture • Muscle Mechanics • Length-Tension Relationship • Muscle Fiber Types • Force-Velocity Relationship • Electromechanical Delay

9. Muscle Architecture • Long axis of muscle determines arrangement of muscle fibers • Reflects muscle force and power • Two basic types • Fusiform: spindle shaped • Pennate: fan-shaped

10. Muscle Fiber Architecture

11. Pennation Effects on Force and Fiber Packing • Pennation allows for packing a more fibers into a smaller cross-sectional area than parallel fibers. •  = surface pennation angle

12. Fusiform Fiber Arrangement Fa = force of contraction of muscle fiber parallel to long axis of muscle SFa = sum of all muscle fiber contractions parallel to long axis of muscle Fa

13. Pennate Fiber Arrangement Fa = force of contraction of muscle fiber parallel to long axis of muscle Fm = force of contraction of muscle fiber  = pennation angle Fa = (cos )(Fm) SFa = sum of all muscle fiber contractions parallel to long axis of muscle Fa Fm 

14. Muscle Mechanics • Active Force through contractile elements: actin and myosin mechanism; • Passive Force through elastic elements: • Series elastic elements (tendons) smooth out force of contraction and reduce effects of external forces from overloads • Parallel elastic elements (fascia) absorb energy input externally if muscle is stretched beyond normal "resting" length.

15. The range of motion and amount of force a muscle can generate is largely determined by the arrangement of the muscle fibers Muscle Mechanics PE = Parallel elastic component SE = Series elastic component CE = Contractile element • Fibers in series • Force production modest, but large range of shortening. • Fibers in parallel • Force production high, but minimal range of shortening.

16. Length-Tension Relationship • Force generation optimized when muscle is slightly stretched. • Due to contribution of elastic components of muscle (primarily the SEC)

18. Human Muscle Fiber Types

19. Human Muscle Fiber Types CharacteristicsNames ST FTa FTd/x SO FOG FG Fibers/Motor Neuron 10-180 300-800 300-800 Motor Neuron Size Small Large Large Nerve Conduction Velocity Slow Fast Fast Contraction Speed (ms) 110 50 50 Type of Myosin ATPase Slow Fast Fast SR Development Low High High Motor Unit Force Low High High

22. Comparison of Maximal Shortening Velocities Between Fiber Types

23. Force and Types of Muscle Contractions Concentric Eccentric Isometric

24. Isotonic Contractions • Muscle changes length (changing angle of joint) and moves a load. • Two types of isotonic contractions • Concentric: Muscle shortens as it contracts • Eccentric: Muscle lengthens as it contracts

25. Isometric Contractions • Tension increases without changes in length • Occurs if the load is greater than the tension the muscle is able to develop

26. Force-Velocity Relationship • Maximal force developed by muscle is governed by its shortening or lengthening velocity - holds true for all muscle types

27. Force Velocity Relationships • Concentric: CON Ability to develop force is greater at slower contraction velocities - allows greater time for cross-bridges to generate tension

28. Force-Velocity Relationship • Eccentric: ECC Greater force with increasing velocity/ acceleration, due to lower metabolic cost, greater mechanical efficiency and greater contribution from series elastic components.

29. Force-Velocity Relationship

30. Electromechanical Delay • Time between arrival of neural stimulus and tension development by muscle • Varies among muscles (20-100 msec) • Short EMDs produced by muscles with high percentage of FT fibers • Not affected by muscle length, contraction type, contraction velocity, or fatigue

31. Electromechanical Delay

32. Development of Muscle Strength • Maturation • Training

33. Maturation and Strength Factors contributing to muscle strength during maturation 100% Adult potential Lean body mass Theoretical fiber type differentiation Testosterone Neural myelination development Birth Puberty Adult Strength primarily via motor patterns Consolidation of strength factors Optimal strength potential Kraemer, 1989

34. Adaptations to Strength Training • Physiological Adaptations •  muscle fiber size and strength; •  connective tissue density and bone integrity. • Muscle fiber type conversion? • Neural Adaptations •  recruitment of motor units; •  in firing rate of motor neurons; • Improved synchronization in motor neuron firing; • Counteraction of autogenic inhibition to allow greater force production.

35. Skeletal Muscle Adaptations • Muscle Fiber Size • Muscle Fiber Type Conversion • Muscular Strength

36. Muscle Fiber Hypertrophy • Increase in numbers of myofibrils and actin and myosin filaments • Allows more cross-bridges. • Increases in muscle protein synthesis during post-exercise period. • Testosterone plays a role in promoting muscle growth. • High intensity training may promote greater fiber hypertrophy than low intensity training.

37. Muscle Fiber Hyperplasia • Muscle fibers may split in half with intense weight training. • Each half may then increases to size of parent fiber. • Satellite cells may also be involved in skeletal muscle fiber generation. • Clearly shown in animal models, but in only a few human studies.

38. Early strength gains influenced by neural factors. Long-term strength gains due to muscle hypertrophy. Process of Strength Gains

39. Mechanisms of Strength Training Adaptations • Mechanical stimuli • CON-only training equally effective as ECC, despite mechanical advantage of ECC (greater forces, muscle damage, etc) • Metabolic Stimuli • Greater metabolic costs with CON • Build-up of metabolic by-products may enhance release of anabolic hormones and lead to greater motor unit activation.

40. Muscular Power • Power = Work/Time = • (Force X Distance)/Time = • Force X Velocity • Maximal power occurs at: • ~ 1/3 max velocity • ~ 1/3 max concentric force • Affected by muscular strength and movement speed; • Main determinant of performance for throwing, jumping, changing direction, and striking activities.

41. Force-Power Relationship • Power generated is greater in muscle with a high % of fast-twitch fibers at any given velocity of movement; • Peak power increases with velocity up to movement speeds of 200-300º•sec-1 • Force decreases with increasing movement speed beyond this velocity

42. Force-Power Relationship

43. MaximumPower Muscle lengthening -10 -20 Muscle Load and Shortening Velocity • Max velocity at minimum load • Max load at velocity 0 30 Power (force x velocity) • Power = 0 at 0 load and max load • Maximal power of muscle occurs at 1/3rd max load, or where Velocity X Load is greatest. 20 Velocity of Contraction (cm/s) 10 0 0.33 0.66 max Load opposing contraction

45. Muscular Endurance • The ability to exert tension over a period of time. • Constant: gymnast in iron cross • Varying: rowing, running, cycling • Length of time dramatically affected by force and speed requirements of activity. • Training involves many repetitions with light resistance.

46. Approaches to Strength Training • Static (isometric) actions • Dynamic actions • Free weights • Gravity dependent • Variable resistance • Isokinetic actions • Plyometrics • Other • Neuromuscular electrical stimulation

47. Free Weights • Gravity dependent • Resistance pattern constant or variable • Concentric and eccentric action of same muscles: antagonistic muscles not utilized • Momentum may be factor in resistance pattern

48. Gravity Dependent Machines • Universal Gym • Resistance moves upward • Round pulleys changes direction of resistance • Constant resistance