Intro to Biomechanics

1. Intro to Biomechanics Professors: Thomas S. Buchanan (lecture) Kurt Manal (lab) TA: Justin Cowder

2. Lab Goal • Muscles are the motors of the human body. Our goal is to explore how muscles’ generate force. Specifically: • How does maximal muscle force change as a function of its length? • How does maximal muscle force change as a function of its velocity?

3. Outline • How muscle works • How muscle force changes with length • How muscle force changes with velocity • How these relate to strength & speed • How we measure these things

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5. Muscles actuate movement by development of tension • That is, muscles pull they don’t push. • Muscles are grouped into antagonist pairs. • Movement involves coordination of many muscles.

6. Muscle Structure Muscle Fascicles Myofibers Sarcomeres

7. Fascicles are groups of fibers • One can dissect out muscle fascicles • Under a light microscope a stripped pattern is seen • A muscle cell may be 10-100mm in diameter and 1-30 cm long

8. Muscle fibers are comprised of myofibrils • Under an electron microscope, one can clearly see individual myofibrils (threads) • The sources of the stripped patterns are also seen

9. Structure of Individual Fiber

10. Sarcomeres • Sarcomeres—the fundamental units of muscle contraction. They are arranged in series, hence the total change in length of a muscle may be great while that of the individual sarcomeres is small. They are comprised of contractile filaments (thick and thin filaments). Together these form a stripped pattern when viewed under a light microscope, which is why skeletal muscle is sometimes called “striated” muscle.

11. Schematic of Sarcomere Schematic of Sarcomere Structure of the Sarcomere Myofibril Electron Microscope View

12. Force is developed at the actin-myosin cross bridge • Thick filament is made of myosin (head and tail) • Actin is the primary component of thin filaments (10nm diameter)

13. Sarcomere structure • A-band • I-band • Z-line • H-zone • M-line or M-region

14. Sarcomere Structrure • A-band • I-band • Z-line • H-zone • M-line or M-region

15. Outline • How muscle works • How muscle force changes with length • How muscle force changes with velocity • How these relate to strength & speed • How we measure these things

16. Length-Tension Relationship • The tension a sarcomere can generate is a function of its length. • When the sarcomeres are very long, only a few of the myosin heads on each thick filament can reach a thin filament, so little force can be exerted.

17. Length-Tension Relationship • At intermediate lengths, all of the myosin heads are within reach of the thin filaments, so maximum force can be exerted.

18. Length-Tension Relationship • With further shortening, the ends of the thin filaments reach beyond the mid-points of the thick ones, to myosin heads that face the wrong direction and push on them instead of pulling. This reduces the force that the muscle fiber can exert.

19. Length-Tension Relationship • Eventually, as shortening continues, the thick filaments collide with the Z-disks. Any further shortening distorts the filaments and the force falls rapidly.

20. Length-Tension Relationship • The tension a sarcomere can generate is a function of its length.

21. Length-Tension Relationship • There is an active and passive component to the L-T relationship. This is at 2.2 mm for a frog. “Optimal length” is about 2.8 mm for humans.

22. Outline • How muscle works • How muscle force changes with length • How muscle force changes with velocity • How these relate to strength & speed • How we measure these things

23. Force-Velocity Relationship • The force a muscle can exert depends upon how fast it is shortening as well as the sarcomere length. • The faster it is shortening, the less force it can exert. This is why you cannot lift heavy weights quickly. • However, a muscle that is being forcibly stretched exerts increased force.

24. Force-Velocity Relationship

25. Concentric Eccentric Force-Velocity Relationship • F-V curve for isotonic contractions. • Note that it is typically plotted with increases in length being negative.

26. Hill Equation • The Hill equation describes shortening muscle:(F + a)v = b(Fo - F) • Here, a and b are constants, Fo is maximum force, F is force, and v is velocity.

27. Outline • How muscle works • How muscle force changes with length • How muscle force changes with velocity • How these relate to strength & speed • How we measure these things

28. Whole Muscle Parameters • Fiber Length • Pennation Angle • Cross Sectional Area • Moment Arm

29. Muscle Architecture

30. Muscle Architecture Parallel Fibered Pennate Muscle

31. Pennation Angle • This is the angle between the muscle fibers and the line of action

32. Cross Sectional Area • Cross Sectional Area is related to the # of muscle fibers in parallel. • Hence, it should take into account the pennation angle, as shown below:

33. Muscle Moment Arms • Moment arm of a muscle is the length from the joint center to the muscle. This is not a constant. • M = r x F

34. For Muscle #1 Angle Angle Angle Joint Moment MA • For each muscle, maximal joint moment changes with joint angle. • It is the product of the moment arm curve and the muscle force curve. • “Strength” is the sum of the joint moment curves for every muscle that acts at the joint. Force Moment

35. Moment Contributions • Note that large Peak Forces (OPF) or large Moment Arm (SAR) does not necessarily result in large Moments.

36. Gastrocnemious has short, pennate fibers and a long tendon Sartorius has long, parallel fibers and very little tendon Static Properties The fiber length and pennation angle of muscles can vary considerably.

37. Excursion vs Force Pennate fibers: Short excursion, Higher forces Parallel fibers: Long excursion, Lower forces

38. Outline • How muscle works • How muscle force changes with length • How muscle force changes with velocity • How these relate to strength & speed • How we measure these things

39. Lab Goal #1 • Q> How does maximal muscle force change as a function of its length? • To answer this we will measure maximal joint moment as a function of joint angle • We will provide estimates of muscle moment arms from literature. • You will take your measured joint moments and the supplied moment arms to estimate muscle force as a function of joint angle.

40. Lab Goal #2 • Q> How does maximal muscle force change as a function of its velocity? • To answer this we will measure maximal joint moment at different velocities • You will take your measured joint moments & velocities and the supplied moment arms to estimate muscle force as a function of velocity.

41. Joint Moment • We will measure joint moment using a Biodex Dynometer • Statically • Dynamically

42. Muscle Parameters • Other muscle parameters can be measured in living people using an ultrasound machine: • Pennation Angle • Fiber length

43. Muscle Activity • Muscle activation can be measured by putting electrodes on (or into) a muscle. • This is called electromyography and the resulting signals are called EMGs.

44. EMG

45. EMG and Muscle Force • EMG is related to muscle force • The relationship is nonlinear • EMG to muscle activation • Muscle force-length relationship • Muscle force-velocity relationship • Mathematical models describing this relationship are the topic of my research (in part)

46. Details • Instructions on how to use the Biodex will be provided at the lab session. • Lab will be in Room 209 Spencer. • Dr. Kurt Manal will coordinate the lab sessions. Many graphics in this talk are from “How Animals Move” by R. McNeil Alexander