Lecture 5

1. Lecture 5 Dimitar Stefanov

2. Mechanical Work, Energy and Power • Definitions: • Purpose of the muscles is to produce tension • Metabolic efficiency – the measure of a muscle’s ability to convert metabolic energy to muscle tension (Example, cerebral palsy patient) • Mechanical efficiency – the ability of the central nervous system to control the tension patterns (the ability of the muscle to perform mechanical work) • Overall muscle efficiency – mechanical efficiency + metabolic efficiency • There are two types of workdone by muscles: • Internal work – the work done by the muscles to move the limb segments through some desired patterns (Example: walking and running) • External work – the work done by the muscles against forces and moments, which are external for the body (Examples: weight lifting, pushing a car) • Net mechanical work – internal work + external work.

3. Positive and negative work of muscles Net work done by the muscles = integral of the power of the muscle over the time of the muscle contraction. Positive work is done when the muscle moment acts in the same direction as the angular velocity of the joint. EXTENSOR FLEXOR Negative work is done when the muscle moment acts in the opposite direction to the movement of the joint. The external force Fextacts on the segment and produces a joint moment greater than the muscle moment.

4. Muscle mechanical power At a given joint, muscle power is a product of the net muscle moment and angular velocity of the of the joint. The sign of the muscle power changes during the movement performance. Example:

5. The work done by a muscle during a period t1 to t2 is: [J] Pm – muscle power Example. Causes of insufficient movement • Co-contractions • Isometric contractions against gravity • Jerky movements • Generation of energy at one joint and absorption at another

6. Energy flows Maintenance heat – amount of metabolic energy to keep the muscles alive

7. Forms of energy storage (1) Potential energy, P.E. Energy due to gravity. It increases with the height of the body above ground. (2)Kinetic energy, K.E. Translational K.E due to the translational velocity and rotational K.E due to the rotational velocity.

8. (3) Total energy and exchange within a segment, Es Energy exchange between segments Example: Mid-stance phase Double support phase Plot of vertical displacement and horizontal velocity of head-arms-trunk (H.A.T.)

9. Total energy of a multi-segment system where: Ei – the total energy of the i-th segment n is the number of the segments Eb is the total body energy. Positive and negative work of the total body Example of a pendulum 100% conservative system!

10. Pendulum system with muscles Muscle is not contracted Muscle is contracted • The total body energy increases when muscles do positive work. • The total body energy decreases when muscles do negative work.

11. Overall efficiency of human movement The major problem is to calculate the internal mechanical work.

12. MUSCLE MECHANICS Motor unit The smallest subunit that can be controlled is called motor subunit because it is separately innervated by a motor axon. • The motor unit consists of: • Synaptic junction in the ventral root of the spinal cord • Motor axon • Motor end plate in the muscle fibers. • The number of the muscle fibers that are under control of 1 motor unit varies from 3 to 20,000 depending on the fineness of the control required. • A muscle fiber is about 100 mm in diameter consisting of fibrils about 1 m in diameter. • Fibrils consist of interacting action and myosin filaments.

13. Muscle fiber Sarcoplasm Sarcolemma – the plasma membrane Sarcomere – repeating functional unit of microfilaments (length 2.6 mm)

14. Basic structure of of the muscle contractive element Myofibril Sarcomere Many filaments are in parallel and many sarcomere elements are in series to make up a single contractive element.

15. Recruitment of motor units Excitation of the motor unit all-or-nothing event • All muscle fibers in a single motor unit contact at the same time; • Muscle fibers in the same muscle but belonging to different motor units may contract at different times. • Two indications of the activation of the motor unit: • Motor unit action potential (electrical indication) • Twitch of tension (mechanical indication). EMG signal from indwelling electrode in a muscle

16. Size principle of recruitment of motor units • How the motor units are recruited? • Which motor units are recruited first? • Are the motor units always recruited in the same order? Hinneman – The size of the newly recruited motor unit increases with the tension level at which it is recruited. • The smallest unit is recruited first and the largest unit last. Because of the smallest units the tension can be changed in finely graded steps. • Movements requiring high forces but not needing fine control are accomplished by recruiting the larger motor units. M.U. = motor unit; M.U.1 – smallest M.U. M.U.3 – largest M.U.

17. The muscle action potential (m.a.p.) increases with the size of the motor unit with which it is associated. WHY? • If the motor unit is larger then: • The motoneuron (that innervates it) is larger • The depolarization potentials associated with the motor end plate is greater. Can we predict the size of a motor unit from the amplitude of the recorded signal? – No, it isn’t possible. Why?

18. Two types of motor units (M.U.): Type I – It considers the smaller units which produce low tension and have low time to peak (60 to 120 msec). (tonic M.U.) Type II – It includes the larger, fast twitch motor units (phasyc M.U.) M.U. controlled by any motoneuron pool form a spectrum of sizes and excitations.

19. Force-length curve of a contractile element 4 mm Influence of parallel connective tissue The connective tissue surround the contractive elements. The connective tissue is called the parallel elastic component. Ft = Fc + Fp Ft – tendon force; Fc – force of the contractive element Fp – force of the parallel elastic component.

20. Series elastic tissue Series elastic element – all connective tissue in series with the contractile component, including the tendon. Isometric contractions