What determines contractile force?

Transcription

1 Movement & Force

2 What determines contractile force? Motor unit size (larger motor units, more fibres) Motor unit fibre type (fast fibres larger) Number of motor units (spatial summation) Frequency of stimulation (fusion of tension) Length: force length relationship Velocity: force velocity curve Series compliance (tendon & cross-bridge)

3 Muscle Actions Force x Displacement

4 Length vs Tension/Force Single muscle fibre Whole muscle In whole muscle, the muscle length is constrained by attachments to joints and is always within an optimal range. But there will be max force output within this range

5 Contributions of active and passive elements to force-length relation for a whole isolated muscle Active Passive Total The effect of length on total force Active tension develops as actin and myosin overlap Passive tension from extracellular collagen develops in the non-contractile components at ~ mid length Effect of passive tension varies depending on muscle

6 Mechanical Effects: Active and Passive Elements Hill Model CE: Contractile Element (Active, elasticity due to cross-bridge formation) SE: Series Elastic Element (Passive, connective tissue) PE: Parallel Elastic Element (Passive, connective tissue) CE generates the force, but it is transmitted through the SE

7 Effects of muscle properties: Length tension relationship means: - The effect of muscle length on the force developed: EG The position of the hand and fingers during a power grip. When you hold a gun you have your wrist extended in order to increase finger flexor force - Stiffness of muscle: increases with the force of muscle contraction, the initial response of a limb to disturbing force. Relaxed muscles provide no resistance to changes in muscle length

8 Velocity of contraction Concentric Contractions: Lifting a load Eccentric Contraction: Unloading Velocity of contraction of the muscle relates to the load that is being lifted Steepest tangent Greatest at light loads Lowest at heavy loads Maximal velocity (V 0 ) At Minimal (zero) load

9 Force vs Velocity The amount of force a muscle can generate varies with velocity of contraction Eccentric Isometric Concentric Lengthening: -ve velocity Shortening: +ve velocity Zero velocity (isometric) Eg Cross-bridge, Actin & myosin attachments and detachments -Rate of actin/myosin detachment slower in lengthening contractions than shortening - Re-attachment of actin/ myosin faster in lengthening>shortening Slower when you lift a heavier loads than when you lift a lighter load Faster when you lower a heavier load than lighter load

10 Increases and decreases in force Changes in neural drive to muscles Coordination of muscles Plasticity in the spinal cord So, peripheral and central changes: different levels of the movement hierarchy can be effected

11 Neuromuscular stimulation Electrical stimulation over skin, generates a.p. in intramuscular nerve branches Large-diameter fibres more easily excited by imposed fields A peripheral change can occur: impacts on recruitment order Neuromuscular stimulation also generates action potentials in sensory receptors. This feedback reaches primary somatosensory cortex, and may lead to central changes Time (days) To calf muscles, enough to produce up to MVC

12 What happens when homologous muscles in two limbs are activated concurrently? Decline in force during MVCs A bilateral deficit (Howard & Enoka 1991) E.g. A task with contractions of triceps brachii muscles with elbow at a right angle Maximum force by each arm reduced EMG reduction in the triceps brachii of each arm

13 Maximal force and EMG of elbow extensor muscles (left and right triceps) during maximal voluntary contractions Reduction Reduction Right Only Right and Left Left Only

14 But, effect of training Bi-lateral interactions are modified with training (Secher, 1975) Howard & Enoka (1991) Comparing MVC during one and 2 limb knee extensor and found: Bi-lateral deficit: Untrained, Cyclists Bi-lateral facilitation: Weightlifters

15 Plasticity in the Spinal Cord, increasing strength: Effects of 12 weeks of training Training was rapid contractions against a moderate load Changes in torque development, EMG and motor unit discharge during rapid submaximal contractions Torque EMG Increase Motor Units Increase

16 Decrease in strength Ageing De-nervation Immobilization Unloading

17 Ageing and decline in strength, motor neurons and motor units

18 Dennervation changes fibre properties If the nerve to a muscle is cut target cells the motor fibres change: Dennervation atrophy: decrease in size (3 days +, all fibres) Necrosis (several fibres, months) Lowered enzyme activity Decline in contractile properties

19 Limb immobilization effect (arm) Healthy humans, with arm immobilized in a cast (Semmler et al 2000) Measurements of EMG for 24 h periods before and during immobilization - EMG activity of biceps brachii declined by 38% and EMG of brachioradialis decreased by 29% Different studies show different effects, different relationships with decline in muscle mass, sometimes male vs female differences

20 Changes in motor units after joint immobolization AFTER: Increase in recruitment threshold Reduction in force Reduction in firing rate Each point is a single motor unit

21 Relation between decline in EMG and reduction in muscle mass? (rat hindlimb unloading) 3 weeks S-L= soleus, long length S-S= soleus, short length S-N= soleus, neutral length M-S=Medial gastrocnemius, short length

22 Summary: Sites of Neural Adaptations For strength : (1) Enhanced output from supraspinal centres imagined contractions (2) Reduced co-activation of antagonist muscles (3)Greater activation of agonist and synergist muscles (4) Enhancing coupling among interneurons (5)Changes in descending drive reducing bilateral deficit (6)Shared input to motor neurons (7)Greater EMG (8)Heighted excitability onto motoneurons IN= interneuron MN= motor neuron (Extensor, Flexor

23 Fatigue Failure to maintain the expected force or power output (Edwards et al 1982) Processes that can be impaired during sustained activity and can contribute to muscle fatigue

24 Factors affecting fatigue Fibre types Intensity and duration Type of contraction Continuous MVC>intermittent MVC Concentric>isometric Eccentric ++ (repeated rapid unloading can cause muscle damage) Training Muscle Length long>short Metabolic state Central drive/reflexes

25 Decrease in force production and velocity of force production with fatigue Twitch Contraction Increase in relaxation time Tetanic Contraction Increase in fusion

26 Maximal shortening and lengthening contractions: ankle dorsiflexor muscles Change in ankle joint angle concentric eccentric Peak torque during first MVC lengthening>shortening Peak EMG during first MVC shortening>lengthening Decline in peak torque and EMG by 150 th MVC shortening>lengthening Concentric more fatiguing BUT repeated eccentric more likely to lead to muscle damage

27 Fibre types and fatigue Healthy Adult Participants Rates of fatigue during continuous maximal isometric contractions of ankle dorsiflexor and plantarflexor muscles Plantarflexors contain higher proportion of slowtwitch fibres and fatigue less rapidly than dorsiflexors

28 Decline in force production during a sustained 2 min MVC with the knee extensor muscles after exercising in normal temperature (open circles) and heat (filled circles) for 1 hour Force Voluntary activation EMG Effect of temperature argued to be an inability to sustain activation of the involved muscles

29 Contra-lateral force matching experiment: Sense of Effort and Fatigue Increase in effort BEFORE decline in force in fatiguing arm One arm sustains a force of 40N for 10 mins (filled circles) The other arm has to perform intermittent contractions to the same level of effort associated with the sustained contractions of the other arm

30 Decline in force during 1 and 2 legged leg extension task, before and after training Dependence on task familarity 150 MVCs, isometric leg extension task, for both conditions after 5 weeks training Rate of decline in force during 150 MVCs was less after training Benefit specific to training regime Selective Fatigue, neural factors

31 Fatigue in Neurological disorders Eg Multiple Sclerosis Patients with MS and healthy subjects perform 45 MVC, force declines by ~45% in patients compared with 20% in controls (Sheean et al 1997) Based on potentials evoked by TMS, no impairment in excitability of primary motor cortex Decline in activation of primary motor cortex?

32 Forms of fatigue: Can be central or peripheral or both

33 Suggested Reading (for this lecture) MacIntosh, BR, Gardiner, PF and McComas, AJ (2006) Skeletal Muscle: form and function (2 nd Ed). Chapter 15 Gordon, A.M, Huxley, A.F. and Julian, FJ (1966) The variation in isometric tension with sarcomere length in vertebrate muscle fibres. Journal of Physiology 184 pp Carpentier et al (2001) motor unit behaviour and contractile changes during fatigue in the first dorsal interosseus. Journal of Physiology 534 pp Carson, R.G et al (2002) Central and peripheral mediation of human force sensation following eccentric or concentric contractions. Journal of Physiology 539 pp Todd G. et al (2005) Hyperthermia: A failure of the motor cortex and the muscle. Journal of Physiology 563 pp