srdce zakl EVALUATION OF MUSCLE CONTRACTION Relationship between load and contraction velocity of skeletal muscle EVALUATION OF CONTRACTION IN SKELETAL MUSCLE load mmax Vmax (m +a)v = (mmax- m) b Hill's equation corresponds to the maximal cycling rate of the cross-bridges constants EVALUATION OF CONTRACTION IN SKELETAL MUSCLE One way to evaluate the contraction of skeletal muscle is based on measurement of relation between muscle load and velocity of muscle contraction. Relation of velocity of muscle contraction to muscle load A skeletal muscle contracts extremely rapidly when it contracts against no load — to a state of full contraction in about 0.1 second for average muscle. When loads are applied, the velocity of contraction becomes progressively less as the load increases, as shown in the graph. When the load is increased to equal the maximum force that the muscle can exert, the velocity of contraction becomes zero and no contraction occurs, despite activation of the muscle fibers. The decreasing velocity of contraction with load is caused by the fact that a load on a contracting muscle is a reverse force that opposes the contractile force caused by muscle contraction. Therefore, the net force that is available to cause velocity of shortening is correspondingly reduced. The data of contraction velocity (v) obtained at different load (m) can be approximated by Hill's equation where m[max] and v[max] stand for the maximal load that the muscle can bear (without lengthening) and maximal velocity of unloaded muscle, respectively. The constants a and b determine the curvature of the relation. Decreased value of v[max ]or increased curvature of the relation may indicate muscle fatigue or disease. Physiological factors affecting relationship between load and contraction velocity of skeletal muscle 1) Initial length of muscle (sarcomeres) contraction from resting length load mmax mmax contraction of shortened or stretched muscle Vmax 2) Number of active sarcomeres load mmax mmax trained muscle mmax untrained muscle Vmax trained muscle untrained muscle sarcomere length [mm] Physiological factors affecting relationship between load and contraction velocity of skeletal muscle 1.Initial length of muscle Since the active force generated by a muscle is determined by the number of all potential actin-myosin interactions, it varies in accordance with the initial sarcomere length. Skeletal muscle can develop maximum force (F[max]) from its resting length if the length of sarcomeres is ca. 2 to 2.2 μm. When the sarcomeres shorten, part of the thin filaments overlap, allowing only forces smaller than F[max] to develop. When a muscle is pre-extended only limited number of actin–myosin bridges is available, which also restrict the generated force (m[max]). 2. Number of active sarcomeres The higher number of sarcomeres (arranged in parallel) is active in a working muscle the higher force can be generated (m[max]). The number of sarcomeres increases with training, so the trained people with bigger muscle mass can develop a higher force than untrained people. 3. Type of muscle fibers The maximal velocity of muscle contraction depends on the prevailing type of muscle fibers in the working muscle. If fast twitch fibers predominate V[max] is higher, if slow twitch fibers predominate V[max] is smaller. 3) Type of muscle fibers predominance of fast twitch fibers load mmax predominance of slow twitch fibers Vmax slow twitch muscle fibers aerobic metabolisms, slow rate of contraction, can be active long time before they fatigue fast twitch muscle fibers anaerobic metabolisms, high rate of contraction, fatigue quickly Note: Depending on the intensity of muscle contraction only certain types of muscle fibbers are activated. What determines how many of each muscle fiber type an individual has? We are genetically programmed to having a certain percentage of each muscle fiber based on your parents’ genes. It is thought that the average person is born with around 60% fast twitch and 40% slow twitch fibers, however some individuals can be born with larger amounts of fast twitch or slow twitch fibers and may therefore be more suited to either high force or long duration activities. Role of hormones Some hormones in our blood affect the muscle fiber properties and fibre growth. Hormone levels naturally fluctuate throughout a person’s lifetime. Males and females have different levels of certain hormones produced and the type of exercise (i.e. light vs heavy weights) we do affect the level of released hormones as well. Some of these hormones are ‘catabolic’, that is they stimulate muscle breakdown, while others are ‘anabolic’, that is they stimulate the growth and repair of muscle tissue. Hence, depending on our age, gender and the type of training we do, the production of certain hormones in our body affects the muscles to contract more slowly or more fast. Role of training There is no evidence as yet to show that fiber type can be changed, however there is evidence to show that fibers adapt to the type of training they are exposed to. It means that if a person with predominantly slow twitch ‘endurance’ fibers will train predominantly with heavy weights the slow twitch fibers will overtime begin to behave more like fast twitch fibers and vice versa. s = 0.07 m Vmean = Exploration of dependence of contraction velocity on skeletal muscle load t = ? s t Exploration of dependence of contraction velocity on skeletal muscle load The experiment in our demonstration consists in measuring the times (t) needed for lifting dumbbell with different weights by 7 cm (s). The ratio s/t then determines the mean velocity of dumbbell lifting (V[mean ]or V ) which is proportional to contraction velocity of biceps brachii. Setup for measurement of contraction velocity of skeletal muscle The measuring setup consists of a dumbbell support with two switches that allow us to measure the time at the beginning of lifting and the time when the dumbbell is lifted by 7 cm. The time difference determining the lifting time (t) is evaluated for loads from 1 to 24 kg. Representative results of measurement Representative results of measurement The results of measurement depicted in the graph show the most often difference between male and female in the contraction velocity – load relation. In the case of male, a higher values of maximal contraction velocity (V[max][,rel ]= 1) and of maximal load (m[max] 44.5 kg) indicate a higher percentage of fast twitch fibers and higher number of sarcomeres, respectively, in the explored muscle. EVALUATION OF CARDIAC MUSCLE CONTRACTILITY EVALUATION OF CARDIAC MUSCLE CONTRACTILITY In cardiac muscle, the term „contractility“ instead of „contraction“ is used when speaking about muscle performance. Contractility represents the performance of the heart at a given preload (left ventricular end-diastolic volume) and afterload (aortic pressure). In recent years, different indices have been proposed for evaluating cardiac contractility. Evaluation of some of them requires an invasive and difficult approach; such indices are therefore used only rarely. In contrast, the non-invasive approaches (based on echocardiography and Doppler ultrasound) that are routinely used in clinical cardiology are simpler, substantially less expensive, and provide a valuable information about cardiac contractility. Invasive indices: ·Index (dP/dt)[max] ·Index [(dP/dt)/P][max] ·Index V[max] ·Index E[max] Non-invasive indices: ·Ejection fraction ·Velocity of circumferential fiber shortening For specification of individual indices see the following slides. Index (dP/dt)max Normal values: 1300-1900 mmHg/s SV Assessment: by means of cardiac catheterization. 1 Index (dP/dt)max represents maximum velocity of left ventricle pressure rise (dP/dt)mean dP dt dP/dt Note.: this index may be affected by the Frank-Starling mechanism (e.g. at hypertension when end-diastolic volume is increased)! Use: mainly for research purposes (difficult and expensive invasive method). Index [(dP/dt)/P]max Assessment: by means of cardiac catheterization. 2 Index [(dP/dt)/P]max represents maximum velocity of cardiac muscle contraction [(dP/dt)/P]max control P 8 mmHg 70 mmHg Note.: this index may be affected by high end-diastolic pressure in left ventricle! Use: mainly for research purposes (difficult and expensive invasive method). ! Normal values: 44±8.4 s-1 Index Vmax Assessment: by means of cardiac catheterization. 3 Index Vmax represents velocity of cardiac muscle contraction at zero pressure P 0 70 Vmax extrapolation 15 Note.: this index may be affected by inaccurate extrapolation! ! 8 [mmHg] P 0 70 Vmax extrapolation 15 8 [mmHg] control control Use: mainly for research purposes (difficult and expensive invasive method). Index Emax Assessment: by means of cardiac catheterization. 5 Index Emax represents slope of the line determined from end-systolic values of P-V diagrams Note.: index Emax is the most exact method for evaluation of cardiac muscle contractility independent on preload and afterload of left ventricle! P V 30 ml 100 ml Emax= DPes DVes 70 110 [mmHg] [ml] aorta closed by a balloon contr. normal Emax smaller Emax lower contractility DVes DVes Pes - end-systolic pressure Ves - end-systolic volume srdce zakl 4 Use: mainly for research purposes (difficult and expensive invasive method). 13 ejekcni frakce obyc schema Ejection fraction (EF) EF = SV EDV SV – stroke volume EDV – end-diastolic volume Normal values: SV » 70 ml, EDV » 100 ml, EF = 50 - 70% SV EDV EF increases under sympathetic stimulation and with increasing inotropic state EF lower than 40 % indicates decreased contractility of cardiac muscle (systolic dysfunction) Assessment: by means of magnetic resonance or echocardiography. 5 Use.: assessment of EF is a non-invasive method commonly used in clinical practice to estimate left ventricular contractility and systolic performance! clinic ejekcni frakce obyc schema Velocity of circumferential fiber shortening (Vcf) Cd – length of inner circumferential left ventricle fiber in diastole Cs – length of inner circumferential left ventricle fiber in systole tef – duration of ejection fraction Normal value: 1.09 ± 0.12 circ · s-1 Cd Assessment: by means of echocardiography 6 Cs clinic Use.: assessment of Vcf is a non-invasive method commonly used in clinical practice to estimate left ventricular contractility! Vcf = (Cd-Cs) Cd · tef