Muscle Mechanics 1 Flashcards
Isometric Contractions
- when a muscle develops tension, but stays the same length because its ends are attached to immovable objects, the contraction is isometric
- length stays the same but tension increases
- work is not done because the load is not moved
- power is zero since work is 0
- involved in maintaining posture
Stretching Series Elastic elements
- stretching of series elastic elements offsets contractile element contraction so there is no movement of load during isometric twitch
- series elastic elements are represented as springs and can be thought as the rubberband-like tendon, although the entire muscle contributes to SE
- upon activation of actin and myosin, and cylcing of the cross bridges, the contractile element shortens while the series elastic elements elongates by an equal distance
- the degree of shortening of the contractile element equals the degree of elongation of the series elastic thereby keeping the overall length of the muscle fiber constant (isometric)
- this sequences of events reverses during relaxation phase of an isometric contraction
Body movements require both isometric and isotonic contractions
- lifting a book
- isotonic- bicep, length change (concentric/ecentric) tension sustained, body movement = book movement
- isometric-back-length constant, tension changes, no movement, postural
Isotonic contractions
- when the force of contraction of the muscle is at least equal to the load so that the muscle shortens, the muscle is said to contract isotonically
- the tension curve for an isotonic twitch shows a plateau during which the force or tension is constant
- the tension curve resulting from an isotonic twitch will look different depending upon the load placed on the muscle
- the greater the load the higher the plateau and the greater the time lag between stimuli and the start of muscle shortening
- if the load exceeds the amount of force the muscle can generate, an isometric twitch results
Shortening and Force during an Isotonic twitch
- upon activation of actin and myosin and cycling of cross bridges the contractile element shortens and series elastic element elongates
- once tension builds up to the point it equals (or lightly exceeds the load, the load begins to move
- then the tension becomes constant
- further shortening of contractile will lift the load, but while the load is being lifted the tension is constant
Four stages of an isotonic twitch
- isometric tension development- tension develops without shortening until the force developed equals the load
- isotonic shortening- shortening of the muscle at constant tension
- isotonic relaxation- elongation to the initial length at constant tension
- isometric relaxation-reduction of tension at the initial length
Isotonic twitch as a function of load
-for isotonic twitches with heavier loads- longer delay (latency) until shortening occurs, initial velocity of shortening,distance of shortening decreases and muscle drops the weight sooner
Isotonic contraction analysis provide load-velocity info
-when a muscle contracts isotonically under increasing loads the contractions display the following effects in the distance shortened vs time plot- the latent period (time lag between stimulation and shortening) increases, the duration of shortening decreases, the velocity of shortening decreases
Po of different muscles
Po is the maximal isometric tension that can be generated at a given initial length, and a given state of contractility
- Po is a function of the number of force generating cross-bridges that can be formed
- the larger the muscle, the larger the number of cross bridges and the larger is Po
- in skeletal muscle Po can increase with frequency modulation, recruitment of additional motor units, and with initial lengths closer to the optimal initial length Lmax, but not by changes in contractility
- in cardiac muscle, Po can increase with an increased initial length, and by increased contractility, but not recruitment
Myofibers produce maximum work and power at intermediate loads
- the speed with which a muscle contracts is also important in movement
- when a muscle is contracted isotonically under successively increasing loads the contractions display the following effects: the latent period (time lag between stimulation and shortening) increases, the duration of shortening decreases, the velocity of shortening decreases
Preload and afterload during an isotonic contraction
- preload is the load required to stretch a muscle from its resting length
- recall that muscles normally operate between 100-115% of resting length, so some load is required to stretch the muscle into its operating range
- once the 1g preload is added and the muscle stretched, the 2g afterload is then added and the muscle is stimulated causing the movement of the preload and the afterload
- the preload+afterload = total load
Skeletal muscle contraction cycle for an isometric contraction
- tension develops at constant length to a maximal tension for that particular initial length
- decrease of tension to 0g at constant length
Skeletal muscle contraction cycle for isotonic contraction
- development of tension at constant length until the tension equals the load
- shortening during the lifting of the of load at constant tension
- lengthening at constant tension during the beginning of relaxation
- decrease of tension during relaxation at constant length
