19.2.13

Types of Muscle Contractions

Total Tension of a Muscle

• Each of these forces will be the sum of active forces (developed by contractile machinery) and passive forces (due to stretching of elastic elements)

• Forcibly stretching a muscle well beyond its resting length will generate a force higher than that produced by active contraction

Types of Skeletal Muscle Contraction

• Isometric contraction• Isotonic contraction

Concentric contraction

Eccentric contraction

Isometric Skeletal Muscle Contraction

• When a muscle is stimulated such that it develops tension but does not shorten or lengthen. This is called an isometric contraction (iso = same, metric = measurement or length).

• This is a contraction in which no movement takes place

• Careful observation reveals that in the isometric contraction, the sarcomeres shorten and stretch the series elastic component even though the muscle as a whole does not shorten.

• Even though the muscle develops tension, but because it does not shorten, it does no external work (work = force x distance moved) but there is internal work being done

• The total tension is the sum of active and passive tension (the curve of total tension is the curve of isometric contraction)

Isotonic Skeletal Muscle Contraction

• When a muscle is stimulated such that the muscle shortens or lengthens with a constant load but its tension remains the same, the contraction is isotonic (iso = same, tonic = tension)

• This is a contraction in which movement does take place, because the tension generated by the contracting muscle exceeds the load on the muscle.

Types of isotonic skeletal muscle contraction

1. Conentric contraction

A concentric contraction is a type of isotonic contraction in which the muscles shorten while generating force such as lifting a weight up (a bicep curl)

2. Eccentric contraction

During an eccentric contraction, the muscle elongates while under tension against an opposing force (load). For example, lowering a load to ground

Force (tension)-velocity relationship of a muscle

• The force a muscle can generate depends upon both the length and shortening velocity of the muscle

• Force declines in a hyperbolic fashion relative to the isometric force as the shortening velocity increases, eventually reaching zero at some maximum velocity.

• The force generated by a muscle depends on the total number of cross-bridges attached.

• Because it takes a finite amount of time for cross-bridges to attach, as filaments slide past one another faster and faster (i.e., as the muscle shortens with increasing velocity), force decreases due to the lower number of cross-bridges attached.

• Conversely, as the relative filament velocity decreases (i.e., as muscle velocity decreases), more cross-bridges have time to attach and to generate force, and thus force increases.

Force (tension)-velocity relationship of a muscle

Types of skeletal muscle contractions at a glance

Cardiac Cells • The heart consists of three special types of cardiac

cells• Pacemaking cells: Have the properties of

automaticity and are capable of generating electrical impulses. These cells are present in the sinoatrial node and entire His-Purkinje system

• Conducting cells: Specialized for rapid conduction of electrical impulses and are present within the entire His-Purkinje system

• Muscle cells: Specialized for contraction and are present in the atria and ventricles

Cardiac Action Potentials

• In cardiac autorythmic cells, membrane does not have a resting potential

• Pacemaker potential - membrane slowly depolarizes “drifts” to threshold, initiates action potential, membrane repolarizes to -60 mV.