Work loop

This technique is primarily used for cyclical contractions such as cockroach walking.,[1] the rhythmic flapping of bird wings[2] or the beating of heart ventricular muscle.

Intuitively, the area enclosed by the loop represents the net mechanical work performed by the muscle during a single cycle.

Classical studies from the 1920s through the 1960s characterized the fundamental properties of muscle activation (via action potentials from motor neurons), force development, length change, shortening velocity, and history dependence.

For instance, if a muscle turns on and off more slowly, the shortening and lengthening curves will be shallower and closer together, resulting in decreased work output.

In 1992, the work loop approach was extended further by the novel use of bone strain measurements to obtain in vivo force.

For example, a muscle that generates force without changing length (isometric contraction) will show a vertical line 'work loop'.

Reciprocally, a muscle that shortens without changing force (isotonic contraction) will show a horizontal line 'work loop'.

[12] Work loop experiments are most often performed on muscle tissue isolated either from invertebrates (e.g. insects[7] and crustaceans[13]) or small vertebrates (e.g. fish,[14] frogs,[10] rodents[15]).

Step 3) Obtain net work (a single number) by numerical integration of muscle power data.

Or, 2) manually by printing a hard copy of the work loop graph, cutting the inner area and weighing it on an analytical balance.

Moreover, usage of the work loop technique as opposed to other modes of contraction, such as isometric, isotonic and isovelocity, allows for a better representation of the changes in mechanical work of the skeletal muscle in response to an independent variable, such as the direct application of caffeine,[17][18][19][20][21] sodium bicarbonate,[22] and taurine[23] to an isolated skeletal muscle, and the changes in work loop power output and fatigue resistance during ageing[24][25][26] and in response to an obesogenic diet.

[27][28] Development of the work loop technique has revealed various functional roles for muscle by simulating more realistic kinematics and activation.

For example, the large basalar (b1) muscle of the blowfly acts as a passive elastic element: it generates little power.

As a “strut”, the muscles generate force isometrically, or evenly while shortening, while allowing the passive elasticity of the tendons to store and release energy.

While this muscle does not change length, it produces high forces and allows the stretch and recoil of tendon to supply the mechanical work.

[33] Originally, work loops imposed a sinusoidal length change on the muscle, with equal time lengthening and shortening.

Imposing these "asymmetrical" stretch-shorten cycles can result in higher work and power outputs, as shown in treefrog calling muscles.

A hypothetical work loop. Arrows indicate the timecourse of the work loop, with stimulation occurring at the red dot. Green shaded area is net work, blue shaded area is work lost to passive and active resistance, and green + blue is total work.
Animation of a hypothetical work loop. Note the counter-clockwise motion of the loop, indicating net work generation by the muscle (as opposed to net work absorption)
In vitro force-length cycles. Each cycle (e.g. gray box) begins with electrical pulses to stimulate force production. While measuring force output, the experimenter controls stimulation and length input to match in vivo rhythmic motions and neural activation.
Strain frequency influences mechanical work output. [ 16 ]
Stimulation phase influences mechanical work output. Work generation vs. work absorption are shown by red vs. blue shading, respectively. Note how stimulation phase influences both the shape and magnitude of force as well as the direction of the work loop. [ 10 ]