page_title: Work loop
section_title: Experimental setup
hierarchical_section_title: Work loop / Work loop experimental approach / Experimental setup
caption_attribution_description: English: Stimulation phase influences mechanical work output
context_page_description: The work loop technique is used in muscle physiology to evaluate the mechanical work and power output of skeletal or cardiac muscle contractions via in vitro muscle testing of whole muscles, fiber bundles or single muscle fibers. This technique is primarily used for cyclical contractions such as the rhythmic flapping of bird wings or the beating of heart ventricular muscle.
  To simulate the rhythmic shortening and lengthening of a muscle, a servo motor oscillates the muscle at a given frequency and range of motion observed in natural behavior. Simultaneously, a burst of electrical pulses is applied to the muscle at the beginning of each shortening-lengthening cycle to stimulate the muscle to produce force. Since force and length return to their initial values at the end of each cycle, a plot of force vs. length yields a 'work loop'. Intuitively, the area enclosed by the loop represents the net mechanical work performed by the muscle during a single cycle.
context_section_description: Following humane procedures approved by IACUC, the muscle is isolated from the animal (or prepared in situ), attached to the muscle testing apparatus and bathed in oxygenated Ringer's solution or Krebs-Henseleit solution maintained at a constant temperature. While the isolated muscle is still living, the experimenter then applies two manipulations to test muscle function: 1) Electrical stimulation to mimic the action of a motor neuron and 2) strain (muscle length change) to mimic the rhythmic motion of a limb.  To elicit muscle contraction, the muscle is stimulated by a series of electrical pulses delivered by an electrode to stimulate either the motor nerve or the muscle tissue itself.  Simultaneously, a computer-controlled servo motor in the testing apparatus oscillates the muscle while measuring the force generated by the stimulated muscle.  The following parameters are modulated by the experimenter to influence muscle force, work and power output:
  Stimulation duration: The time period over which the muscle receives electrical stimulation
  Stimulation pulse frequency: The number of stimulation pulses per stimulation duration
  Stimulation phase: The time delay between the onset of stimulation and muscle length change
  Strain amplitude: The difference between the maximum and minimum values of the length oscillation pattern
  Strain/cycle frequency: The number of shortening-lengthening periods per time period
caption: 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.[20]