muscle explain 1

 Muscles require energy both to prepare for a contraction and then to sustain a contraction after stimulation.  Muscle cells have a few different strategies available for generating ATP to power contractions.  Of special relevance to our friend with her new interest in exercise, those energy harnessing strategies change depending on the type and length of exercise.
Strategy 1: CP

For the initial energy needs during a contraction, muscles can take advantage of a quick form of stored energy in the form of creatine phosphate (CP).  At rest, the muscle cell uses some of its ATP to add a phosphate group to creatine, creating CP.  As a result, muscle cells have a reserve of CP ready to use when energy demands grow.  The phosphate group on CP can very quickly be transferred to ADP, creating ATP to power contractions.  Only one ATP molecule is created for each CP molecule used and CP is depleted after a matter of seconds.

 Strategy 2: Glycolysis

Beyond 15 seconds or so of contraction, muscles will need another strategy besides CP use to generate ATP.  One option is glycolysis.  Glycolysis is a series of chemical reactions in the cytoplasm that use energy stored within carbohydrates from the blood or from muscle cell reserves to generate ATP.  This is still a relatively fast set of chemical reactions that does not require oxygen and it produces 2 ATP molecules per glucose molecule used.  It also produces lactic acid, which itself can be converted back to glucose and used in glycolysis.  It turns out lactic acid is not a primary cause of muscle fatigue or pain as was popularly believed.  Since glycolysis only produces 2 ATP molecules per cycle, it is not efficient enough to sustain contractions beyond a couple minutes.

 Strategy 3: Aerobic Respiration

A third strategy for ATP generation is aerobic respiration.  This is a more extensive set of chemical reactions that requires the participation of the mitochondrion.  The reactions require oxygen to proceed and the process is much slower than CP usage or glycolysis.  However, aerobic respiration is advantageous in that it more efficiently harnesses the stored energy in nutrient molecules to produce 30+ ATP molecules per cycle.  As a result, aerobic respiration can sustain contractions for hours.

 A Time to Think & Put the Pieces Together

Before moving on, take a moment to think about the above information in relation to our hypothetical friend's question.  Our friend started a new workout program where "she cycles quickly through various dumbbell exercises for a few minutes before resting and repeating later."  Consider the intensity and duration of this workout.

    What ATP generating strategies would her muscles initially use when beginning the workout?  Why?
    What ATP generating strategies might her muscles use later on in the workout?  Why?