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HMS, BM EQ-Bank 763

A competitive CrossFit athlete completes a workout consisting of 5 rounds of: 400m run, 15 heavyweight power cleans, and 15 ring dips, taking approximately 20 minutes to complete.

Critically analyse how the availability and utilisation of different fuel sources affect their performance throughout this high-intensity, mixed-modality workout.   (8 marks)

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Sample Answer

  • During the power cleans and initial acceleration of each run, phosphocreatine is the primary fuel source through the ATP-PCr system, providing immediate energy for these explosive movements.
  • The 400m runs predominantly utilise the glycolytic system, using muscle glycogen and blood glucose as fuel sources due to the high-intensity nature lasting 60-90 seconds each round.
  • As the athlete progresses through multiple rounds, incomplete recovery between efforts causes greater reliance on the aerobic system, even during higher-intensity segments.
  • The varied nature of the workout creates a significant oxygen debt during the weightlifting portions, leading to elevated EPOC (excess post-exercise oxygen consumption) during the runs.
  • By rounds 4-5, glycogen stores in the specific muscle groups become significantly depleted, resulting in reduced power output during the weightlifting movements.
  • The accumulation of lactic acid from the glycolytic system progressively affects the athlete’s ability to maintain proper technique in the ring dips and power cleans as the workout continues.
  • The athlete’s metabolic efficiency in switching between energy systems becomes crucial for maintaining performance across all rounds, with those having greater aerobic capacity typically preserving anaerobic resources more effectively.
  • Nutritional preparation and increased glycogen storage prior to the workout directly impact the athlete’s ability to sustain power output throughout all five rounds of the high-intensity circuit.
Show Worked Solution

Sample Answer 

  • During the power cleans and initial acceleration of each run, phosphocreatine is the primary fuel source through the ATP-PCr system, providing immediate energy for these explosive movements.
  • The 400m runs predominantly utilise the glycolytic system, using muscle glycogen and blood glucose as fuel sources due to the high-intensity nature lasting 60-90 seconds each round.
  • As the athlete progresses through multiple rounds, incomplete recovery between efforts causes greater reliance on the aerobic system, even during higher-intensity segments.
  • The varied nature of the workout creates a significant oxygen debt during the weightlifting portions, leading to elevated EPOC (excess post-exercise oxygen consumption) during the runs.
  • By rounds 4-5, glycogen stores in the specific muscle groups become significantly depleted, resulting in reduced power output during the weightlifting movements.
  • The accumulation of lactic acid from the glycolytic system progressively affects the athlete’s ability to maintain proper technique in the ring dips and power cleans as the workout continues.
  • The athlete’s metabolic efficiency in switching between energy systems becomes crucial for maintaining performance across all rounds, with those having greater aerobic capacity typically preserving anaerobic resources more effectively.
  • Nutritional preparation and increased glycogen storage prior to the workout directly impact the athlete’s ability to sustain power output throughout all five rounds of the high-intensity circuit.

HMS, BM EQ-Bank 762

After completing a marathon, an athlete describes "hitting the wall" at around the 32-kilometre mark. Analyse this experience with reference to the fuels used by the aerobic energy system.   (5 marks)

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Sample Answer

  • “Hitting the wall” refers to the point where muscle and liver glycogen stores become significantly depleted, forcing greater reliance on fat metabolism.
  • The aerobic system initially uses a mixture of carbohydrates and fats as fuel sources, with carbohydrates being the preferred and more efficient fuel.
  • After approximately 2 hours of running, glycogen stores (which total about 350g) become increasingly depleted, necessitating a shift toward greater fat utilisation.
  • Fat metabolism requires more oxygen per unit of ATP produced compared to carbohydrate metabolism, resulting in increased breathing rate and perceived exertion.
  • The transition from predominantly carbohydrate to predominantly fat metabolism is accompanied by a noticeable drop in sustainable pace due to the slower rate of ATP production from fats.
  • The athlete’s body temperature and respiratory rate typically increase when shifting to fat as the primary fuel, contributing to the subjective feeling of suddenly increased difficulty.
  • Trained athletes develop metabolic adaptations that enhance their ability to use fat as fuel, potentially delaying or minimising the effects of “hitting the wall.”
Show Worked Solution

Sample Answer 

  • “Hitting the wall” refers to the point where muscle and liver glycogen stores become significantly depleted, forcing greater reliance on fat metabolism.
  • The aerobic system initially uses a mixture of carbohydrates and fats as fuel sources, with carbohydrates being the preferred and more efficient fuel.
  • After approximately 2 hours of running, glycogen stores (which total about 350g) become increasingly depleted, necessitating a shift toward greater fat utilisation.
  • Fat metabolism requires more oxygen per unit of ATP produced compared to carbohydrate metabolism, resulting in increased breathing rate and perceived exertion.
  • The transition from predominantly carbohydrate to predominantly fat metabolism is accompanied by a noticeable drop in sustainable pace due to the slower rate of ATP production from fats.
  • The athlete’s body temperature and respiratory rate typically increase when shifting to fat as the primary fuel, contributing to the subjective feeling of suddenly increased difficulty.
  • Trained athletes develop metabolic adaptations that enhance their ability to use fat as fuel, potentially delaying or minimising the effects of “hitting the wall.”

HMS, BM EQ-Bank 761

Explain why a 400-metre runner experiences severe fatigue in the final stretch of the race, with reference to fuel sources and ATP production.   (3 marks)

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Sample Answer

  • The 400-metre race predominantly uses the glycolytic energy system, which relies on muscle glycogen and blood glucose as fuel sources.
  • The high intensity of the race causes rapid depletion of ATP and PCr stores in the first 10-15 seconds, requiring increased reliance on anaerobic glycolysis.
  • The anaerobic breakdown of carbohydrates produces lactic acid as a by-product, which accumulates faster than it can be removed from the muscles.
  • The accumulation of lactic acid reduces the muscle’s ability to contract forcefully by interfering with enzyme function and the process that connects nerve stimulation to muscle contraction.
  • As the race progresses, the incomplete breakdown of glucose provides increasingly insufficient ATP to maintain the initial pace, resulting in the characteristic slowing in the final stretch.
Show Worked Solution

Sample Answer 

  • The 400-metre race predominantly uses the glycolytic energy system, which relies on muscle glycogen and blood glucose as fuel sources.
  • The high intensity of the race causes rapid depletion of ATP and PCr stores in the first 10-15 seconds, requiring increased reliance on anaerobic glycolysis.
  • The anaerobic breakdown of carbohydrates produces lactic acid as a by-product, which accumulates faster than it can be removed from the muscles.
  • The accumulation of lactic acid reduces the muscle’s ability to contract forcefully by interfering with enzyme function and the process that connects nerve stimulation to muscle contraction.
  • As the race progresses, the incomplete breakdown of glucose provides increasingly insufficient ATP to maintain the initial pace, resulting in the characteristic slowing in the final stretch.

HMS, BM EQ-Bank 760

The body turns over ATP equivalent to 75% of its bodyweight during a 24-hour period, and much more during intense exercise. Evaluate the significance of fuel source availability and efficiency for a long-distance triathlete's performance.   (7 marks)

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Sample Answer

  • Long-distance triathletes primarily rely on the aerobic energy system, which can utilise carbohydrates, fats, and proteins as fuel sources, providing metabolic flexibility during extended competition.
  • Carbohydrate availability (approximately 350g of glycogen) is critical as it provides the most efficient source of ATP per unit of oxygen consumed during moderate to high-intensity efforts.
  • Fat stores provide virtually unlimited energy (compared to carbohydrates) but require more oxygen for metabolism, making them most suitable for lower intensity portions of the race.
  • The glycogen sparing effect, where well-trained athletes utilise fat earlier in exercise, preserves carbohydrate stores for higher intensity segments like hill climbs or sprint finishes.
  • Protein contributes minimally (less than 5%) to energy production during endurance events but increases in contribution when glycogen stores become significantly depleted.
  • Efficient fuel utilisation through training adaptations allows athletes to maintain higher work rates at lower relative intensities, preserving glycogen and delaying “hitting the wall.”
  • Nutritional strategies that maximise glycogen stores before competition and provide carbohydrate replenishment during the event directly impact performance by ensuring optimal fuel availability for ATP production.
Show Worked Solution

Sample Answer 

  • Long-distance triathletes primarily rely on the aerobic energy system, which can utilise carbohydrates, fats, and proteins as fuel sources, providing metabolic flexibility during extended competition.
  • Carbohydrate availability (approximately 350g of glycogen) is critical as it provides the most efficient source of ATP per unit of oxygen consumed during moderate to high-intensity efforts.
  • Fat stores provide virtually unlimited energy (compared to carbohydrates) but require more oxygen for metabolism, making them most suitable for lower intensity portions of the race.
  • The glycogen sparing effect, where well-trained athletes utilise fat earlier in exercise, preserves carbohydrate stores for higher intensity segments like hill climbs or sprint finishes.
  • Protein contributes minimally (less than 5%) to energy production during endurance events but increases in contribution when glycogen stores become significantly depleted.
  • Efficient fuel utilisation through training adaptations allows athletes to maintain higher work rates at lower relative intensities, preserving glycogen and delaying “hitting the wall.”
  • Nutritional strategies that maximise glycogen stores before competition and provide carbohydrate replenishment during the event directly impact performance by ensuring optimal fuel availability for ATP production.

HMS, BM EQ-Bank 759

A netball centre typically performs high-intensity bursts of activity throughout a 60-minute game. Analyse how the fuel sources used by this athlete would change during different phases of the game.   (6 marks)

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Sample Answer

  • During explosive movements (jumping, quick directional changes), the centre primarily uses phosphocreatine through the ATP-PCr system for immediate energy.
  • In fast breaks or intense rallies lasting 10-30 seconds, the glycolytic system becomes dominant, using muscle glycogen and blood glucose as fuel sources.
  • During lower intensity periods of the game, the aerobic system predominates, using carbohydrates as the primary fuel source.
  • As the game progresses and glycogen stores become partially depleted, the body increases its reliance on fat metabolism during recovery phases between high-intensity efforts.
  • The interplay of energy systems allows the centre to perform repeated high-intensity efforts throughout the game by recovering phosphocreatine stores during periods of lower intensity.
  • The athlete’s ability to spare glycogen through efficient use of fats during lower intensity phases helps maintain carbohydrate availability for high-intensity efforts in the latter stages of the game.
Show Worked Solution

Sample Answer 

  • During explosive movements (jumping, quick directional changes), the centre primarily uses phosphocreatine through the ATP-PCr system for immediate energy.
  • In fast breaks or intense rallies lasting 10-30 seconds, the glycolytic system becomes dominant, using muscle glycogen and blood glucose as fuel sources.
  • During lower intensity periods of the game, the aerobic system predominates, using carbohydrates as the primary fuel source.
  • As the game progresses and glycogen stores become partially depleted, the body increases its reliance on fat metabolism during recovery phases between high-intensity efforts.
  • The interplay of energy systems allows the centre to perform repeated high-intensity efforts throughout the game by recovering phosphocreatine stores during periods of lower intensity.
  • The athlete’s ability to spare glycogen through efficient use of fats during lower intensity phases helps maintain carbohydrate availability for high-intensity efforts in the latter stages of the game.

HMS, BM EQ-Bank 758

Compare and contrast the efficiency of ATP production in the three energy systems and explain how this affects the duration and intensity of activities where each system is predominant.   (6 marks)

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Sample Answer

  • ATP-PCr system produces ATP rapidly but has limited stores (approximately 90g of ATP and 120g of PCr), allowing only 10-15 seconds of high-intensity activity.
  • The glycolytic system produces ATP at a moderate rate by partially breaking down glucose anaerobically, supporting 30-45 seconds of high-intensity activity.
  • The aerobic system produces ATP efficiently but more slowly, enabling sustained energy production for activities lasting minutes to hours.
  • The ATP-PCr system supports maximal intensity (>95% of maximum heart rate) activities due to rapid ATP production, but quickly fatigues due to limited PCr stores.
  • The glycolytic system supports high, sub-maximal intensity (85-95% of maximum heart rate) activities but accumulates lactic acid, limiting duration to approximately 30 seconds at peak output.
  • The aerobic system supports sub-maximal intensity (\(\leq\)85% of maximum heart rate) activities due to its efficiency in completely metabolising fuels with oxygen, allowing for sustained energy production.
  • The interplay between rate of ATP production and total ATP production capacity determines the specific performance profile of each energy system.
Show Worked Solution

Sample Answer 

  • ATP-PCr system produces ATP rapidly but has limited stores (approximately 90g of ATP and 120g of PCr), allowing only 10-15 seconds of high-intensity activity.
  • The glycolytic system produces ATP at a moderate rate by partially breaking down glucose anaerobically, supporting 30-45 seconds of high-intensity activity.
  • The aerobic system produces ATP efficiently but more slowly, enabling sustained energy production for activities lasting minutes to hours.
  • The ATP-PCr system supports maximal intensity (>95% of maximum heart rate) activities due to rapid ATP production, but quickly fatigues due to limited PCr stores.
  • The glycolytic system supports high, sub-maximal intensity (85-95% of maximum heart rate) activities but accumulates lactic acid, limiting duration to approximately 30 seconds at peak output.
  • The aerobic system supports sub-maximal intensity (\(\leq\)85% of maximum heart rate) activities due to its efficiency in completely metabolising fuels with oxygen, allowing for sustained energy production.
  • The interplay between rate of ATP production and total ATP production capacity determines the specific performance profile of each energy system.

HMS, BM EQ-Bank 757

Identify the fuel sources used by the three energy systems and explain why each energy system uses its particular fuel source(s).   (6 marks)

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Sample Answer

  • The ATP-PCr system uses phosphocreatine as fuel because PCr can rapidly donate a phosphate group to ADP to reform ATP without requiring oxygen.
  • The glycolytic (lactic acid) system uses carbohydrates in the form of glucose (from blood) and glycogen (stored in muscles) because these can be quickly broken down anaerobically to produce ATP.
  • The aerobic system can use carbohydrates, fats, and proteins as fuel sources because oxygen is available for complete breakdown of these nutrients.
  • PCr is used for the ATP-PCr system due to its high concentration in muscle cells (five times that of ATP) allowing for quick energy availability.
  • Carbohydrates are the only fuel source that can be metabolised without oxygen, making them essential for the glycolytic system.
  • The aerobic system’s ability to use multiple fuel sources, particularly fat, allows for sustained energy production during prolonged activities.
Show Worked Solution

Sample Answer 

  • The ATP-PCr system uses phosphocreatine as fuel because PCr can rapidly donate a phosphate group to ADP to reform ATP without requiring oxygen.
  • The glycolytic (lactic acid) system uses carbohydrates in the form of glucose (from blood) and glycogen (stored in muscles) because these can be quickly broken down anaerobically to produce ATP.
  • The aerobic system can use carbohydrates, fats, and proteins as fuel sources because oxygen is available for complete breakdown of these nutrients.
  • PCr is used for the ATP-PCr system due to its high concentration in muscle cells (five times that of ATP) allowing for quick energy availability.
  • Carbohydrates are the only fuel source that can be metabolised without oxygen, making them essential for the glycolytic system.
  • The aerobic system’s ability to use multiple fuel sources, particularly fat, allows for sustained energy production during prolonged activities.

HMS, BM EQ-Bank 756 MC

During a marathon, which statement accurately describes what happens to an athlete's fuel usage after approximately 2 hours of running?

  1. The athlete continues to rely exclusively on the ATP-PCr system
  2. The glycolytic system becomes the predominant energy provider
  3. The body shifts from predominantly using glycogen to using more fat
  4. Protein becomes the primary fuel source for ATP production
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\(C\)

Show Worked Solution

Consider Option C: 

  • During a marathon, after approximately 2 hours of running, glycogen stores become depleted, causing the body to shift from predominantly using glycogen to increasingly relying on fat as a fuel source.

Other Options:

  • A is incorrect: The ATP-PCr system is only predominant for activities lasting 1-10 seconds, not endurance events like marathons.
  • B is incorrect: The glycolytic system is dominant for activities lasting 30-45 seconds, not for prolonged endurance events.
  • D is incorrect: While protein can be used as a fuel source in the aerobic system, it is not the primary fuel source, even in prolonged exercise; fat becomes the primary alternative fuel once glycogen is depleted.

\(\Rightarrow C\)

HMS, BM EQ-Bank 755 MC

During a 100-meter sprint, an athlete primarily uses which energy system and fuel source in the first 2 seconds?

  1. ATP-PCr system using phosphocreatine
  2. Glycolytic system using glucose
  3. Aerobic system using carbohydrates
  4. Aerobic system using fats
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\(A\)

Show Worked Solution

Consider Option A: 

  • In the first 2 seconds of a 100-meter sprint, the body relies on the ATP-PCr system, which uses phosphocreatine as its fuel source to rapidly generate ATP for the explosive muscular contraction needed at the start of the sprint.

Other Options:

  • B is incorrect: The glycolytic system becomes predominant after approximately 10 seconds once PCr supplies begin to deplete.
  • C is incorrect: The aerobic system using carbohydrates is not the predominant energy system during the initial explosive phase of a sprint.
  • D is incorrect: The aerobic system using fats is used primarily during longer duration, lower intensity activities, not the explosive start of a sprint.

\(\Rightarrow A\)

HMS, BM EQ-Bank 152

Describe the process of mobilising fat as a fuel source during submaximal exercise.   (5 marks)

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Sample Answer

  • During submaximal exercise, stored triglycerides are broken down into free fatty acids and glycerol in response to hormonal stimulation.
  • The mobilisation process is oxygen-dependent and requires specific enzymes which enables fatty acid transport across cellular membranes.
  • Specialised transport proteins actively move fatty acids across the mitochondrial membrane where energy production occurs.
  • Once inside the mitochondria, beta oxidation breaks down fatty acids producing acetyl-CoA.
  • This process provides substantial energy for prolonged submaximal exercise (< 65% VO2 max), yielding more ATP per molecule than glucose but requiring greater oxygen availability and processing time.
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Sample Answer

  • During submaximal exercise, stored triglycerides are broken down into free fatty acids and glycerol in response to hormonal stimulation.
  • The mobilisation process is oxygen-dependent and requires specific enzymes which enables fatty acid transport across cellular membranes.
  • Specialised transport proteins actively move fatty acids across the mitochondrial membrane where energy production occurs.
  • Once inside the mitochondria, beta oxidation breaks down fatty acids producing acetyl-CoA.
  • This process provides substantial energy for prolonged submaximal exercise (< 65% VO2 max), yielding more ATP per molecule than glucose but requiring greater oxygen availability and processing time.

HMS, BM EQ-Bank 151

Analyse how the availability of different fuel sources affects an athlete's performance in a 90-minute soccer match.   (6 marks)

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Sample Answer

  • The ATP-PCr system provides immediate energy for explosive sprints and tackles in the first few seconds of each high-intensity effort through stored creatine phosphate.
  • During the match, muscle glycogen serves as the primary fuel source, supporting both aerobic activity for general play and anaerobic bursts for sprinting or rapid directional changes.
  • As the match progresses and muscle glycogen becomes depleted, the body increasingly relies on fatty acid oxidation for energy production during lower intensity periods of play.
  • The maintenance of blood glucose levels through liver glycogen breakdown becomes crucial in the latter stages of the match to sustain both physical and mental performance.
  • Players often experience fatigue in the final 15 – 20 minutes of the match due to significant depletion of muscle glycogen stores, resulting in reduced sprint capacity and skill execution.
  • Athletes who engage in carbohydrate loading protocols prior to the match can increase their muscle glycogen stores by up to 50%, thereby delaying the onset of fatigue and maintaining performance intensity throughout the full 90 minutes.
Show Worked Solution

Sample Answer

  • The ATP-PCr system provides immediate energy for explosive sprints and tackles in the first few seconds of each high-intensity effort through stored creatine phosphate.
  • During the match, muscle glycogen serves as the primary fuel source, supporting both aerobic activity for general play and anaerobic bursts for sprinting or rapid directional changes.
  • As the match progresses and muscle glycogen becomes depleted, the body increasingly relies on fatty acid oxidation for energy production during lower intensity periods of play.
  • The maintenance of blood glucose levels through liver glycogen breakdown becomes crucial in the latter stages of the match to sustain both physical and mental performance.
  • Players often experience fatigue in the final 15 – 20 minutes of the match due to significant depletion of muscle glycogen stores, resulting in reduced sprint capacity and skill execution.
  • Athletes who engage in carbohydrate loading protocols prior to the match can increase their muscle glycogen stores by up to 50%, thereby delaying the onset of fatigue and maintaining performance intensity throughout the full 90 minutes.

HMS, BM EQ-Bank 150

Explain how glucose is used differently as a fuel source in the glycolytic and aerobic energy systems.   (4 marks)

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Sample Answer

Glycolytic System:

  • Breaks down glucose without oxygen in sarcoplasm
  • Produces 2 – 3 ATP molecules through rapid glycolysis
  • Results in lactic acid as by-product
  • Powers high-intensity exercise 10 – 60 seconds

Aerobic System:

  • Complete glucose breakdown with oxygen in mitochondria
  • Produces 36 – 38 ATP through Krebs cycle/electron transport chain
  • Results in CO2 and H2O as by-products
  • Powers prolonged exercise 2+ minutes
Show Worked Solution

Sample Answer

Glycolytic System:

  • Breaks down glucose without oxygen in sarcoplasm
  • Produces 2 – 3 ATP molecules through rapid glycolysis
  • Results in lactic acid as by-product
  • Powers high-intensity exercise 10 – 60 seconds

Aerobic System:

  • Complete glucose breakdown with oxygen in mitochondria
  • Produces 36 – 38 ATP through Krebs cycle/electron transport chain
  • Results in CO2 and H2O as by-products
  • Powers prolonged exercise 2+ minutes

HMS, BM EQ-Bank 149 MC

Which statement best describes why fatty acids cannot be the primary fuel source for high-intensity exercise?

  1. Fatty acids require too much oxygen for breakdown
  2. Fatty acids are not stored in muscle tissue
  3. Fatty acids produce less ATP than glucose
  4. Fatty acids are too complex to break down quickly
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\(A\)

Show Worked Solution

Consider Option A:  Fatty acids require too much oxygen for breakdown

  • Fatty acid metabolism requires significant oxygen

Other Options:

  • B is incorrect: Fatty acids are stored as intramuscular triglycerides
  • C is incorrect: Fatty acids actually produce more ATP than glucose
  • D is incorrect: Complexity isn’t the primary limitation

\(\Rightarrow A\)

HMS, BM EQ-Bank 148 MC

A marathon runner hits "the wall" after 30 km of running. Which fuel source has most likely been depleted?

  1. Creatine phosphate
  2. Blood glucose
  3. Muscle glycogen
  4. Plasma proteins
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\(C\)

Show Worked Solution

Consider Option C:  Muscle glycogen

  • Muscle glycogen depletion occurs in prolonged exercise

Other Options:

  • A is incorrect: CP depletion occurs in seconds
  • B is incorrect: Blood glucose is maintained through liver glycogen
  • D is incorrect: Proteins are not a primary fuel source

\(\Rightarrow C\)

HMS, BM EQ-Bank 144 MC

Which row correctly identifies the fuel source and duration of the ATP-PCr energy system?

\begin{align*}
\begin{array}{l}
\rule{0pt}{2.5ex} \ \rule[-1ex]{0pt}{0pt}& \\
\rule{0pt}{2.5ex}\textbf{A.}\rule[-1ex]{0pt}{0pt}\\
\rule{0pt}{2.5ex}\textbf{B.}\rule[-1ex]{0pt}{0pt}\\
\rule{0pt}{2.5ex}\textbf{C.}\rule[-1ex]{0pt}{0pt}\\
\rule{0pt}{2.5ex}\textbf{D.}\rule[-1ex]{0pt}{0pt}\\
\end{array}
\begin{array}{|l|l|}
\hline
\rule{0pt}{2.5ex}\textbf{Fuel Source}\rule[-1ex]{0pt}{0pt}& \textbf{Duration} \\
\hline
\rule{0pt}{2.5ex}\text{Glucose}\rule[-1ex]{0pt}{0pt}&\text{30 - 60 seconds}\\
\hline
\rule{0pt}{2.5ex}\text{Creatine Phosphate}\rule[-1ex]{0pt}{0pt}& \text{0 - 10 seconds}\\
\hline
\rule{0pt}{2.5ex}\text{Fatty Acids}\rule[-1ex]{0pt}{0pt}& \text{2+ minutes} \\
\hline
\rule{0pt}{2.5ex}\text{Creatine Phosphate}\rule[-1ex]{0pt}{0pt}& \text{30 - 60 seconds} \\
\hline
\end{array}
\end{align*}

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\(B\)

Show Worked Solution

Consider Option B: Creatine Phosphate 0 – 10 seconds

  • CP is the fuel source and system operates 0 – 10 seconds

Other Options:

  • A is incorrect: Glucose is fuel for glycolytic system
  • C is incorrect: Fatty acids fuel aerobic system
  • D is incorrect: Duration is wrong for ATP-PCr system

\(\Rightarrow B\)