Exercise physiology

Exercise physiology US Medical PG Question 1: A 35-year-old woman presents to the clinic for a several-month history of heat intolerance. She lives in a small apartment with her husband and reports that she always feels hot and sweaty, even when their air conditioning is on high. On further questioning, she's also had a 4.5 kg (10 lb) unintentional weight loss. The vital signs include: heart rate 102/min and blood pressure 150/80 mm Hg. The physical exam is notable for warm and slightly moist skin. She also exhibits a fine tremor in her hands when her arms are outstretched. Which of the following laboratory values is most likely low in this patient?

• A. Triiodothyronine (T3)
• B. Thyroxine (T4)
• C. Calcitonin
• D. Glucose
• E. Thyroid-stimulating hormone (Correct Answer)

Exercise physiology Explanation: ***Thyroid-stimulating hormone*** - The patient's symptoms (heat intolerance, weight loss, tachycardia, hypertension, warm/moist skin, fine tremor) are classic for **hyperthyroidism**. - In primary hyperthyroidism, the thyroid gland overproduces T3 and T4, which **negatively feedbacks** on the pituitary, leading to a **low TSH** level. *Triiodothyronine (T3)* - In hyperthyroidism, **T3 levels are typically elevated**, not low, as the thyroid gland is overactive. - T3 is one of the primary thyroid hormones responsible for the patient's metabolic symptoms. *Thyroxine (T4)* - In hyperthyroidism, **T4 levels are typically elevated**, not low, alongside T3. - T4 is the other key thyroid hormone produced in excess, contributing to the hypermetabolic state. *Calcitonin* - Calcitonin is a hormone involved in **calcium regulation** and is produced by the parafollicular C cells of the thyroid gland. - Its levels are not directly affected by hyperthyroidism and would not be consistently low in this scenario. *Glucose* - While hyperthyroidism can affect glucose metabolism, causing increased gluconeogenesis and glycogenolysis, it more commonly leads to **elevated or normal glucose levels**, not consistently low levels. - Low glucose would typically suggest other conditions like insulinoma or adrenal insufficiency.

Exercise physiology US Medical PG Question 2: A scientist is trying to design a drug to modulate cellular metabolism in the treatment of obesity. Specifically, he is interested in understanding how fats are processed in adipocytes in response to different energy states. His target is a protein within these cells that catalyzes catabolism of an energy source. The products of this reaction are subsequently used in gluconeogenesis or β-oxidation. Which of the following is true of the most likely protein that is being studied by this scientist?

• A. It is stimulated by epinephrine (Correct Answer)
• B. It is inhibited by glucagon
• C. It is inhibited by acetylcholine
• D. It is inhibited by cortisol
• E. It is stimulated by insulin

Exercise physiology Explanation: ***It is stimulated by epinephrine*** - The protein described is likely **hormone-sensitive lipase (HSL)**, which catabolizes **triglycerides** in adipocytes to **glycerol** and **fatty acids**. - **Epinephrine** (and norepinephrine) stimulates HSL activity via a **cAMP-dependent protein kinase A (PKA)** pathway, leading to increased fatty acid release for energy. *It is inhibited by glucagon* - **Glucagon primarily acts on the liver** to promote gluconeogenesis and glycogenolysis, but it does **not directly inhibit HSL** in adipocytes. - While glucagon has a lipolytic effect, it doesn't inhibit the enzyme that releases fatty acids. *It is inhibited by acetylcholine* - **Acetylcholine** is a neurotransmitter involved in the **parasympathetic nervous system**, which generally promotes energy storage. - It does **not directly inhibit HSL**; its effects on lipid metabolism are indirect and typically involve other pathways. *It is inhibited by cortisol* - **Cortisol**, a glucocorticoid, generally **promotes lipolysis** (breakdown of fats) in certain contexts, particularly during stress to provide energy substrates. - Therefore, it would **not inhibit HSL**; rather, it often enhances its activity or provides a permissive effect for other lipolytic hormones. *It is stimulated by insulin* - **Insulin** is an **anabolic hormone** that promotes energy storage, including **lipogenesis** (fat synthesis) and inhibits lipolysis. - Insulin **inhibits HSL activity** by activating phosphodiesterase, which reduces cAMP levels, thus deactivating PKA and preventing HSL phosphorylation.

Exercise physiology US Medical PG Question 3: A healthy 22-year-old male participates in a research study you are leading to compare the properties of skeletal and cardiac muscle. You conduct a 3-phased experiment with the participant. In the first phase, you get him to lift up a 2.3 kg (5 lb) weight off a table with his left hand. In the second phase, you get him to do 20 burpees, taking his heart rate to 150/min. In the third phase, you electrically stimulate his gastrocnemius with a frequency of 50 Hz. You are interested in the tension and electrical activity of specific muscles as follows: Biceps in phase 1, cardiac muscle in phase 2, and gastrocnemius in phase 3. What would you expect to be happening in the phases and the respective muscles of interest?

• A. Increase of tension in experiments 2 and 3, with the same underlying mechanism
• B. Increase of tension in all phases (Correct Answer)
• C. Recruitment of large motor units followed by small motor units in experiment 1
• D. Fused tetanic contraction at the end of all three experiments
• E. Recruitment of small motor units at the start of experiments 1 and 2

Exercise physiology Explanation: ***Increase of tension in all phases*** - In **phase 1**, lifting a 2.3 kg weight requires the **biceps** to contract, generating sufficient force (**tension**) to overcome gravity. - In **phase 2**, the **cardiac muscle** increases its contractile force (**tension**) to meet the metabolic demands of **exercise**, leading to a heart rate of 150/min. - In **phase 3**, electrical stimulation of the **gastrocnemius** at 50 Hz triggers muscle contraction, leading to an increase in **tension**. *Increase of tension in experiments 2 and 3, with the same underlying mechanism* - While tension increases in phases 2 and 3, the **underlying mechanisms differ**: cardiac muscle tension increases due to increased sympathetic stimulation and preload, while skeletal muscle tension increases due to unfused or fused tetanus from electrical stimulation. - Cardiac muscle contraction is regulated by **calcium-induced calcium release**, while skeletal muscle involves direct coupling of DHP receptor and ryanodine receptor. *Recruitment of large motor units followed by small motor units in experiment 1* - **Motor unit recruitment** follows the **size principle**, meaning smaller, more easily excitable motor units are activated first, followed by larger ones as more force is needed. - Therefore, in phase 1, **small motor units** would be recruited first, not large ones. *Fused tetanic contraction at the end of all three experiments* - **Fused tetanic contraction** occurs in **skeletal muscle** when stimulation frequency is high enough that individual twitches summate completely, leading to sustained contraction. - This phenomenon is **not possible in cardiac muscle** due to its long **refractory period**, which prevents sustained contraction and allows for adequate filling time. *Recruitment of small motor units at the start of experiments 1 and 2* - **Motor unit recruitment** applies to **skeletal muscle** (phase 1) and involves recruiting small motor units first for fine or gentle movements. - **Cardiac muscle** (phase 2) does not have motor units; instead, it relies on the **Frank-Starling mechanism** and hormonal/nervous regulation to adjust its contractile force as a syncytium.

Exercise physiology US Medical PG Question 4: An 83-year-old male presents with dyspnea, orthopnea, and a chest radiograph demonstrating pulmonary edema. A diagnosis of congestive heart failure is considered. The following clinical measurements are obtained: 100 bpm heart rate, 0.2 mL O2/mL systemic blood arterial oxygen content, 0.1 mL O2/mL pulmonary arterial oxygen content, and 400 mL O2/min oxygen consumption. Using the above information, which of the following values represents this patient's cardiac stroke volume?

• A. 30 mL/beat
• B. 70 mL/beat
• C. 40 mL/beat (Correct Answer)
• D. 60 mL/beat
• E. 50 mL/beat

Exercise physiology Explanation: ***40 mL/beat*** - First, calculate cardiac output (CO) using the **Fick principle**: CO = Oxygen Consumption / (Arterial O2 content - Venous O2 content). Here, CO = 400 mL O2/min / (0.2 mL O2/mL - 0.1 mL O2/mL) = 400 mL O2/min / 0.1 mL O2/mL = **4000 mL/min**. - Next, calculate stroke volume (SV) using the formula: SV = CO / Heart Rate. Given a heart rate of 100 bpm, SV = 4000 mL/min / 100 beats/min = **40 mL/beat**. *30 mL/beat* - This answer would result if there was an error in calculating either the **cardiac output** or if the **arteriovenous oxygen difference** was overestimated. - A stroke volume of 30 mL/beat with a heart rate of 100 bpm would yield a cardiac output of 3 L/min, which is sub-physiologic for an oxygen consumption of 400 mL/min given the provided oxygen content values. *70 mL/beat* - This stroke volume is higher than calculated and would imply either a significantly **lower heart rate** or a much **higher cardiac output** than derived from the Fick principle with the given values. - A stroke volume of 70 mL/beat at a heart rate of 100 bpm would mean a cardiac output of 7 L/min, which is inconsistent with the provided oxygen consumption and arteriovenous oxygen difference. *60 mL/beat* - This value is higher than the correct calculation, suggesting an error in the initial calculation of **cardiac output** or the **avO2 difference**. - To get 60 mL/beat, the cardiac output would need to be 6000 mL/min, which would mean an avO2 difference of 0.067 mL O2/mL, not 0.1 mL O2/mL. *50 mL/beat* - This stroke volume would result from an incorrect calculation of the **cardiac output**, potentially from a slight miscalculation of the **arteriovenous oxygen difference**. - A stroke volume of 50 mL/beat at 100 bpm would mean a cardiac output of 5 L/min, requiring an avO2 difference of 0.08 mL O2/mL, which is not consistent with the given values.

Exercise physiology US Medical PG Question 5: A 27-year-old man is running on the treadmill at his gym. His blood pressure prior to beginning his workout was 110/72. Which of the following changes in his cardiovascular system may be seen in this man now that he is exercising?

• A. Decreased blood pressure
• B. Decreased systemic vascular resistance (Correct Answer)
• C. Increased systemic vascular resistance
• D. Decreased stroke volume
• E. Decreased heart rate

Exercise physiology Explanation: ***Decreased systemic vascular resistance*** - During dynamic exercise, metabolic vasodilation in exercising muscles leads to a substantial **decrease in systemic vascular resistance (SVR)** to accommodate increased blood flow. - This vasodilation overrides the systemic vasoconstriction driven by the sympathetic nervous system, resulting in a net decrease in overall SVR. *Decreased blood pressure* - While SVR decreases, **systolic blood pressure typically increases** during exercise due to increased cardiac output. - **Diastolic blood pressure** usually remains stable or may slightly decrease, but overall blood pressure, specifically the mean arterial pressure, is generally maintained or elevated. *Increased systemic vascular resistance* - This is incorrect as **vasodilation in active muscles** causes a significant decrease in overall systemic vascular resistance. - An increase in SVR would typically hinder blood flow to working muscles and is not a characteristic cardiovascular response to dynamic exercise. *Decreased stroke volume* - Stroke volume generally **increases significantly** during exercise due to enhanced venous return, increased contractility, and reduced afterload (from decreased SVR). - A decreased stroke volume would limit cardiac output and exercise performance. *Decreased heart rate* - Heart rate **increases proportionally with exercise intensity** to boost cardiac output and oxygen delivery to active muscles. - A decreased heart rate would counteract the body's physiological demand for increased blood flow during physical activity.

Exercise physiology Flashcard 1 of 5

Question: In the muscle stretch reflex, some Ia and II afferent fibers stimulate _____ muscles (agonist or antagonist) causing muscle contraction

Answer: agonist

Exercise physiology Flashcard 2 of 5

Question: How does Pao2 change in response to exercise?_____

Answer: No change

Exercise physiology Flashcard 3 of 5

Question: How does Paco2 change in response to exercise?_____

Answer: No change

Exercise physiology Flashcard 4 of 5

Question: Type _____ muscle fibers are increased in proportion after endurance training

Answer: I

Exercise physiology Flashcard 5 of 5

Question: The primary function of autoregulation of blood flow to the skin is _____ regulation

Answer: temperature