Muscular System Structure and Contraction (3B) - Bio & Biochem

1. In a controlled experiment, a skeletal muscle fiber is exposed to a drug that selectively inhibits the sarcoplasmic reticulum $\text{Ca}^{2+}$-ATPase (SERCA) by 80% without directly affecting ryanodine receptors, troponin, or myosin ATPase. A single muscle action potential is elicited. Assume resting cytosolic free $\text{Ca}^{2+}$ is $\sim 100\ \text{nM}$ and peak free $\text{Ca}^{2+}$ during a twitch is $\sim 10\ \mu\text{M}$ under control conditions. Which statement best describes the role of calcium in this process?
   Answer: Reduced SERCA activity prolongs elevated cytosolic $\text{Ca}^{2+}$, delaying relaxation by maintaining troponin C occupancy This question tests understanding of SERCA's role in muscle relaxation and calcium homeostasis. SERCA (sarcoplasmic reticulum Ca2+-ATPase) is responsible for pumping calcium from the cytosol back into the sarcoplasmic reticulum after contraction, which is essential for muscle relaxation. When SERCA is inhibited by 80%, calcium removal from the cytosol is severely impaired, causing cytosolic calcium to remain elevated for a prolonged period after the initial release. Since calcium binding to troponin C is what keeps the myosin-binding sites on actin exposed, this prolonged elevation of cytosolic calcium maintains troponin C occupancy and delays relaxation. The question confirms that ryanodine receptors function normally, so initial calcium release and contraction can still occur. Choice D is incorrect because ATP is still required for cross-bridge cycling regardless of calcium levels, and high calcium actually promotes continued cycling rather than detachment. When analyzing muscle relaxation problems, remember that relaxation requires both calcium removal (by SERCA) and ATP for cross-bridge detachment - disrupting either process will impair relaxation.
2. In a skeletal muscle fiber, extracellular Ca$^{2+}$ is reduced from 1.2 mM to 0.1 mM while Na$^+$ and K$^+$ remain normal. A single motor nerve stimulus still produces a normal-amplitude muscle action potential and near-normal twitch force. Which statement best describes the role of calcium in this process?
   Answer: Skeletal muscle contraction depends primarily on Ca$^{2+}$ released from the SR; extracellular Ca$^{2+}$ influx is not required for a single twitch. This question tests the source of activator calcium in skeletal muscle contraction. Skeletal muscle relies on SR calcium release triggered by T-tubule depolarization, not extracellular influx for single twitches. Reducing extracellular calcium minimally affects twitch force, as SR provides sufficient calcium. Choice A rightly states SR dominance, explaining preserved contraction. Choice B is wrong because it overstates extracellular calcium's role, applicable to cardiac but not skeletal muscle. For ion dependency questions, distinguish tissue-specific calcium sources. Validate by considering prolonged activity, where extracellular calcium might influence refilling.
3. A research group studies excitation–contraction coupling in permeabilized skeletal muscle fibers in which the sarcolemma is bypassed and intracellular ionic conditions can be clamped. Fibers are exposed to solutions with either low or high free cytosolic calcium while ATP is held constant at 5 mM and pH at 7.2. Troponin C is intact. The dissociation constant for $\text{Ca}^{2+}$ binding to troponin C under these conditions is $K_d = 1\ \mu\text{M}$. In one condition, free cytosolic $\text{Ca}^{2+}$ is set to $0.1\ \mu\text{M}$; in another, it is set to $10\ \mu\text{M}$. Which statement best describes the role of calcium in this process?
   Answer: Calcium binds to troponin C, shifting tropomyosin to expose myosin-binding sites on actin and increasing cross-bridge formation This question tests understanding of calcium's role in the troponin-tropomyosin regulatory system of skeletal muscle contraction. In skeletal muscle, calcium binds to troponin C, which is part of the troponin complex that sits on the thin filament along with tropomyosin. When calcium binds to troponin C, it causes a conformational change that shifts tropomyosin away from the myosin-binding sites on actin, allowing myosin heads to bind and form cross-bridges. The experimental setup with permeabilized fibers and controlled calcium concentrations (0.1 μM vs 10 μM) relative to the Kd of 1 μM demonstrates how calcium occupancy of troponin C directly controls the availability of binding sites. Choice B is incorrect because calcium does not bind directly to myosin ATPase in skeletal muscle, and increased ATP hydrolysis causes contraction, not relaxation. To distinguish skeletal from smooth muscle mechanisms, remember that skeletal muscle uses the troponin-tropomyosin system while smooth muscle uses calcium-calmodulin activation of myosin light chain kinase.
4. A skeletal muscle is stimulated with two identical supramaximal impulses separated by 20 ms. The second twitch produces greater peak force than the first, even though the second action potential is unchanged. Cytosolic Ca$^{2+}$ after the second stimulus is higher due to incomplete Ca$^{2+}$ reuptake. Which statement best describes the role of calcium in this process?
   Answer: Residual Ca$^{2+}$ increases troponin occupancy, increasing the fraction of available actin sites and enhancing force (temporal summation). This question explores temporal summation in muscle force generation through calcium dynamics. Residual calcium from prior stimuli adds to new releases, increasing troponin occupancy and force. Closer impulses cause summation, enhancing the second twitch. Choice D correctly describes summation via higher calcium and more sites. Choice B errs by saying calcium promotes detachment, but it enables attachment. In summation queries, relate calcium kinetics to force potentiation. Check by varying intervals, expecting less summation with longer gaps.
5. A patient develops fluctuating muscle weakness that worsens with repeated use (e.g., repeated stair climbing) and improves after rest. Electromyography shows a decremental response during repetitive nerve stimulation. A serum assay detects IgG that binds the postsynaptic membrane at the NMJ, reducing functional nicotinic acetylcholine receptor density. Assume presynaptic acetylcholine release per action potential is normal and that acetylcholinesterase activity is unchanged. Which factor is most likely to limit muscle contraction in the described conditions during sustained activity?
   Answer: Reduced safety factor for neuromuscular transmission, causing some muscle fibers to fail to generate action potentials This question tests understanding of myasthenia gravis pathophysiology and the concept of neuromuscular transmission safety factor. In this autoimmune condition, antibodies reduce the density of functional nicotinic acetylcholine receptors at the neuromuscular junction, which decreases the amplitude of end-plate potentials even though presynaptic acetylcholine release is normal. During sustained activity, the already compromised EPPs may fall below the threshold needed to trigger muscle action potentials in some fibers, especially as slight depletion of readily releasable vesicles occurs with repetitive stimulation. This reduction in the safety factor (the margin by which EPP amplitude exceeds action potential threshold) explains the characteristic fatigability and decremental response seen on electromyography. Choice B is incorrect because the problem is insufficient depolarization, not excessive calcium release, and the weakness improves with rest rather than showing signs of tetanic fatigue. To analyze neuromuscular junction disorders, always consider whether the defect is presynaptic (affecting transmitter release) or postsynaptic (affecting receptor function), and how this impacts the safety factor for transmission.
6. In an experiment on skeletal muscle, extracellular Mg$^{2+}$ is increased substantially. The primary observed effect is reduced quantal content of acetylcholine release at the NMJ, with smaller end-plate potentials. Muscle fiber resting potential is unchanged. Which factor is most likely to limit muscle contraction in the described conditions?
   Answer: Reduced presynaptic Ca$^{2+}$ entry and vesicle fusion probability, decreasing acetylcholine release per nerve impulse. This question tests understanding of the neuromuscular junction (NMJ) and the role of ions in neurotransmitter release during skeletal muscle contraction. The key physiological principle is that calcium influx into the presynaptic terminal triggers vesicle fusion and acetylcholine (ACh) release, with magnesium ions acting as antagonists to calcium channels, reducing this influx. In the scenario, elevated extracellular Mg²⁺ reduces quantal content of ACh release, leading to smaller end-plate potentials without altering muscle fiber resting potential, indicating a presynaptic effect. The correct answer, choice D, follows logically because decreased presynaptic Ca²⁺ entry due to Mg²⁺ blockade limits ACh release per impulse, thereby restricting the depolarization needed for muscle contraction. A common distractor, choice B, is incorrect as it misattributes the effect to postsynaptic troponin C inhibition, ignoring the primary presynaptic reduction in quantal content described. For similar questions, always distinguish between pre- and postsynaptic mechanisms by noting effects on potentials or release metrics. Additionally, verify ion roles by recalling antagonists like Mg²⁺ primarily affect voltage-gated calcium channels at the NMJ.
7. In a skinned skeletal muscle fiber, ATP is clamped at 5 mM. When free Ca$^{2+}$ is raised to 1 $\mu$M, force develops. A nonhydrolyzable ATP analog (e.g., AMP-PNP) is then substituted for ATP at the same concentration. Force rapidly declines despite unchanged Ca$^{2+}$. Which factor is most likely to limit muscle contraction in the described conditions?
   Answer: Myosin head cycling because ATP hydrolysis is required to re-cock the myosin head for repeated power strokes. This question explores ATP's necessity in the cross-bridge cycle, distinguishing hydrolysis from mere presence. ATP hydrolysis by myosin provides energy for the power stroke and enables repeated cycling. Substituting a nonhydrolyzable analog prevents hydrolysis, halting cycling and causing force decline despite high calcium. Choice D correctly pinpoints limited myosin head cycling as the issue, aligning with ATP's role in re-cocking. Choice B errs by stating ATP binds troponin for site exposure, but calcium does that, misassigning ATP's function. In biochemical manipulation queries, differentiate substrate roles in energy versus regulation. Check by predicting if hydrolyzable ATP restores force, confirming the hydrolysis requirement.
8. A researcher compares two conditions in isolated skeletal muscle fibers at the same sarcomere length (2.1 µm) and temperature (30°C). Condition 1: intracellular ATP = 5.0 mM. Condition 2: ATP is rapidly depleted to 0.05 mM while intracellular free Ca2+ is held at 1.0 µM in both conditions. Under Condition 2, fibers develop high passive stiffness and fail to relax even after Ca2+ is lowered. The physiological process assessed is ATP's role in the contraction–relaxation cycle. Which factor is most likely to limit muscle contraction in the described conditions?
   Answer: Inability of myosin to detach from actin due to insufficient ATP binding, leading to rigor-like persistent cross-bridges This question tests understanding of ATP's essential role in cross-bridge detachment during the contraction-relaxation cycle. When ATP is depleted to very low levels, myosin heads that are bound to actin cannot detach because ATP binding to myosin is required to break the actin-myosin bond. This creates a rigor state where cross-bridges remain persistently attached, causing high muscle stiffness even when calcium is lowered. The muscle cannot relax because the mechanical linkages between thick and thin filaments cannot be broken without ATP. Option D is incorrect because SERCA activity would be reduced (not increased) with low ATP, and this would impair calcium removal rather than accelerate relaxation. To understand ATP's roles in muscle function, remember it is needed for: cross-bridge detachment, myosin head resetting, and active calcium pumping by SERCA.
9. At the NMJ, a competitive antagonist partially blocks nicotinic acetylcholine receptors such that the peak end-plate potential is reduced by 40% but remains above threshold at rest. During high-frequency stimulation, some stimuli fail to trigger muscle action potentials. Which factor is most likely to limit muscle contraction in the described conditions?
   Answer: Reduced safety factor for neuromuscular transmission, making transmission failures more likely during repetitive activity. This question assesses the safety factor in neuromuscular transmission during high-frequency activity. Partial receptor blockade reduces end-plate potentials, which may stay above threshold at rest but fail during fatigue or high demand. The antagonist lowers the safety margin, leading to transmission failures at high frequencies. Choice A properly identifies the reduced safety factor as the limiting element, explaining intermittent action potential failures. Choice B is incorrect because nicotinic receptors are postsynaptic ion channels, not SR release channels, confusing junctional and intracellular roles. Approach transmission reliability questions by considering amplitude margins and frequency effects. Validate by noting if low-frequency stimulation succeeds, highlighting frequency dependence.
10. A skinned skeletal muscle fiber is activated at constant intracellular free Ca2+ (1.0 µM) and then exposed to a myosin ATPase inhibitor that markedly slows ATP hydrolysis but does not affect actin binding directly. ADP and Pi are initially low, and total ATP is clamped at 5.0 mM. The physiological process assessed is ATP utilization in the cross-bridge cycle during contraction and relaxation. Which factor is most likely to limit muscle contraction in the described conditions?
    Answer: Reduced frequency of power strokes because ATP hydrolysis is required to cock the myosin head for force generation This question tests understanding of ATP's role in the cross-bridge cycle during muscle contraction. ATP hydrolysis by myosin ATPase is essential for cocking the myosin head into its high-energy conformation, preparing it for the power stroke that generates force. The scenario describes an ATPase inhibitor that slows this hydrolysis step, which would reduce the frequency of power strokes because fewer myosin heads can be reset to their pre-power stroke state. With calcium present and binding sites exposed, myosin can still bind actin, but without efficient ATP hydrolysis, the cycling rate decreases, limiting force generation. Option A is incorrect because myosin head detachment requires ATP binding (not hydrolysis), and calcium's role is to expose binding sites, not directly affect detachment. When analyzing cross-bridge cycle questions, trace through each step: binding, power stroke, ATP binding for detachment, and ATP hydrolysis for resetting.
11. A skeletal muscle fiber is treated with a drug that increases leak of Ca$^{2+}$ from the SR into the cytosol. SERCA activity is unchanged but must work harder to restore SR Ca$^{2+}$. During prolonged activity, ATP declines faster than in control fibers. Which factor is most likely to limit muscle contraction in the described conditions?
    Answer: ATP depletion due to increased SERCA workload, impairing both Ca$^{2+}$ reuptake and myosin detachment during relaxation. This question explores metabolic consequences of SR calcium leak. Increased leak demands more SERCA ATP use, accelerating depletion and impairing reuptake and detachment. Choice A identifies ATP depletion's dual impact. Choice C overlooks depletion effects. For leak-induced fatigue, link calcium handling to energy. Verify with faster ATP decline rates.
12. A researcher measures sarcomere geometry in a single skeletal muscle fiber during an isometric contraction at optimal length. At rest, the sarcomere length is 2.2 µm. During maximal isometric activation (intracellular free Ca2+ ~ 1 µM; ATP 4 mM), the sarcomere length remains 2.2 µm, but the I band width decreases and the A band width is unchanged. The physiological process assessed is the sliding filament theory during force generation without overall shortening. Based on the scenario, which outcome is most consistent with the sliding filament theory?
    Answer: The H zone decreases because thin filaments slide toward the M line while thick filament length remains constant This question tests understanding of the sliding filament theory and sarcomere band patterns during isometric contraction. According to the sliding filament theory, muscle contraction occurs when thin filaments slide past thick filaments without either filament changing length. During isometric contraction, the muscle generates force without overall shortening, but internal sliding still occurs as cross-bridges form and pull thin filaments toward the M line. The H zone (region of thick filaments without thin filament overlap) decreases as thin filaments slide inward, while the A band (entire thick filament length) remains constant because thick filaments don't change length. Option A is incorrect because thick filaments do not depolymerize during normal contraction; their length remains fixed. To analyze sarcomere questions, remember that A bands represent thick filament length (constant), I bands represent regions with only thin filaments, and H zones represent regions with only thick filaments.
13. During repetitive stimulation of a skeletal muscle at 50 Hz, intracellular ATP falls from 5.0 mM to 2.0 mM while inorganic phosphate rises. The muscle begins to fail to relax fully between stimuli, producing a sustained increase in baseline tension. Assume cytosolic free Ca$^{2+}$ transients remain normal in amplitude. Which factor is most likely to limit muscle contraction in the described conditions?
    Answer: Impaired detachment of myosin from actin due to insufficient ATP binding to myosin heads. This question probes the role of ATP in the cross-bridge cycle during sustained muscle activity, particularly in relaxation and repeated contractions. ATP binding to myosin heads causes detachment from actin, allowing relaxation and re-cocking for new cycles. During repetitive stimulation, falling ATP and rising phosphate impair this detachment, leading to incomplete relaxation and sustained tension. Choice A rightly attributes the limitation to impaired myosin detachment, consistent with the cross-bridge cycle requiring ATP for release. Choice D is misleading as it claims ATP activates troponin, but troponin is calcium-dependent, not ATP-dependent, confusing regulatory mechanisms. To solve similar fatigue-related queries, review metabolic changes and their impact on contractile steps. Validate by confirming if calcium transients are normal, isolating the issue to post-activation processes.
14. A skeletal muscle fiber is placed in a solution containing high extracellular K$^+$ (20 mM; normal 4 mM) while extracellular Na$^+$ and Ca$^{2+}$ are normal. The resting membrane potential becomes less negative, and subsequent nerve stimulation produces smaller or absent muscle action potentials. Which factor is most likely to limit muscle contraction in the described conditions?
    Answer: Reduced excitability due to depolarization-induced inactivation of voltage-gated Na$^+$ channels. This question tests how altered resting potential affects muscle excitability. High extracellular potassium depolarizes the membrane, inactivating sodium channels and reducing action potential amplitude. This limits contraction by impairing propagation. Choice A correctly attributes it to inactivation. Choice D misstates driving forces, as depolarization reduces sodium influx. In ion imbalance scenarios, calculate potential changes and channel effects. Confirm with normal ion responses.
15. A toxin is applied to the presynaptic terminal at the NMJ and selectively cleaves SNARE proteins required for synaptic vesicle fusion. Motor neuron action potentials still invade the terminal, and voltage-gated $\text{Ca}^{2+}$ channels open normally. The muscle fiber membrane and contractile apparatus are otherwise intact. Resting membrane potential is $-90\ \text{mV}$, and a typical EPP in control is sufficient to trigger a muscle action potential. Based on the scenario, which outcome is most consistent with the sliding filament theory?
    Answer: Sarcomeres fail to shorten because insufficient postsynaptic depolarization prevents $\text{Ca}^{2+}$ release needed to permit actin–myosin interaction This question tests understanding of synaptic transmission failure and its consequences for muscle contraction. When SNARE proteins are cleaved (as occurs with botulinum toxin), synaptic vesicles cannot fuse with the presynaptic membrane despite normal calcium influx, preventing acetylcholine release. Without acetylcholine release, there is no activation of postsynaptic nicotinic receptors, no end-plate potential generation, and therefore no muscle fiber depolarization. Without reaching the action potential threshold, there is no propagation through T-tubules, no calcium release from the sarcoplasmic reticulum, and consequently no calcium available to bind troponin C. Without calcium-troponin C binding, tropomyosin remains blocking the myosin-binding sites on actin, preventing cross-bridge formation and sarcomere shortening. Choice A is incorrect because it ignores the requirement for neural activation - sarcomeres cannot contract without the excitation-contraction coupling cascade initiated by neurotransmitter release. When analyzing neuromuscular junction blockade, trace the complete pathway from presynaptic vesicle fusion through to sarcomere activation to identify where the cascade is interrupted.
16. A patient with fatigable weakness is suspected to have myasthenia gravis. Repetitive nerve stimulation at 3 Hz shows a decrement in compound muscle action potential amplitude. ACh release per nerve impulse is normal, but the postsynaptic membrane has fewer functional nicotinic acetylcholine receptors. Which factor is most likely to limit muscle contraction in the described conditions?
    Answer: Reduced end-plate current leading to failure to reach threshold for a muscle action potential in some fibers. This question examines the pathophysiology of myasthenia gravis and its impact on neuromuscular transmission in skeletal muscle. Functional nicotinic receptors at the end plate are crucial for acetylcholine-mediated depolarization to reach action potential threshold. Fewer receptors reduce end-plate potential amplitude, causing a decrement during repetitive stimulation despite normal acetylcholine release. Choice D correctly identifies reduced end-plate current as the limiter, potentially failing to trigger action potentials in some fibers. Choice C is incorrect because fewer receptors would decrease, not increase, signaling, misapplying receptor dynamics to cross-bridge formation. For related autoimmune or transmission disorders, trace signal amplitude from synapse to muscle response. Assess by checking if direct muscle stimulation bypasses the defect, confirming synaptic involvement.
17. A single skeletal muscle fiber is activated by a brief depolarizing pulse that triggers a normal action potential and sarcoplasmic reticulum $\text{Ca}^{2+}$ release. In one trial, the intracellular ATP concentration is experimentally lowered to 0.2 mM (from a typical ~5 mM), while inorganic phosphate and ADP are held low by buffering systems. In a second trial, ATP is maintained at 5 mM. Temperature and pH are constant. The fiber is allowed to contract and then relax in each trial. Which factor is most likely to limit muscle contraction in the described conditions when ATP is lowered to 0.2 mM?
    Answer: Reduced detachment of myosin from actin, slowing cross-bridge cycling and impairing relaxation This question tests understanding of ATP's role in cross-bridge cycling, specifically in the detachment phase. During normal muscle contraction, ATP binding to myosin is required for the myosin head to detach from actin after the power stroke - without sufficient ATP, myosin remains bound to actin in a rigor state. When ATP is lowered from 5 mM to 0.2 mM, there is insufficient ATP available for efficient myosin detachment, which slows cross-bridge cycling and impairs both the speed of contraction and subsequent relaxation. The question specifies that calcium release is normal, ruling out issues with excitation-contraction coupling, and that ADP and phosphate are kept low, preventing product inhibition. Choice C is incorrect because low ATP would reduce, not increase, calcium binding efficiency to troponin C, and would decrease rather than increase shortening velocity. When analyzing muscle contraction problems involving ATP depletion, focus on the cross-bridge cycle steps that require ATP: myosin detachment and the subsequent cocking of the myosin head for the next cycle.
18. A permeabilized skeletal muscle fiber is placed in solutions with controlled free cytosolic Ca2+ while ATP is maintained at 5.0 mM and Mg2+ at 1.0 mM. At baseline (pCa 8; $[\mathrm{Ca^{2+}}] = 10^{-8}$ M), the fiber generates negligible tension. When free Ca2+ is increased to pCa 5 ($[\mathrm{Ca^{2+}}] = 10^{-5}$ M), tension rises rapidly without changing sarcomere length. The physiological process assessed is Ca2+-regulated activation of the thin filament during contraction. Which statement best describes the role of calcium in this process?
    Answer: Ca2+ binds to troponin C, shifting tropomyosin to expose myosin-binding sites on actin This question tests understanding of calcium's role in regulating skeletal muscle contraction at the molecular level. In skeletal muscle, calcium binds to troponin C on the thin filament, causing a conformational change that shifts tropomyosin away from myosin-binding sites on actin. The experiment uses a permeabilized fiber where calcium concentration can be directly controlled, bypassing the neuromuscular junction and membrane depolarization steps. When calcium increases from $10^{-8}$ M to $10^{-5}$ M, it binds troponin C and exposes actin binding sites, allowing myosin heads to bind and generate force. Option B is incorrect because calcium does not bind directly to myosin heads; rather, myosin ATPase activity is constitutive when binding sites are available. For similar questions, remember that calcium's primary regulatory role in skeletal muscle is through the troponin-tropomyosin complex on thin filaments, not through direct effects on thick filaments.
19. At the neuromuscular junction, a toxin cleaves synaptobrevin (a vesicle SNARE protein) in the presynaptic motor neuron terminal. In an ex vivo preparation, motor neuron action potentials are recorded as normal, and extracellular Ca2+ is 2.0 mM. However, muscle end-plate potentials are absent and no twitch force is produced. The physiological process assessed is neurotransmitter release–dependent activation of skeletal muscle. Which factor is most likely to limit muscle contraction in the described conditions?
    Answer: Failure of acetylcholine vesicle fusion with the presynaptic membrane, preventing neurotransmitter release into the synaptic cleft This question tests understanding of SNARE protein function in neurotransmitter release at the neuromuscular junction. Synaptobrevin (VAMP) is a vesicle SNARE protein essential for calcium-triggered fusion of acetylcholine-containing vesicles with the presynaptic membrane. When this protein is cleaved by toxin, vesicles cannot fuse and release their contents even though the motor neuron fires normally and calcium enters the terminal. Without acetylcholine release, there is no activation of postsynaptic receptors, no end-plate potential, and consequently no muscle contraction. Option B is incorrect because the problem occurs before any postsynaptic events; calcium regulation of troponin occurs much later in the excitation-contraction sequence. For neurotoxin questions, identify which step of synaptic transmission is affected: action potential propagation, calcium entry, vesicle fusion, or postsynaptic response.
20. A researcher measures force in a single skeletal muscle fiber while holding sarcomere length constant. When cytosolic Ca$^{2+}$ is clamped high (1 $\mu$M), the fiber generates steady force. A myosin ATPase inhibitor is then added; Ca$^{2+}$ remains high, but force declines over time. Which factor is most likely to limit muscle contraction in the described conditions?
    Answer: Reduced cross-bridge cycling because myosin cannot hydrolyze ATP to perform repeated power strokes. This question investigates ATP's role in sustaining force via myosin ATPase activity. Myosin ATPase hydrolyzes ATP to drive repeated power strokes in cross-bridge cycling. Inhibiting it prevents energy release, causing force decline despite high calcium. Choice D correctly identifies reduced cycling as the limiter, tied to ATPase function. Choice B misconnects ATPase to troponin binding, but troponin is calcium-regulated, not ATPase-dependent. In enzymatic inhibition studies, link the enzyme to its contractile step. Confirm by ensuring calcium clamping isolates the issue to myosin function.

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