Story A cyclist pedals hard up a long hill for exactly two m…
Story A cyclist pedals hard up a long hill for exactly two minutes. Right after she stops, scientists take a tiny sample of her thigh muscle and measure these things inside and around the muscle cells: Thing Measured Before the hill After the hill ATP inside the cell 5.2 mM 4.9 mM (hardly changed) Acid level (pH) inside 7.1 7.0 (almost the same) Inorganic phosphate (Pi) inside 1.0 mM 3.2 mM (goes up a little) Potassium (K⁺) outside the cell 4 mM 11 mM (jumps way up) Size of the calcium burst during a twitch Normal Normal Force of a single twitch — drops to 50 % of normal Question Which change most likely explains why her muscle force fell to half of normal? A. She nearly ran out of ATP, so the cross‑bridges got stuck.B. Too much potassium built up outside the fibers, making it hard to start a new electrical signal.C. Extra phosphate inside the fibers blocked the power stroke of myosin.D. Calcium bursts got smaller, so fewer binding sites opened on actin.E. The small drop in pH damaged myosin proteins.