Radha was running on a treadmill at a great speed for $15$ minutes continuously. She stopped the treadmill and abruptly came out. For the next few minutes,she was breathing heavily/fast. Answer the following questions.
$(a)$ What happened to her muscles when she did strenuous exercise?
$(b)$ How did her breathing rate change?

(N/A) Due to continuous strenuous exercise,her muscles undergo anaerobic respiration because the oxygen supply is insufficient to meet the high energy demand. This leads to the accumulation of lactic acid in the skeletal muscles,causing muscle fatigue and pain.
$(b)$ Her breathing rate increases significantly from the normal resting rate. During exercise,the muscles require more $O_2$ for the production of $ATP$ through aerobic respiration. Therefore,the breathing rate increases to intake more $O_2$ from the atmosphere and to eliminate the excess $CO_2$ produced.