How do your arm muscles work to lift a mug of coffee to your lips? A. The biceps muscle contracts while the triceps muscle relaxes. Only then can the forearm move up to lift the mug. B. The muscle tendons in your arms stretch to their limit as a result of the flexibility exercises that lift the mug. C. Your muscles rapidly increase the speed at which they twitch to trigger your arm movement. D. Your biceps and triceps muscles first relax, and then the arm muscle raises the mug.