When ever you are straining your body or performing repetitive motions over a period of time it is always good to have the habit of taking a break and stretching. Stretches for warm-up and preparing the body you generally are only holding the stretch approx. 5 seconds and or performing active range of motion which most of the postings are describing that I have read.
For what you are describing, it will be important for you to stretch your pectoralis major/ minor muscles, latissimus dorsi, trapezius, and anterior serratus. The following stretches I'll be describing will address all of these muscle groups. Note: do not be aggressive with these stretches- more is not better! It's best to go into the stretch sslloowwllyy, that way you are paying attention to how your body is responding. I found that it is best to hold the stretch 20 - 30 seconds with 3 reps, alternating the stretch for each side of the body if applicable (for a total of 6 stretches). This helps to relax and balance the body while increasing flexibility. If you hold the stretch for less than 15 seconds, your muscles are not going to get the full benefit of the stretch and the objective will not be met.
Stretch for your Pecs: Stand next to a wall/ tree/ or vehicle to where you are parallel to it and you can rest your elbow against it comfortably. Your forearm will be vertical and hand should also be comfortably resting on the wall. Your upper arm should be at the same height as your shoulder and your elbow on the wall should be aligned with your shoulder (not in front of your shoulder or behind it). At this point your back is neutral, also take note of your feet - they should be parallel to the wall. Slightly rotate your feet away from the wall to a 20 degree angle - this prevents any hyperextension of the lower back. Then simply rotate your upper body away from the wall. This stretch specifically targets the pec muscles and prevents rotation of the shoulder that would intern access the anterior deltoid muscle. When performed correctly, you will notice that your spine/ back is in alignment, the stretch will be comfortable.
For your Latissimus dorsi, anterior serratus, and upper trapezius: Stand and face the wall while being about a step (24 - 30 inches) away from it. Place your hands on the wall and above your head, your arms should be straight and not locked. As you slowly lean into the wall, arch your back slowly by bending at the hip so you can look up towards the ceiling. You should feel a nice extension and lengthening of your spine. If your legs are straight and you are leaning in with your hips with the bend beginning at your ankles, stop, and begin again with the bend starting at the hip. If you're at a field, you can get down on your knees and walk your hands out to where you are on all fours and sit back slowly. This becomes "child's pose" in yoga.
As for your calf muscle, the best stretch that I can recommend that not only goes into your calf muscle (gastrocnemius/ and soleus), it also goes into your hamstrings and glutes. This is important as it addresses the entire muscle group and stretches your sciatic nerve; plus, it loosens your hips - this is a good thing.
For this stretch all you need is a belt, rope, or towel. When you are lying on the ground/ floor/ bed/, simply lasso your foot through the arch of your foot and slowly pull your foot up towards the ceiling/ sky. Keep your leg straight and knee soft, not locked. You want to be able to hold this stretch comfortably for 20 - 30 seconds; if you find that the stretch is becoming too much, just lower your leg to where it is more comfortable. If it's painful: stop.
For more information, tips, stretching modifications, please send me an email.