If you do not have any weights at hand, but still want to perform flyes and reverse flyes, take a look at the study that sports scientists at the Norwegian University of Science and Technology published in the Journal of Human Kinetics. According to this research you can perform these exercises with an elastic band instead of free weights. However, exercises with an elastic band stimulate the chest muscles and rear delts a little bit less than exercises with dumbbells.
The Norwegians had 29 test subjects perform the fly and the reverse-fly with a load with just 10 reps possible. On one occasion, the subjects performed the exercises with dumbbells, the other time with elastic bands.
The researchers placed electrodes on the skin of the test subjects with which they could see how hard the muscles underneath had to work.
In the figures below, the researchers cut the concentric and eccentric parts of the movement into two halves [CON1 and CON2, ECC1 and ECC2].
In general, both versions of the flyes and the reverse flyes have approximately the same effect on the pectoralis major and the posterior deltoid, respectively. But if you look more precisely, you will see that the dumbbell exercises provide a bit stronger stimuli than the elastic band exercises.
On the other hand, the stimulation of the supporting muscle groups during flyes and reverse flyes, such as the anterior deltoid and the deltoid medius at the reverse flyes, is greater when the subjects used an elastic band.
“In conclusion, elastic resistance bands induced slightly lower muscle activity in the muscles most people aim to activate during flyes and reverse flyes, i.e., pectoralis major and deltoideus posterior, respectively”, the Norwegians write.
“However, elastic resistance bands increased the muscle activation level substantially in the deltoideus anterior in flyes, and deltoideus medius and trapezius descendens in reverse flyes, possibly due to elastic bands being a more unstable resistance modality.”
“Overall, the results show that elastic resistance bands can be considered a feasible alternative to dumbbells in flyes and reverse flyes.”