Dorsal neck muscles such as the rhomboids and splenius contract isometrically and eccentrically to control the extension and lengthening of the neck into a contact and to resist gravity, as well as supporting the weight of the head and neck. These muscles work extremely hard to maintain a consistent elevated outline during dressage and this increases as you ask your horse to perform more advanced or elevated movements or to hold the contact for longer durations.
The goal is to strengthen these muscles, but not overwork them. Here are some key tips on how to work these muscles correctly, sustainably and to avoid fatigue and/or muscular damage.
Training methods, which restrain the horse into a contact, will exacerbate the strain on these dorsal muscles as they resist the forces placed on them. This in turn can lead to breakdown of the dorsal muscular chains and further disruption in the ventral chain. This has been explained in the diagram below.
Horse Movement: Structure, function and rehabilitation by Gail Williams & Alexa McKenna.