The function of a muscle can be categorised based on the density of type I and type II muscle fibres.
Type I muscle fibres are slow-twitch oxidative muscles, which means that they contract at a low level for longer periods of time and are resistant to fatigue. Muscles which are predominantly type I fibres are responsible for postural type work and are involved in maintaining position rather than actual movement.
Type II muscle fibres are fast-twitch glycolytic muscles, which means that they contract at a high level for shorter periods of time and are more likely to fatigue. Muscles which are predominately type II fibres are involved in producing movement for locomotion. Training and breed can influence the distribution of the muscle fibre types.
The best way to look at the distribution of muscles fibres is visually. You can see that a major proportion of locomotor muscles are located in the hindquarters, which is why horses are known as “rear wheel drive” animals. The muscles in the hindquarters drive the horse forward under powerful contraction. It is important to remember that these muscles are much more prone to fatigue and soreness in the muscle can lead to a lack of impulsion and strength, which may cause issues for all disciplines such as difficulty performing collective and technical movements (pirouette, passage, piaffe), difficulty taking off before a jump or refusal, struggling with uphill work, inconsistencies working in a contact, hollowing of the back.
You can see that the splenius and the rhomboid muscles mainly comprise of type I muscle fibres and are therefore predominantly postural muscles. They aid in maintaining the position of the head and neck during exercise and are under much strain when a horse is asked to work consistently in a contact. Soreness and trigger points are very common in these muscles especially in the dressage horse. Giving your horse several breaks during your training session and allowing the horse to stretch into long and low contact will help to relax and prevent overworking these muscles. As a tight muscle cannot strengthen!