Hamstring Group

I'm going to change things a bit this week. Instead of focusing on just one muscle, I'm going to focus on the muscle group known as the hamstrings.
Horses involved in collection like dressage horses, cutting horses, reining horses, jumpers, any discipline that requires the horse to flex and engage the hind end needed for great forward momentum and collected movements are prone to soreness and tension in the hamstring group of muscles.
There are three muscles comprising of the hamstring group. These three significant hard-working muscles are the biceps femoris, the semitendinosus and semimembranosus. They are influential driving muscles that push the horse forward.
L O C A T I O N:
The biceps femoris is located on the outer portion of the hip. Adjacent to the biceps femoris lies the semitendinosus and contiguous to the inner hind leg is the semimembranosus. All three connect at the sacrum. When a problem occurs in the hamstring group the issue can be confused with a back issue. However, tight hamstrings can torque on the sacrum. It is worth noting that soreness in the hocks can potentially cause tension in the hamstrings. (Note here that sore hocks can lead to us thinking "back issues." It can cause bucking, roach back, kicking etc. See how connected the body is? Just from hock pain?)
R O L E:
The biceps femoris basically extends the hip, stifle and hock and abducts the limb and rotates it outward. Abduction is movement AWAY from the mid-line of the body. Tight biceps could viably cause the hind toe to graze along the surface of the ground.
The Semitendinosus is fundamentally responsible for hip, stifle and hock extension during the weight bearing phase resulting in a thrusting force as the leg pushes off. Tension in the semitendinosus could produce a shortened stride.
The semimembranosus is instrumental in adduction and aiding in flexing the hip joint. Adduction is movement TOWARD the mid-line of the body, also refers to movements inwards and across the body. Restriction or soreness in the semimembranosus could present as a shortened stride, hindquarters tracking to the inside and possibly resistance to lateral movement.
D Y S F U N C T I O N:
Main causes: Injury though hyperextension is most common in this group, insufficient warm ups, impaired circulation to the muscles, and poor or insufficient training. The most commonly injured muscle of the group is the semimembranosus as it is the most stressed muscle. Hind fetlock pain can also lend to hamstring injuries as they can predispose this area to injury under pressure.
S Y M P T O M S  O F  P A I N :
• Reduced hindlimb stride length.
• Reduced hip extension & flexion.
• Poor collection and impulsion.
• Refusal to jump.
• Stressed with lateral work
• Dragging a hind toe
R E S T O R I N G  M U S C L E  S Y M M E T R Y:
Make sure to offer the horse a chance to stretch periodically during collected and intense work thus giving the muscle a chance to stretch. The muscles must be able to relax and contract easily to maintain elasticity and a healthy balance. The horse cannot go forward with full impulsion and thrust if the driving muscles are in a constant state of contraction; the rider may complain the horse is not in front of the leg. Consider massage therapy to assist in restoring and maintaining muscle balance. Your horse will thank you.