Hamstring Injuries 101

Hamstring injuries are the most common type of muscle injury. I’m sure you can attest to that if you are reading this article! Hamstring strains/tears afflict both the novice and the competitive athlete. Interestingly, data collected from professional athletes indicates hamstring muscle injuries are the most prevalent muscle injuries in all of soccer, Australian football, American football, and track and field. Sadly, the data also confirms recurrence rates of up to 30% and that’s in professional athletes that do everything possible to avoid re injury.

What muscles make up the hamstring?

The hamstring muscle group is composed of the biceps femoris, semitendinosus, and semimembranosis.

How do muscles tear?

Muscle fibers fail in tension. This means that if they are stretched too much they tear.

 

Most muscle tears occur at the junction of muscle and tendon. Marked by red X’s in the schematic below.

 

What are common risk factors for hamstring injury?

  1. Previous injury-The primary risk factor for hamstring injury is a previous hamstring injury. This occurs because the residual scar tissue left behind from your previous injury is much weaker and much stiffer than normal muscle. Thus, this “scar ball” is at higher risk for tearing during activity.
  2. Fatigue-Both mental and physical fatigue disrupts some of the reflex mechanisms that your body relies upon to protect your hamstrings from stretching too far. If these failsafe controls deactivate the hamstring elongates too much and tears. Additionally, just like any mechanical device if you use your hamstrings too much it tends to break down.
  3. Muscle & nerve imbalance-Imbalances between the nerves and muscle groups around the hip, thigh and knee can lead to overuse, underuse, and over-stretching of the hamstrings. Weak hamstrings, overused hamstrings, and over stretched hamstrings are at greater risk for injury.

What are some anatomic risk factors for hamstring injuries?

The hamstring muscles possess unique attributes that predispose them to injury:

  • Hamstring muscles cross both the hip and knee joint. Most muscles only cross one joint, not two. The additional joint crossing can cause extra elongation of the hamstring that leads to tearing.
  • Hamstring muscles contain a high portion of type II muscle fibers. Type II muscle fibers are important for speed, strength, and power. The high force demand made on these fibers sets them up for breakdown.
  • The biceps femoris is controlled by two different nerves, while most muscle are controlled by just one nerve. Any lack of coordination between the nerves controlling the biceps femoris can lead to injury.
  • Most people’s pelvis is tilted forward. Since the hamstrings originate from the back side of the pelvis, this forward tilting can overstretch them leading to micro-tears that can easily become a big tear with rigorous activities like sprinting or heavy lifting.

During what type of movement patterns do hamstring tears occur?

There are two general type of movement patterns that can lead to hamstring injury: type 1 and type II.

Type 1

Type 1 occurs during high speed running. Injury to the hamstring muscle has been shown to occur during the terminal swing phase of running, when the hamstring muscles eccentrically contract (muscle activates while it is elongating) to decelerate the leg and prepare for foot strike.

The long head of the biceps femoris is most commonly involved.

Type II

Type II hamstring injuries occur during excessive lengthening of the hamstrings.These types of hamstring tears are most common in activities such as dancing, slide tackling and high kicking that combine hip flexion with knee extension.

Type II hamstring strains commonly involve the semimembranosus.

How do hamstrings heal?

Hamstrings heal by a three phase process: destruction, repair, and remodeling.

Phase 1-Destruction

  • Muscle fibers rupture
  • Blood clots form
  • Inflammation is initiated
  • Stage of greatest pain
  • Usually lasts 1 to 3 days

 

Phase-2 Repair

  • Breakdown of injured muscle
  • New scar production
  • New blood supply to injured muscle
  • Usually lasts 3 to 4 weeks

Phase-3 Remodeling

  • Scar organizes and matures to mimic normal muscle mechanical properties
  • New muscle formation
  • New muscle tendon attachments
  • Lasts anywhere from 3 to 23 months

The remodeling phase is the critical stage of the recovery period. In the absence of sufficient new muscle growth an overabundance of scar tissue remains in the injured area. As previously noted, scar tissue is weaker and stiffer than normal muscle. Therefore, residual scar puts you at high risk for a recurrent hamstring injury and lingering weakness, pain, and stiffness.

I encourage you to read my article on Hamstring Injury Prevention and Rehabilitation to learn the best strategies to minimize lingering scar tissue and maximize new, robust hamstring muscle.