The Hamstring muscle complex injuries: types and classification

This article aims to provide a conceptual clarification, classification and basic knowledge about one of the most common injuries in sport: the hamstring injury.

Muscle function

The Hamstrings is a biarticular muscle group; It is the main knee flexor and is one of the hip join extensors. In addition, it also provides medial and lateral stability to the knee (1). 

Anatomy

The hamstrings muscle complex (Figure 1) are composed by:

• Semimembranosus (SM)
• Semitendinosus (ST)
• Biceps femoris (BF)
  • Long head (BFlh)
  • Short head (BFsh)

The muscle origin and insertion of each of the hamstring complex are presented in Table 1.

Muscle	Origin	Insertion
SM	ischial tuberosity	proximal medial tibia
ST	ischial tuberosity (joint tendon with BFlh)	proximal anteromedial tibia, forming the Pes Anserinus complex together with the sartorius and gracilis muscles
BFlh	ischial tuberosity (joint tendon with ST)	External tuberosity fibula
BFsh	linea aspera, lateral intermuscular septum, and condyloid ridge of the posterior femur	External tuberosity fibula

Classification of Hamstring injuries

The hamstring may be injured in the muscle, tendon or in the ischial tuberosity. Therefore, the injuries are also classified according to the affected area:

• Proximal injury (ischial origin): tendinosis, partial tendon tear, or complete tendon tear with or without avulsed osseous fragment. 
• Distal injury: involves the distal tendons or insertion of the tendons in a similar fashion.
• Central hamstring injury: involve muscle usually at the proximal or distal musculotendinous junction. There are the most common injuries.

Muscular injuries

On the muscle belly or musculotendinous junction, the hamstring muscle injuries are the most common injuries in soccer (2–4). Ekstrand et al. (4) found in their study with elite soccer players that these injuries represent the 12% of total injuries during the season and the sprinting and/or accelerations and decelerations actions are the greatest injury mechanisms. In addition, a recent systematic review showed a mean percentage of reinjury of 22.9% (2).

Muscular injury classification according to the severity:

Grade 0

• Discomfort sensation caused by overloading or contractures. DOMS. 
• Slight fiber structural-damage. Edema (but non hematoma).
• Prognosticated recovery time: 1-5 days.

Grade I

• Muscle strain (microtear) of the muscle fibers. Edema.
• It mostly occurs at the musculotendinous junction, and more often in the proximal area. 
• Higher reinjury incidence compared with others grades (5).
• Healing: RICE and adequate training program.
• Prognosticated recovery time: 1-2 weeks (6).

Grade II

• Partial macroscopic muscle fiber disruption.
• Edema and immediate functional impotence after the injury.
• Prognosticated recovery time: 3-4 weeks (6).

Grade III

• Complete disruption of the myotendinous unit, often with retraction and a gap between the torn ends.
• It necessitates prompt surgical correction. Delayed reattachment is associated with loss of full function and therefore increased morbidity. 
• Prognosticated recovery time: ≥ 3-4 weeks (6).

There are different grades of severity based on recovery time (4):

• Minimal injury: 1-3 days
• Mild injury: 4-7 days
• Moderate injury: 8-28 days
• Severe injury: >28 days

Tissue damage

However, the injury normally comprises a spectrum of tissue damage (i.e. different grades and areas) and not only in a single grade injury (1). On the other hand, there are factors which increases the recovery time: presence of injury to the BF, the cross-sectional area (as a percentage score), the length of the injury, and injury outside the musculotendinous junction.

Tendon injuries

Although it is less common, tendons may also avulse from their distal insertions or proximal origins with or without an associated avulsed osseous fragment.

• Tendinosis (chronic injury caused by an accumulation of small tears in the tendon that have failed to heal properly over the time).
• Partial tear
• Complete tear

Besides, the ischiatic tuberosity (muscle origin) injuries may be described according to a three-tiered method:

• Enthesopathic change (osseous area where the tendon is attached).
• Avulsion < 2 cm displacement of an osseous fragment 
• Avulsion > 2 cm displacement of an osseous fragment (normally requires surgical intervention)

The causes of enthesopathic changes may be due to a contusion, osteomyelitis (bone infection), or a more aggressive though rare process such as neoplasm (abnormal formation of new tissue of a tumoral, benign or malignant nature) should be considered with marrow signal change. In addition, they may be associated with a thickening of the cortical area of the bone or periostitis.

Finally, high-grade muscle, tendon, and chronic osseous avulsion injuries can lead to muscular atrophy over time (1).

I will post another publication about the recommendations and training program of hamstring injury treatment.

References

1. Hancock CR, Sanders TG, Zlatkin MB, Clifford PD, Pevsner D. Flexor femoris muscle complex: grading systems used to describe the complete spectrum of injury. Clin Imaging. 2009; 33(2): 130–5.
2. Pfirrmann D, Herbst M, Ingelfinger P, Simon P, Tug S. Analysis of injury incidences in male professional adult and elite youth soccer players: A systematic review. J Athl Train. 2016; 51(5): 410–24. 
3. Dupont G, Nedelec M, McCall A, McCormack D, Berthoin S, Wisløff U. Effect of 2 soccer matches in a week on physical performance and injury rate. Am J Sports Med. 2010; 38(9): 1752–8. 
4. Ekstrand J, Hägglund M, Waldén M. Injury incidence and injury patterns in professional football: The UEFA injury study. Br J Sports Med. 2011;45(7):553–8. 
5. Petersen J, Hölmich P. Evidence based prevention of hamstring injuries in sport. Br J Sports Med. 2005;39(6):319–23. 
6. Revista AMEF. Asociación Española de Médicos de Equipos de Fútbol. 2007.

Berni Guerrero-Calderón

S&C Coach | Rehab Therapist | Sport Scientist 

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This article has been made based on the references showed, other studies reviewed but not showed and according to the experience and knowledge of the author. In this way, it may include subjective ideas and opinions not contrasted in the research.