In part one of this blog series, I discussed the prevalence of muscular injuries in general. In this, we discuss the specific need for further examination of the hamstrings – and why this muscle group is so predisposed to injury.
Askling et al (2002) stated that 80% of muscle strains occur in the lower limbs, with 47% of these being hamstring injuries. This study also found that 10% of Major League soccer players encountered a hamstring injury during a season. Hamstring injuries have been one of the most prevalent ailments over the last eight to ten years (Brockett et al, 2001), the most common and problematic of soft tissue injuries (Dadebo et al, 2004). In Australian Rules Football, hamstring injuries account for 13% of all injuries and contribute to 16% of playing time missed (Orchard et al 1996), the highest percentage of any injury (Dadebo et al, 2004). Hamstring injuries represent 11% of all soccer injuries and a third of all muscle strains in this sport (Dadebo et al, 2004), and 11% of all running injuries (Running Research News). In addition, approximately 14% of hamstring injuries were re-injuries (Dadebo et al, 2004), the highest rate of all soft tissue injuries (Crosier et al, 2002), the significance of which will be later discussed.
The debilitating nature of hamstring injuries exists as a biomechanical double edged sword. Not only do the statistics point to such injuries as a major concern, but the nature of the injury itself further adds to their performance and function negating nature. Hamstring injuries frequently heal slowly and have a tendency to recur (Agre, 1985), once again alluded to by previous statistics. As the hamstring muscles play a principal role in running and sprinting, “…an injury of this muscle group may cause severe functional loss” (Jonhagen, 2005). In addition, hamstring injuries are most common in top level athletes (Jonhagen, 2005), perhaps suggesting a widely occurring training deficiency.
Hamstring injuries have one of the highest recurrence rates of all injuries suggesting current rehabilitation and prevention programs are having an insufficient effect, possibly due to a lack of bias towards eccentric exercise (Camilla et al, 2001). A conflicting view is that the recurrence of an injury is not due to the presence of the injury itself, but rather, to an increased susceptibility to hamstring injuries in certain individuals (Johnagen, 2003). Askling et al (2002) found that the site of recurrence is often at the site of previous injuries, providing evidence against Jonhagen’s view. It has been proposed that a plausible explanation for the recurring nature of posterior thigh injuries is that the optimum length of previously injured muscles is shorter, leading to a greater susceptibility to eccentric damage (Brocket et al, 2004). A similar theory has been proposed in that the threshold for muscle injury is lowered by the presence of a previous injury, resulting in stress levels which may once have had no effect, now causing further tissue damage (Mueller and Maluf, 2002). It is proposed that the physiological mechanism behind the high reinjury rate is the development of connective scar tissue between the ruptured myofibers which persists indefinitely. Stress immediately following an injury can hinder tissue regeneration, once again emphasising the need for correct rehabilitation. Implications for the high reinjury rate include the need for a greater onus to be put on determining injury history in athletes (Askling et al, 2003) and for rehabilitation to focus on restoration of function, not just relief of symptoms (Jonhagen, 2004).
Both a previous history of muscle injury, and the effect of fatigue can be viewed in light of the effects of eccentric activity and the subsequent delayed onset muscle soreness (DOMS). In essence, eccentric muscle contraction produces injuries, ranging from the micro-tears associated with DOMS, to tendinous injury and muscle rupture (Middleton and Montero, 2004). The association between eccentric exercise and DOMS is apparent with subjects demonstrating more discomfort after muscle disorders with isokinetic eccentric testing (Rochcongar, 2004). A further association between eccentric exercise and the incidence of muscle tears has been suggested by Camilla et al (2001). It has been proposed that “…the initial microscopic damage grows and leads to a muscular tear”. The amount of microscopic damage depends on the muscle’s optimum length for active tension (Brockett et al, 2004), and thus the optimum length of a muscle is a measure of susceptibility for muscle strains, a point which will be expanded on in regard to eccentric training. Thus, a training program which protects an individual against the micro-tears associated with DOMS, can be hypothesised to possess a similar protective effect against gross muscle tears.
It is not exclusively a hamstring weakness which poses a risk for hamstring injuries, but rather, a muscular imbalance, or strength deficiency between the hamstring and quadriceps muscle groups; a quadriceps to hamstring ratio. A poor ratio, when combined with fatigue (Sports Injury Bulletin) poses the main risk factor for hamstring injury (Askling et al, 2003). Additionally, this study indicated a favourable ratio has also been shown to correlate with an increase in performance and the identification of a poor ratio can be used to predict injury.
Early studies into the topic (Bennell et al, 1998) have found this ratio by “…dividing the maximal knee flexor (hamstring) moment by the maximal knee extensor (quadriceps) moment measured at an identical angular velocity and contraction mode” (Aagaard et al, 1998). In that same study, an improved mechanism of calculating the ratio, based on a more functional movement pattern was proposed. It was suggested that a comparison of eccentric hamstring strength and concentric quadriceps strength (or vice versa) would provide a more functional ratio due to the presence of this biomechanical combination in functional activities.
Brockett et al (2000) found that antagonist muscles such as the hamstring in the knee joint are at a greater risk of injury than the agonistic muscle themselves. As is the case with this particular joint, antagonist muscles are forced to contract to provide a decelerating force and stabilising effect (Coombs and Garbut, 2002), thus contracting during a rapid lengthening phase. The lesser mass of the hamstrings (when compared to the quadriceps) has been suggested to provide a lesser visco-elastic effect and thus a reduced ability to passively control and decelerate a knee extensor moment (Coombs and Garbut, 2002). It was found that muscles performing this eccentric contraction will experience both delayed onset muscle soreness (DOMS) and an increased risk of injury. A low ratio may imply an increased risk of injury due to the large torque produced concentrically by the quadriceps which exceeds the stopping force produced eccentrically by the hamstrings. “…a low ratio…could exert powerful forces during knee extension which would outstrip the hamstrings’ abilities to control the action, leading to potential hamstring damage.” (Running Research News).
The anatomy of the hamstrings places this muscle group in a situation where injuries are likely to occur. The hamstrings consist of three muscles (semitendinosus, semimembranosus and biceps femoris) which act over two joints, the hip and the knee (apart from the short head of the biceps femoris, which only acts over the knee).
This hamstring anatomy allows the muscle group to both flex the knee and also act as a hip extensor (Sports Injury Bulletin). The role of the hamstrings in gait and functional movement patterns is another variable which can be examined in order to identify injury mechanisms. In a functional gait pattern, the hamstring controls and decelerates the leg in an eccentric contraction (Sports Injury Bulletin). This eccentric action in itself generates high levels of force, once again increasing hamstring injury rates. Muscle strains occur most often when the hamstring is exposed to tensile stress while it is activated (contracting). “Forces on the muscle are greater in an eccentric contraction than in a…concentric contraction, which is why so many strains occur from eccentric overloading” (Sports Injury Bulletin). A proposed reason for this occurrence credits the heterogeneity of sarcomere lengths (Croisier et al, 2002). A weakness or strength deficiency during this eccentric phase will result in an inability to control deceleration and absorb shock upon landing, further contributing to the injury mechanism. Suggestions to combat this deficiency will be discussed later in the report. The eccentric role of the hamstrings is emphasized in the fact that the length-tension properties of the hamstrings are unaltered at differing velocities, suggesting this muscle group has a role in eccentric breaking at the end of the swing phase (Coombs and Garbutt, 2002).
In part three of this series we will explore methods to screen, prevent and rehabilitate hamstring injury.
