30% Figure 2: Diagram showing the phases of walking and running
to work concentrically to prepare for weight-bearing. As the heel strikes, the hamstrings contract to stabilize the knee as knee flexion commences. The hamstrings then continue to contract concentrically during toe off. The hamstrings are then passive until the cycle begins again (2).
Given the important role which the hamstrings play in opposing the powerful quadriceps in the second half of the swing phase it is little wonder that they are so commonly injured during this eccentric loading phase as they aim to decelerate the thigh.
Predisposing factors It is important to highlight a number of predisposing factors to hamstring injury so that these can be addressed at the rehabilita- tion stage.
A lot of research exists in the supposition of pre-
disposing factors but little evidence has conclusively defined them. Predisposing factors which have been highlighted by various authors include: ● Decreased hamstring strength ● Reduced flexibility ● Muscle imbalance between hamstrings and quadriceps ● Inadequate warm-up/preparation ● Hamstring fatigue ● Previous hamstring injury ● Poor posture/running technique
Research findings 1. In a study of Swedish soccer players it was reported that players doubled their chances of suffering a more severe hamstring injury if they had had a minor strain within the previous two months (3). A possible explanation of recurrence being improper or insufficient rehabilitation and an early return to sport.
2. Hamstring injuries are most commonly seen in the last quarter of a game where fatigue of the muscles will almost certainly be present.
3. A further study investigated the affect of fatigue on muscle function, which found that an alteration of the gait cycle was induced (4). The biggest change was seen during the swing phase, which was noticeably increased due to a decrease in stride rate.
4. It has been found that knee extension increases during the swing phase due to the hamstrings being unable to limit the end point. In addition the activation of the hamstrings to decelerate the leg during the swing phase was delayed, causing it to absorb a greater amount of force in a shortened period of time (2).
5. Increased hamstring activity during the stride due to early deactivation of rectus femoris was also reported
6. The above points have been suggested as protective mechanisms and failure of these may lead to hamstring injury (4).
7. In another study the author was able to correctly predict hamstring injuries in professional football players based on muscle strength imbalances between the quadriceps and hamstrings (5). He found that players were more susceptible to injury if they demonstrated hamstring strength of less than 60% of the quadriceps.
In addition, he suggested that hamstring
injuries were more likely in those who had a difference of more than 10% of hamstring strength between the two legs. This study would certainly emphasise the need for pre-season testing.
8. Research also supports a need for a pre-requisite minimum isokinetic concentric hamstring/quadriceps ratio of .60 at 60° per second for college football players (6). This finding was also echoed by a second study of Australian rules football players (7).
9. Many studies have investigated the relationship between hamstring injury and hamstring flexibility. However the research remains an area of great debate and therefore cannot be dismissed as a possible pre-disposing factor.
10.Posture is another contentious issue, several studies have reported a change in posture in those with hamstring injuries. However since these studies were retrospective it would be