 Risk Factors for Muscle Strain Injury Click
to read the .pdf version Invited paper for Sportslink IntroductionMuscle strains are
amongst the most common injuries in football players and sprint athletes, with
the most common strains being to the long muscles of the lower limb,
particularly the hamstring muscle group, the rectus femoris (quadriceps group)
and gastrocnemius (calf group) (1,
2)
. The assessment of risk factors is best made through a combination of
epidemiological studies and biomechanical analysis (3)
. It is known that strain injury is the result of excessive forces (which
can be either externally applied or passive internal forces, due to strain) (3)
. However, there is no established model to determine the external force
(or stress) that individual muscle groups or muscle fibres are subject to at any
given time of the gait cycle, making it very hard to assess why strain injury
actually occurs. Bahr and Holme have recently reviewed most of the studies examining hamstring risk factors and have concluded that most of the studies have too small a sample size to properly assess risk factors that do not have strong associations (4) . Proposed risk factors for gastrocnemius (calf) strain injury1. Sports involving repetitive push-off (acceleration) movements Gastrocnemius muscle strains are known to commonly occur late during the single-leg stance phase of a push-off manoeuvre. Clinical and anecdotal evidence is in agreement with direct video evidence that has recently been reported (5) . ‘Tennis leg’ in which the gastrocnemius suffers strain injury during the push-off movement after a tennis serve, was described over 30 years ago (6) . Other sports with regular push-off movements are also those most likely to result in calf strains. 2. Increasing age Calf strains are known to be far more common in older players (2) . 3. Past history of calf strain Previous calf muscle strain injury is a strong risk factor for future strain injury to the calf (2) . 4. Lumbar spine nerve entrapment at L5 level Lumbar spine degeneration changes which affect the L5 nerve root may be partially responsible for the age-related bias towards calf injuries (7) . In anatomical dissections, a structure in the pelvis called the lumbosacral ligament shows a correlation between L5/S1 degenerative changes and compression of the L5 nerve root (by the ligament) (8) . 5. Dehydration and fatigue There is a trend towards increased risk of calf muscle strain on warmer days (2) . It is also known anecdotally that calf cramps are far more common on warmer days early in the football season. It is possible that dehydration and fatigue play a role in changing the mechanics of the gastrocnemius muscle and increase the risk of calf strain. 6. Harder grounds Calf strains (like many other non-contact lower limb injuries) are relatively
more likely on northern AFL grounds, where shoe-surface traction is generally
greater (9)
. There is an almost significant trend towards more calf strains on
harder grounds, as measured by the Penetrometer in the AFL (10)
. Proposed risk factors for hamstring strain injury1. Sports involving maximum speed movements and/or the requirement to bend to pick up a ball whilst running Although hamstring muscle strains are common injury, authors have disagreed about whether strains occur during late swing or early stance during sprinting (5, 11, 12) . Video analysis of hamstring strains, whilst not revealing the time of injury during the gait cycle, shows that they are likely to occur during overstriding when close to maximum speed and trying to maintain speed. During this type of movement, the hamstring muscles are relatively more stretched than during a stride of normal length at maximum speed, although they do not reach maximum muscle length. Hamstring strains are more common in Australian football than in the rugby codes (13) and are the most common injury in 100m sprinters. It is likely that the greater propensity to reach maximum sprinting speed (which occurs after 30m of running) and the regular task of picking the ball up on the run are the reasons for hamstring strains being so common in Australian football. Bahr and Holme have recently presented a comprehensive analysis of the studies examining hamstring risk factors (4) . 2. Increasing age Hamstring strains are known to be far more common in older players (2, 14, 15) . 3. Past history of hamstring strain Previous hamstring muscle strain injury is a strong risk factor for future strain injury to the hamstring (2, 14-16) . 4. Low hamstring strength Many prospective studies have found a significant correlation between poor strength and risk of hamstring injury (17-19) . However, none of these studies measured past injury history as a confounder, and it is possible that low H:Q ratio was simply a marker for those athletes that had a past history of hamstring strain. A more recent study with superior sample size to those cited previously suggested that H:Q ratio was not a risk factor for hamstring strain (16) . This paper used an eccentric protocol, which has become standard for new generation isokinetic machines. However, a possible problem with this methodology is that an eccentric protocol on an isokinetic machine can actually cause a hamstring strain itself (20) , so athletes may not have given 100% effort when performing the eccentric protocol out of self-preservation. Although the weight of studies at least suggest that low H:Q ratio may be a risk factor for hamstring strain, it is not readily apparent that attempting to reverse H:Q deficits can reduce the incidence of hamstring strain. 5. Race Verrall has found a correlation between aboriginal race and hamstring injury in Australian football (14) . 6. Lumbar spine L5 nerve entrapment Lumbar spine degeneration changes which affect the L5 nerve root may be partially responsible for the age-related bias towards hamstring injuries (7) . 7. Lack of stretching and/or warm-up A recent survey of stretching practices
amongst professional soccer teams in the 8. Softer or more slippery playing surface Hamstring strains are only non-contact
lower limb injury in the AFL which defy the trend for greater risk in the
northern states and in fact are more common on 9. Excessive aerobic (compared to sprint) training There is anecdotal evidence to suggest that excessive aerobic training (as opposed to more field training) in Australian football teams leads to a greater risk of hamstring strain (22) . Proposed risk factors for rectus femoris (quadriceps) strain injury1. Sports involving kicking on the run and/or deceleration movements Quadriceps strains are a common injury in Australian football (23) and soccer (24) . The most commonly injured of the four quadriceps muscle is rectus femoris (25) . No study has fully answered the question of whether they primarily occur during ball contact, back swing, or ground contact during the step before kicking (2) , although it is unlikely that quadriceps strains occur during ball contact (3) . If a rectus femoris strain injury occurred during the ball contact phase of kicking, it would be when the muscle was in a relatively shortened state, inactive and shortening further (26, 27) , apparently conditions where muscle is able to withstand greater force. A comparative set of conditions in the upper limb, where the shortening triceps muscle is resisted by the ball when serving or spiking in volleyball, does not lead to muscle strain injury. Another anecdotal observation which suggests that the rectus femoris is not susceptible to strain injury by ball contact is that the speed of the run-up, rather than the distance the ball is kicked, is associated with a greater risk of strain (3) . The amount of foot-ball impact force is roughly proportional to the distance that the ball travels, yet long kicks with a slow run-up do not tend to cause rectus femoris strain. In Australian football, quadriceps strains are rare on long kick-outs from goal or kicking after a mark. They are also rare in punters in American football and goalkickers in rugby union and league, where kicks for maximum distance are often attempted. However, punters and kickers in these sports use a short and/or slow run-up. In contrast, rectus femoris strain injuries often occur during short kicks in Australian football when the player is running at high speed. Video analysis of rectus femoris strain injuries shows similarity between the quadriceps strains occurring during running and those occurring during kicking. The common factor is deceleration of the kicking or standing leg with a relatively short stride. Normally during deceleration the leg providing the loss of momentum overstrides and lands well in front of the body, directing a strong ground reaction force backwards. However, this method cannot be used by the stance leg during a running kick, as the hips would also lower during this manoeuvre and the kicking leg could not swing through without hitting the ground. In fact, the opposite effect is needed, with a slightly short step in order to raise the hips to provide clearance for the kicking leg. Although video evidence suggests that the ‘under-stride’ during deceleration is the gross mechanism for rectus femoris strain, as with the hamstring strain it is not clear whether the actual muscle failure occurs during the ground contact phase or swing phase. Like the hamstring, the swing phase is associated with the greatest muscle length (stretch), whereas the ground phase is associated with the greatest potential impact of external force (ground reaction force). In contrast to the hamstring strain injury, with rectus femoris strain injury the ground contact phase of risk precedes, rather than follows, the swing phase of greatest stretch. 2. Dominant kicking leg Quadriceps strains in
football players are more common in dominant kicking leg (RR 2.13, 95% CI
1.59-28.6), whereas hamstring and calf injuries are fairly evenly distributed (2)
. 3. Past history of quadriceps strains Previous hamstring muscle strain injury is a strong risk factor for future strain injury to the quadriceps (2) . 4. Higher traction grounds or drier playing surface Quadriceps strains are more common in
matches where there has been low rainfall over the previous week, which suggests
a ground contact rather than a ball contact mechanism (2)
. If quadriceps strains occurred during ball contact, it would be
expected that these injuries would be more common on wet days, when the ball may
be heavier. Quadriceps strains
(like many other non-contact lower limb injuries) are relatively more likely on
northern AFL grounds, where shoe-surface traction is generally greater (9)
. 5. Recent history of hamstring strain Quadriceps strains are also more likely
after a recent hamstring strain (2)
. Hamstring strains follow
the opposite trend to quadriceps strains and are more common on southern grounds
(9)
. It is possible that on grounds with less traction available,
alterations are made to gait including over-striding to increase ground contact
time, which make hamstring strains more likely and quadriceps strains less
likely. In grounds where traction is greater, the stride length may be
relatively shorter, which makes an under-stride, and consequently a quadriceps
strain, more likely. It is possible that during recovery from a hamstring
strain, alterations are made to gait (28)
, which include reducing the stride length, protecting the weakened
hamstring muscle from re-strain but increasing the chance of a secondary
quadriceps strain. 6. Repetitive short kicking during training A relatively high proportion of quadriceps strain injuries occur during training compared to matches (23) suggesting that repetitive kicking may be a risk. References
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