A strained hamstring is one of the most common injuries amongst those whose sport involves sprinting and / or jumping but despite extensive research the exact cause of hamstrings strains remains inconclusive therefore it is likely there are a number of factors involved. The hamstrings are three muscles (biceps femoris, semitendinosis and semimembranosus) situated at the back of the thigh. Some authorities consider the posterior portion of the adductor magnus to be the forth hamstring since, like the main hamstring muscles, it extends the hip. EMG analysis shows that increased running speed is accompanied by increased activity (force and speed of contraction) of the proximal hip muscles* as well as increased movement about the hip. The hip extensors were dominant during back swing and the first half of the stance phase of running while the hip flexors were dominant during the early swing phase and the second half of the stance phase. During studies in 1986 Mann et al showed that during sprinting, the iliacus (a major hip flexor muscle) produced 86% of total hip flexion range of motion but only 46% during jogging, concluding that to increase running speed the athlete must increase the speed and range of hip flexion. The hip flexors also decelerate the terminal stage of back swing. The three main muscles for forward propulsion are the hamstrings, gluteus maximus and adductor magnus (muscles that extend the hip). This was shown in a study by Ito et al in 1993 in which they showed that world class sprinters pull their leg through the back swing phase of gait (hip extension) faster than other sprinters. This greatly contributed to the greater sprint speed. Mann (1998) showed that elite sprinters have a small horizontal distance between their body’s centre of gravity and the front support foot during the stance phase of running. This reduces ground reaction forces and thus reduces the risk of injury to the hamstrings, knees, hips and lumbar region as does reducing the contact time of the stance leg. This also increases speed. Over striding places the foot too far ahead of the body’s centre of gravity at foot contact and therefore increases ground reaction forces and brings about a braking effect. Sprinters who over stride may over compensate by trying to pull their body over the leg which potentially over works (strains) the hip extensors (specifically the hamstrings). EMG studies have shown that the hamstrings are the most active muscles during sprinting with increased activity coming during forward swing as they contract eccentrically to control hip flexion and knee extension and concentrically to bring about hip extension and knee flexion. It has also been shown that greater run speeds require increased hamstring activity. The gluteus maximus and adductor magnus are synergists in hip extension but the adductor magnus also neutralizes the abduction of the gluteus maximus through its role as an adductor. It is important that the synergy between these two muscles is correct as poor synergy can adversely affect the hamstring. Weak adductors forces the medial hamstrings to stabilize the femur in adduction as well as extending it while a weak gluteals forces the lateral hamstring to stabilize the femur in abduction while extending it. This extra work could over load the hamstring and increase the risk of injury.
EMG Analysis of One Complete Sprint Gait Cycle (right leg) Fig. 1
Muscle length appears to be an important factor in hamstring injury. Short (possibly hypertonic) hip flexors and quadriceps pulls the pelvis into an anterior tilt which lengthens the hamstrings and causes a degree of inhibition of these muscles thereby forcing them to work harder. This extra work load increases the risk of injury. Muscle strength is important too and not just the hamstring itself. Weak hip flexors produces less drive through the stance phase resulting in a compensation during which excessive use of the hamstrings is employed to produce this drive. This excessive use potentially overloads the hamstring and increases the risk of injury. Routine assessment of the flexibility, strength and balance (synergy) of a sprinter’s proximal hip muscles (or motion) is essential if an athlete is to increase speed and remain injury free. Other crucial areas to consider are sprinting technique (e.g. knee lift, stride length and foot placement) and training method (e.g. the proper application and implementation of plyometrics).
*The proximal hip muscles and actions during sprinting.
The principle hip motions in sprinting and what to assess
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Final note Hamstring strains can stem from imbalances further down the kinetic chain. Let’s take over pronation of the ankle as a simple example. Over pronation results in the excessive lowering of the medial arch of the foot during the stance phase of gait. The ankle roles inwards, the forefoot abducts and dorsiflexes while the rear foot everts. This causes the tibia to internally rotate. This in turn pulls the femur into internal rotation. This internal rotation pulls the pelvis into an anterior pelvic tilt which shortens the hip flexors and thus lengthens and inhibits the hip extensors. This inhibition means the hamstrings have to work harder to fulfil their roles in sprinting which increases the risk of hamstring injuries. Of course over pronation can cause other injuries e.g. patellar tendinopathies, patelllofemoral pain, ITB syndrome, Achilles tendinopathies and plantar fasciitis. |
Anatomy Chart
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Fig. 1 Explained The hip extensors are dominant in the back swing and first half of the stance phase. From left to right of the EMG... The hip extensors contract powerfully (concentrtically) in the early part of the support phase to drive the support leg back and to flex the knee behind the body during early flight (forward swing). During mid flight (float) they become less active as the hip flexors dominate in order to powerfully flex the hip. When the hip reaches the teminal stage of hip flexion (its highest point) the hip extensors then contract eccentrically to decelerate hip flexion (in the late flight stage or back swing) and they decelerate knee extension in preparation for foot contact (support). |
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Head Neck Shoulder Upper Arm Elbow Wrist & Hand Back Abdominals Hip & Groin Thigh Knee Lower Leg Ankle Foot Miscelaneous |
The Sports Injuries & Fitness Clinic
