This article is an adaptation of a seminar presented at the Rugby Football Union Conference on injuries in the Zurich Rugby Premiership in April 2003. The theme of this presentation was causative factors in hamstring injuries. It highlights a number of risk areas and predisposing features pertaining to hamstring injuries that a physician/therapist must consider when addressing possible cause and effect relationships with hamstring injuries. The four key areas discussed are;
It is accepted that this list does not exhaust all the possible elements that may feature in the multi-factorial nature of hamstring injuries. It is, however, a solid starting point for clinicians to begin their management of hamstring injuries.
This area is further broken down into three key areas:
The possible pathologies having a direct cause and effect relationship with hamstring injuries or being indirectly associated with hamstring injuries include:
The possible mechanisms by which lumbar spine pathologies may contribute to hamstring problems may be described from a mechanical perspective or a neurological perspective.
The ‘mechanical’ argument suggests that a restriction of intersegmental movement at the lumbar spine due to a pathology in a spinal segment, will shift relative movement away from the lumbar spine and on to areas of least resistance. For example, the hip joint may need more extension to make up for a lack of lumbar spine extension (and therefore anterior pelvic tilt) at the end of stance phase of gait (a movement requiring both hip and lumbar spine extension). The hamstring may potentially become overactive in assisting the gluteal muscles in pulling the hip into active extension, possibly leading to overactivity and associated increase in tone/spasm. This may lead to sudden injury causing cramp or excessive fatigue leading to hamstring injury.
The ‘neurological’ perspective highlights that altered afferent input due to pain and pathology in the lumbar spine (due to a disc, ligament, muscle, nerve), reflexively alters efferent output in the form of ‘upregulated’ muscle tone or muscle inhibition. This is commonly known as a ‘facilitated spinal segment’. The most overt example of this is a direct nerve root impingement caused by a protruding L5/S1 lumbar spine disc. This will lead to lower-limb pain, paraesthesia, reduced reflexes and reactive muscle tone in the hamstrings. Overactivity due to increased tone may push the resistance of the muscle to stretch towards injury levels.
The most common pelvic muscle tone pattern found in athletes is a right tensor fascia latae (TFL), iliacus and psoas muscle that is hypertonic and a corresponding hypertonic left gluteus medius and adductor magnus. These asymmetries are common in the majority of athletes and this right anterior/left posterior pattern will fit approximately 80% of the athletic population. Most athletes will operate reasonably well within small tone increases with no apparent problems; however, the occasional few will develop musculo-skeletal problems when the muscles fall out of a ‘normal window’ of tone. The reason for this asymmetrical tone pattern is multi-factorial and includes predominately a) habitual muscle use in the form of repeated contraction (due to repetitive execution of movement) and b) sustained positioning such as hip flexion in sitting. This pattern is not necessarily consistent with an athlete’s handedness. Left-footed kickers may still present with this right anterior/left posterior pattern.
The rationale for this line of thought is derived from schools of thought in osteopathic medicine and exponents of ‘muscle energy’ concepts. The buzzword for these therapists is ‘balancing’ pelvic muscle tone. The pelvis forms the cornerstone of all kinetic chain movements. The ‘unbalancing’ of muscle tone in the pelvis is driven by the nervous system that works to upbeat (hypertonic/spasm) and downbeat (hypotonic/inhibition) resting muscle tone via neurological pathways. In essence, the nervous system works as a graphic equaliser to regulate muscle tone, whereby some muscles go up in tone and others go down in tone to keep a balanced system.
The carry-on effect is for the hypertonic muscle patterns to affect motion and kinematics of the lumbar spine, lumbosacral, and sacroiliac areas. The three-dimensional movement patterns of these joints are altered, leading to inefficient movement and altered force production in the involved muscles. Furthermore, muscle tone of the entire kinetic chain distal to the pelvis is then affected, the hamstrings included. For example, a hypertonic hamstring is often associated with increased tone in the gluteals, adductor magnus, spine extensors and quadratus lumborum on the same side. Hypotonic muscles lead to poor joint stability, increasing shear force and tensile force on a joint system, potentially leading to injury. As a consequence, other surrounding muscles (such as the hamstrings for a hypotonic gluteal) may work excessively hard to first stabilise and then attempt to move a system.
The concept of stability and joint control, or as strength coaches call it ‘core stability’, has been derived from a number of significant research areas. First and foremost are the research findings from Paul Hodges and Julie Hides with the work they have done on the association between transversus abdominus function and multifidus function and low back pain. This material is presented very well elsewhere and it is accepted that most therapists will have some exposure to the essential concepts discussed. It will not be discussed any further other than to mention that poor segmental control of the lumbar spine can lead to increased exposure to shear force on the vertebral segments, leading to breakdown and pathology (see pre-existing lumbar spine conditions above). Furthermore, poor stability leads to the pelvic muscles operating off an unstable base of support, the pelvis.
The research findings have been extrapolated to discuss the role these ‘core stability’ muscles have in the athletic population. It is proposed that a poorly functioning inner core unit of muscles may predispose an athlete to a myriad of musculo-skeletal problems, including hamstring injuries. This area requires a lot more direct research rather than the assumption that all hamstring problems have associated poorly functioning transverses abdominus.
Recent research that may have an interesting application in the concept of joint control is the work by Vleeming and Snijders in Holland regarding the concept of the ‘myofascial slings’. The sling system they describe may potentially be used to provide a basis for lumbo-pelvic joint control. They discuss the concept of ‘form closure’ of the sacro-iliac joints using these sling systems, and the implications this has for the athletic population is exciting. (We intend to expand this in a further issue of SIB on ‘New Advances in Trunk Stability’.)
Much has been published recently on the issue of static flexibility and stretching programmes, and the influence these have on injury rates. The most recent published papers have involved military recruits and particularly the effect that stretching has on overuse-based injuries. The findings have not been particularly favourable for those preaching the benefits of stretching in injury prevention. However, readers must be aware that the athletic population has a different set of injury mechanisms to military recruits, in particular acute muscle injuries. The findings of such studies must be viewed with caution when comparing them with athletes.
What is of particular interest is the role that hamstring flexibility has on the potential for hamstring injury. It is this author’s experience that muscle flexibility in the hamstrings is a poor predictor in determining hamstring muscle injury. The reasons for this opinion are multi-faceted.
First, most running injuries to the hamstrings occur in mid ranges of stretch and generally occur with an element of rapid velocity change – that is, acceleration and deceleration and the crossover from swing phase of gait to stance phase of gait. In this instance, the hamstring does not approach anywhere near the range it may possibly have. Having great muscle flexibility will definitely help an athlete who happens to get caught in an awkward position, such as a full slump position while under the load of opposing players. However, these instances are quite rare.
Second, athletes who attempt to suddenly increase muscle flexibility through the use of regular and consistent static flexibility programmes may have an increased risk of injury in the initial period. The possible reasons most likely relate to altered length-tension relationships in the muscle, and excessive neurological input due to repeated stretch causing proprioceptive changes and the innate ability of the body to control muscle force through range, leading to altered force production. However, this is not to say that an athlete with poor flexibility should not at least make an attempt to gradually increase flexibility while allowing the body to adapt force production to new-found length changes over the course of a competitive year. Actively encouraging athletes to improve flexibility will, in effect, prolong their playing careers due to the flow-on benefits experienced elsewhere in the body.
Third, athletes with below-average muscle flexibility tend to be those who never suffer a hamstring injury. They tend to function with tight restricted hamstrings quite well. The athletes who are hamstring restricted and do suffer hamstring injuries are often those with a pattern of global muscle inflexibility around the pelvis, not just the hamstrings.
What may be more predictive of the potential for hamstring injury is hip flexor tightness on the same or opposite side of the body to a hamstring injury. The possible reasons for this are either mechanical and/or neurological. Lack of hip extension due to hip-flexor tightness may cause the gluteals and hamstrings to work excessively in order to actively pull the femur into extension with running. This may lead to fatigue in the hamstrings, poor force production and potential for damage. The neurological perspective has already been discussed under the headings of pre-existing lumbar spine pathology and pelvic muscle tone.
A number of important elements apply when considering hamstring injuries and the development of a strength programme. These include:
Strength athletes all over the world tend to suffer from the same muscle imbalance problems in the lower limbs: quadriceps strength dominance over posterior chain strength (back extensors, gluteals, adductor magnus, hamstrings). This is a direct result of years of accumulated weight training focusing on squat lifts and leg pressing over deadlifts and all the variations of posterior chain exercises. I know most readers are thinking that squats still work these posterior chain muscles to some degree. That is true in concept, but in practice the majority of strength-trained athletes will squat with an emphasis on shifting the leverage on to the knee and thus quadriceps, rather than the hip and posterior chain muscle system. The effect of this is to overdevelop the quadriceps in relation to gluteals, erector spinae and more importantly the hamstrings. What we see are quad-heavy athletes with tightness and restriction in the quadriceps (poor knee flexion) and hip flexors (poor hip extension). The pulling strength of the supposedly ‘powerhouse’ muscles in the posterior chain does not match and is limited by the strength and restriction of the quadriceps. This may also be reflected in imbalances in quadriceps/ hamstring ratios as measured on an isokinetic dynometer. Isokinetic knee flexion/extension strength measures are not particularly functional tests for assessing hamstring function as it occurs in human locomotion; however, research findings measuring imbalances in quads/hams and side-to-side differences appear to be useful in predicting hamstring injury over the course of a competitive season. In a forthcoming issue of SIB I will discuss in detail some gym-based exercises relevant to developing strength in the posterior chain.
It is also important to consider strength development in terms of absolute strength and fatigue tolerance. It is critical that weight-training programmes focus on the development of hamstring strength at maximal levels in order to fully prepare the muscle for the forces encountered in acceleration and deceleration phases of sprinting. Typical programmes of three sets of 10 reps are probably not adequate in load levels to place a demand on the hamstrings in order to develop the necessary strength. Phasing a conditioning programme into maximal resistance will be necessary to stress the hamstrings enough to generate an adaptive response needed to meet the demands of top-end sprinting. Furthermore, fatigue tolerance in the hamstrings in also important, especially during the late stages of a game. The cycling of conditioning programmes must also consider the ability of the hamstring to generate force under fatigue situations.
Eccentric strength appears to be the more dominant contraction type in hamstring injury, particularly the cross over between eccentric contraction at the end of swing phase into dynamic concentric contraction at the start of stance phase. Training all contraction modes with emphasis on fast-speed eccentric is also a necessary component of a strength programme at some stage of the preparatory period. Again, this can be done via the use of posterior chain dominant exercises.
Finally, a brief mention on the specificity of conditioning running programmes in team sports needs to be addressed. Top-end speed is only achieved for a small percentage of time in most team-based sports; however, it is these small exposures that tend to place hamstrings in a vulnerable state for injury. Conditioning programmes and skill drills must involve a component whereby the athlete is attempting to execute skill/performance at maximal speed. Far too often the speed of a movement is slowed to accommodate the learning effect of a skill, much to the detriment of the injury potential of the muscle system. Coaches need to be educated by those dealing with the physical preparation of a team, that a small amount of time at top-end speed is necessary at some stage in order to fully prepare a muscle for the demands placed on it by fast ballistic movement.
The key issue to consider here is overstriding. A longer foot strike position compared to sprinters, along with a lower centre of gravity of the body with decreased knee height during leg recovery, is a key adaptive response of trained team-sport athletes as compared to sprinters. This is necessary, as locomotion in team sports involves rapid changes of direction and external forces acting on the body in an attempt to knock a player off balance. However, overstriding is described as a greater than 10-15cm foot placement in front of the hip joint at the start of stance phase of gait. The fundamental biomechanical problem with overstriding is that it increases the breaking force acting on a body and thus causes deceleration of the moving body, and it increases ‘clawing’ of the ground with the planted foot in order to bring the foot under the body so that propulsion may occur. The force for this ‘clawing’ is generated by the hamstrings and gluteals. Excessive ‘clawing’ will lead to early hamstring fatigue, leading to potential for injury.