BRINGING SCIENCE TO TREATMENT

Hamstrings: why ‘long and strong’ is best

Chris Mallac explains why a ‘long’ biceps femoris fascicle length and good strength in the Nordic hamstring exercise correlate quite well to reduced injury risk in the biceps femoris, a commonly injured muscle in athletes.

Injuries to the hamstrings are a common injury in athletes who are exposed to high-speed running such as sprint athletes, footballers, rugby players, AFL, NFL and hockey players. Hamstrings may also be injured in over-stretch situations or due to strong contractions, for example those encountered with aggressive change of direction.

It has been proposed that 80% of hamstring injuries involve the long head of the biceps femoris muscle, with the remaining 20% being a mix between the short head of the biceps muscle, the semitendinosus and the semimembranosus(1-5). Furthermore, reinjury rates in certain sports can be reasonably high. In thirteen seasons of AFL injury surveys, it was found that 27% of hamstring injuries were re-occurrences(6).

Strength and conditioning coaches and physiotherapists involved in these sports spend a lot of time and resources attempting to prevent these injuries through interventions such as stretching and strength work. In the event of an injury, the rehabilitation of these injuries requires a great deal of strength training. Strength training has always been the mainstay in the management of hamstring injuries to get the athlete robust enough in recovery to return to sport.

A group of researchers in Australia have presented some research papers that promote the ‘Long and Strong’ approach to strength training to both prevent injury and recover strength following injury. In this article, we’ll explore this concept and the implications for clinicians.

Anatomy and biomechanics

The hamstrings are comprised of the four following muscles;

  1. Biceps femoris long head (BFLH).
  2. Biceps femoris short head (BFSH).
  3. Semitendinosus (ST).
  4. Semimembranosus (SM).

All four muscles are involved in knee flexion, with the BFLH, SM and ST involved in hip extension (BFSH is not involved in hip extension). The ST and BFLH share a common proximal origin that is anteromedial on the ischial tuberosity and the SM originates more proximal and posterolaterally on the ischial tuberosity (see figure 1).

Injuries to the hamstring muscles caused by sprinting usually happen during the terminal swing phase and early stance phase; however they may also be caused by an over-stretch (such as stretching out to trap a ball with the foot) or due to a strong contraction (such as with hard stepping/cutting movements). These injuries are characterised by acute pain and dysfunction in the posterior thigh. The muscle fibres will become disrupted in the grade-2 and 3 versions(2).

The biceps femoris (BF) is the most commonly injured of the hamstring muscles(1,3,7), with the muscle-tendon junction and adjacent muscle fibres being the most common sites of disruption(3,8). Recent interest has been directed towards the involvement of the ‘central tendon’ in hamstring injuries, and the typical protracted rehabilitation period and high re-occurrence rate when these exist(9). Figure 2 shows the structure of the central tendons, and the extension of these tendons in relation to the pennate muscle belly of the biceps femoris.

The terminal swing phase of running and early stance phase of running are particularly hazardous as the following motions occur:

  • The hamstrings are required to contract forcefully whilst lengthening – in order to decelerate the extending knee and flexing hip during the terminal swing phase(10-11).
  • In terminal swing, the hamstrings reach their maximum length(10, 11), with the BFLH undergoing the greatest stretch (almost 110%), whilst SM and ST reach 107.5% and 108.2%, respectively(11).
  • The maximum torques for hip extension and knee flexion are found to occur during ground contact in overground sprinting(12). During this phase, the hamstrings are acting primarily concentrically to extend the hip(13).

Muscle architecture and the variation in the morphology of different hamstrings may predispose the muscles to injury. The BFSH possess much longer fascicles but a much smaller physiological cross-sectional area compared with the BFLH(14). Longer fascicles allow for greater muscle extensibility and reduce the risk of over lengthening during eccentric contraction. This is a potential dilemma for the BFLH as it is required to undergo the greatest lengthening of all the hamstrings during sprinting(15). However the fascicles are shorter compared with the BFSH, and this may predispose the BFLH to repetitive over lengthening and accumulated muscle damage(16, 17).

Why the ‘Nordic hamstring exercise’?

There exists a plethora of exercises used to develop hamstring strength in the athlete. However, many of these exercises have never been researched and validated as effective hamstring strength exercises. Most of the exercise selections have been driven by historical strength training methods, strength coach bias, popular exercises shown on social media and ‘bro science’.

The ‘Nordic hamstring exercise’ (NHE – see figure 3) has been a commonly-used hamstring exercise by all types of athletes for the last 18 years when they were popularised in the early 2000s. In fact, a version of the exercise was first described by author and health writer George Herbert Taylor in 1880. In its simplest form the exercise is performed in the following way:

  1. Start with kneeling on floor/bench/customised NHE device.
  2. The heels need to be held steady by a partner or a fixed strap to prevent the ankle moving.
  3. Fold the arms across the chest.
  4. Keep the hip locked into a neutral position or slight flexion.
  5. Start to lean forward slowly so that the knees begin to extend.
  6. Move as far as you can until the sensation you are about to fall.
  7. Allow yourself to drop to the floor and catch yourself with your hands.

The NHE described above is the classic eccentric-only version. Other versions involve holding an isometric contraction and returning to the start point (eccentric and then concentric). Other versions start in 90 degrees hip flexion and extend all the way out into full knee extension (‘razor Nordics’).

The NHE has polarised strength and conditioning coaches and physiotherapists; some claim that it is the best exercise for reducing hamstring injuries, while others claim that it is non-specific as it is essentially a knee dominant movement. It is also claimed that due to its eccentric nature, it induces so much muscle damage and delayed onset muscle soreness (DOMS) that many coaches and athletes do not like to use them in season due to the potential for muscle soreness.

However, the NHE has been shown to be effective in both increasing eccentric knee flexor strength(18)and significantly reducing hamstring injuries in football players(19-23)and Rugby Union players(24). Furthermore, it has been found that re-injuries in the hamstring were reduced by 85% in a group that performed the NHE program as part of the rehabilitation program(19). This idea can be extrapolated to all running-based sports.

The NHE has been shown to preferentially activate the ST and BFSH and to a lesser degree the BFLH(25,26). Conversely, exercises that require more hip extension as the focus (such as the 45-degree hip extension and Romanian deadlift) tend to preferentially activate the BFLH and the SM(27). Finally, the NHE preferentially increases biceps femoris fascicle length and is on par with the hip extension exercise in this regard(26). This will be discussed below.

The obvious question and argument from all this is that if the NHE preferentially activates the BFSH and ST, but most muscle injuries occur in the BFLH, then why do NHE at all? The clinical argument is that by making the ST and BFSH extremely strong, then this will offset the relative contribution of the BFLH in producing the bulk of the hip extension force and slowing down the knee extension during terminal swing phase of running. However, it can still be argued that the NHE does recruit the BFLH, just not to the same degree as the ST.

What is ‘fascicle length’?

The length of the BF muscle fascicle is measured on ultrasonography along the longitudinal axis of the muscle belly, and measuring the distance between the superficial and intermediate aponeuroses(26). It has been found that athletes (soccer players) with short fascicle length have a 4-fold increase in hamstring injury risk than players who have longer fascicles(28). For every 0.5cm increase in fascicle length, there is a 74% decreased chance of a hamstring injury in football players.

Eccentric-based exercises have been shown to increase fascicle length, whereas concentric-only exercise has not shown similar increases in fascicle length(28,29). The increase in fascicle length is a result of a sarcomerogenesis(17). It has been proposed that this increase in serial sarcomeres accounts for both a rightward shift in a muscle’s force-length relationship(30), while also reducing its susceptibility to damage(17,31). However, it is also at least theoretically possible that fascicle lengthening occurs as a result of increased tendon or aponeurotic stiffness(32).

The ‘quadrant of doom’

Based on the information above regarding increased fascicle length of the BFLH and the increase in strength experienced with NHE training, and how these correlate to injury rates in athletes, the ‘Quadrant of Doom’ has been developed that schematically represents how these factors interrelate. This correlation can be seen in the plot graph of fascicle length and NHE strength (figure 3). Those who did not suffer a hamstring injury are represented by green circles. Those that did suffer a hamstring injury are represented by red crosses). It can be clearly seen that there is a heavy weighting for red crosses in the bottom left corner (‘short and weak’), which represents the short fascicles with weak NHE strength(33). Those in this so-called ‘quadrant of doom’ are 39% more likely to suffer an injury compared with the ‘long and strong’ quadrant where there is a 4% chance (see figure 4).

Dosage and frequency of Nordic hamstring training

The suggested dosage of NHE needed to induce a strength change has not been fully elucidated. However, the program used in one of the studies to improve both strength and fascicle length used the program shown in table 1(18). This program is used over a 10-week period, and is planned to be performed twice per week. For example, in week 1 the athlete uses 2 x 6 reps twice per week, and in week 10 it is 5 sets of 5 reps. This program would most practically be performed at the end of a training session to minimise fatigue in the hamstring prior to any running based training. It has been found that strength training has a time-dependant effect on eccentric hamstring strength and that strength training after training decreases the negative influence of fatigue(34).

Practically speaking, it is likely that a program such as this will need to commence as the athlete/footballer is on an off-season break. As no equipment is required, it is simple enough to commence whilst on holidays. In a typical footballer’s calendar, where they have a 4 week off-season, weeks 1-4 are started independently, and once pre-season training starts the program would pick up from week 5. Once the ten weeks are complete, the athlete only needs small exposures to maintain strength and fascicle length, for example 2 x 4, once per week.

Together with observations that long-length concentric hamstring training can shorten muscle fascicles(35), the current findings are consistent with the possibility that the combination of concentric and eccentric contractions somewhat dampens the elongation of BFLH fascicles. The advantage of the NHE may be its almost purely eccentric or eccentrically-biased nature. Therefore, there may be ‘interference’ if other concentric or isometric based exercises are also performed as part of the routine strength training session. This idea needs future studies to confirm that an interference effect may exist.

Conclusion

Recent evidence strongly suggests that the NHE facilitates longer fascicles in the BFLH and that NHE training improves strength in this movement that is productive for injury prevention in hamstring injuries. It can be argued that it is a necessary inclusion in all strength programs for athletes in running based sports.

References

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