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Exercise Protocols For Hamstring Injury Prevention In Athletes - How Effective?

Several factors have been associated with the risk of hamstring injuries in athletes. How effective are the various exercise protocols for hamstring injury prevention?

Alexander McCaslin
Jan 19, 2024125 Shares17832 Views
There exist different exercise protocols for hamstring injury preventionin athletes.
Hamstring strain, the most prevalent non-contact injury in many team sports, refers to any acute physical injury in the region of the posterior thigh, irrespective of the requirement for medical attention or time lost from sporting activities.
In team sports, such as rugby, football, and cricket, the incidence of hamstring injuries has shown an upward trend over the past two decades, and currently represents between 12% to 17% of total injuries, with a high rate of recurrence.
To prevent the incidence of hamstring injuries, several training methods involving have been proposed:
  • eccentric exercises
  • stretching
  • unstable agility exercises
However, there are still some inconsistencies regarding the type of training programs and their subsequent effects.
The lack of uniform criteria for implementing injury prevention exercise programs could be also associated with the empirical beliefs or unfounded concerns about the low compliance, time required, and perceived negative effects, such as:
  • excessive hypertrophy
  • loss of velocity
  • power
In this critical review, after analyzing the functional actions of hamstring and their injury mechanisms, we examine the effects of different exercises used to elicit protective adaptations to potentially reduce the incidence of hamstring injuries in athletes.
Additionally, we will address some practical recommendations for integrating an injury prevention program into the athlete’s periodized training plan.

Functional Actions Of Hamstring

The biarticular nature of the hamstrings allows them to act simultaneously as hip extensors and knee flexors during concentric contraction or eccentrically during concurrent hip flexion and knee extension, as observed in running and kicking.
However, the requirements of the hamstrings in termsof force, velocity and power are limited during submaximal or low-speed movements compared to the significantly greater levels occurring in the late swing phase of the sprint or an explosive kick.
Such eccentric muscle activity may predispose the hamstrings to injury as the lengthening may exceed the mechanical limits of the muscle or lead to the accumulation of microscopic muscle damage.
In addition, when the hamstrings are considered within a functional kinetic chain, this muscle group appears to be associated with both upper body (pelvis, spine, shoulder, and skull) and the lower-limb alignment and stabilization.
An outline of the human body with different parts pointing in the skull, upper torso, knee, ankle and foot
An outline of the human body with different parts pointing in the skull, upper torso, knee, ankle and foot
These anatomical links may be the reason by which the lack of stability of the back and/or pelvis has been proposed as one of the main contributors to increased risk of hamstring injury.
Additionally, the two heads of the biceps femoris are innervated by different nerve branches of the sciatic nerve, with the tibial portion innervating the long head and the common peroneal branch innervating the short head.
This dual innervation could possibly result in uncoordinated contraction and poor motor control leading to an increased rate of hamstring injury observed as fatigue progresses during exercises.

Mechanism And Localization Of Non-Contact Hamstring Injury

Hamstring strains occur through an over acute strain episode that could be represented by a continuum from microfiber damage (as seen in association with delayed onset muscle soreness) through a partial strain or complete muscle rupture.
This injury has been strongly associated with high-eccentric muscular tension exceeding the mechanical limits of the tissue.
Based on the injury mechanisms and localization, two types of hamstring injuries have been differentiated as follows:
1. Muscle fiber injuries, mainly in the biceps femoris long head and less often in the distal semitendinosus muscle, resulting from explosive high-speed actions, such as:
  • kicking
  • sprinting
2. Tendon injuries, particularly the proximal semimembranosus tendon, occurring when performing slow-controlled stretching to an extreme position.
Potentially, an interrelationship exists between eccentric force and muscle strain that dictates whether a muscle is injured.
For example, the risk of strain injury would be significantly reduced when performing submaximal stretching exercises involving moderate-to-high levels of strain and low eccentric force.
Similarly, the risk would be also reduced when controlling a heavy load during the eccentric phase while performing typical resistance exercises, such as:
  • bench press
  • squat
Therefore, when athletes are regularly exposed to activities that simultaneously produce both high-strain and eccentric forces without appropriate preparation, the risk of injury is increased.

Potential Risks Factors Associated With Hamstring Strain In Athletes

Hamstring injury is a complex and multifactorial phenomenon eventually sustained by the interaction of the following two types of factors.
1. Alterable
fatigueoptimal peak torque angle
strength deficitslow flexibility
muscular imbalancepoor motor control or technique
2. Non-alterable
agemuscular fiber composition
ethnicityprevious injuries
anthropometric relationships--
In order to further understand the sequence of events that eventually lead to an injury, the causative effects of both alterable and non-alterable factors should be considered within an integrated approach.
For example, the combination of age, with the lack of strength, would dramatically increase the risk of injury in some athletes.
Therefore, under some circumstances (e.g., being an older or previously injured athlete), the effects of some modifiable risk factors (e.g., the lack of strength or poor motor control) will increase the risk of injury with respect to a younger or previously uninjured athlete.
A successful hamstring injury prevention program would attempt to reduce the negative influence of the alterable risk factors.
However, as stated by David A. Opar, Morgan D. Williams, and Anthony J. Shield in their study published in 2012 by the journal Sports Medicine, a risk factor cannot be considered causative unless there is a reduction in the rate of hamstring injury following an intervention focused on ameliorating them.
The following sections will discuss the effects of the most proposed exercises and protocols for reducing hamstring injuries in athletes.

Stretching Exercises

Whilst it has been reported that there is no relationship between static stretching and the risk of hamstring strain[13], recent reports have suggested positive effects of stretching protocols involving several repetitions of more than 30 s (4 min in total) for reducing muscle strains[6]. In subjects with limited flexibility, it appears that the protective effects of stretching include a shift in the optimal muscle length at which the peak torque occurs towards a more open position i.e., a more extended joint angle[6]. However, caution should be taken when performing stretching exercises as a part of warm up, during and after training because a high volume of excessive hamstring stretching can be the cause of hamstring tendon strains[14]. The risk of this type of injury appears to be even greater after exercise, when a fatigued muscle-tendon complex may be at more risk of damage by extensive stretching.

Active Lengthening Exercises

Active lengthening occurs when muscles are stretched while trying to contract.
Studies suggest that the optimum muscle length at which peak tension is recorded would be the best indicator for identifying athletes at risk of hamstring strain.
Therefore, the more open the angle at which the optimal peak torque is achieved, the less the muscle will be at risk of injury.
The functional and structural adaptations elicited by eccentric exercises depend on the magnitude of the resistance, movement velocity and amplitude, i.e., the magnitude of muscle length achieved throughout the exercise.
In both eccentric and concentric actions, as the magnitude of the resistance increases the capacity to modulate, the velocity decreases.
Therefore, when using light to moderate loads (<80% 1 RM), the muscle fibers will progressively be extended during eccentric muscular action.
This is the case of some active lengthening movements, such as kicking or the terminal swing of running, where biceps femoris long-head fibers would be extended close to 110% of their original length.
However, when using a heavy resistance (>80% 1 RM) throughout an eccentric action, muscle fibers will contract along a quasi-isometric action that is compensated by a greater elongation of the elastic components (tendons and elastic sarcomeric proteins).
Thus, as the movement progresses, and the muscle-tendon unit continues to lengthen beyond its optimum length, the tension levels will decrease.
Therefore, if we consider that the maximal peak torque occurs always at the same fiber length:
  • the lighter the resistance, the more open the position the optimal peak torque will be produced; and
  • conversely, the heavier the resistance, the closer position the optimal peak torque will be localized
With respect to movement amplitude, it has been demonstrated that the protective effects of eccentric exercises will be only produced over the trained range of motion, but not at longer amplitudes.
Thus, if eccentric exercises are performed over short ranges - smaller than those where the injury commonly occurs (e.g., the last 30° of terminal swing phase) - the optimal peak torque would be moved towards a closer knee angle, and therefore, instead of being protective, might even be increasing the risk of hamstring strain.
In order to obtain the optimum benefits, eccentric exercises should be conducted throughout the largest possible range of motion.
Different eccentric exercises using a flywheel eccentric leg curl, Nordic Curl alone combined with other strengthening or stretching exercises, have been successfully applied for:
  • reducing the incidence of hamstring injuries
  • enhancing hamstring eccentric strength
  • shifting the optimal peak torque in non-previous injured athletes
However, the loading patterns and the appropriate training protocols continue to be determined.
Nordic Curls have recently been criticized because of their lack of specificity, with respect to the mechanical action where the injuries occur.
This exercise requires athletes from kneeling position to gradually lower their trunk, representing about 90% of body weight, towards the ground.
For many not well-conditioned athletes, the overload placed on the hamstrings will be excessive, especially when the trunk approaches the end of the movement.
As a consequence, when the athlete is unable to control the movement, they will stop applying force and fall forward.
Performing uncontrolled Nordic Curls would not elicit positive adaptations for reducing hamstring strain, because the effective trained range of motion:
  • will be restricted to the closer angles (<30° of knee extension)
  • will not be transferred to the longer muscle length
In fact, it has been observed that the biceps femoris long head and semimembranosus were significantly less active than the semitendinosus and gracilis during a heavily loaded eccentric leg curl.
Therefore, in order to maintain an appropriate activation on the biceps femoris long head and semimembranosus, other alternative could be recommended exercises, such as:
  • the eccentric single or two stiff-legged deadlifts
  • assisted band Nordic Curl
Three scenes of a person with face covered in black, doing single and double stiff-leg deadlifts and Nordic Curls
Three scenes of a person with face covered in black, doing single and double stiff-leg deadlifts and Nordic Curls

Unstable Core And Sport-Specific Exercises

An effective injury prevention program should consider the specific condition that more accurately reflects the situation where injuries occur.
In addition, to the active lengthening observed during sprints or kicking, the hamstring acts as knee stabilizers counteracting and balancing the actions of the quadriceps during sport-specific activities, such as:
  • landing
  • deceleration
  • change of direction
Therefore, specific drills have been proposed as effective methods for preventing hamstring injuries, such as:
  • repeated sprints
  • jumps
  • hops combined with unstable core exercises (e.g., one-leg squat or lunges on unstable surfaces)
In their study published in 2005 by the British Journal of Sports Medicine, Geoffrey M. Verrall, John P. Slavotinek, and Peter G. Barnes reported a significant reduction of hamstring injuries in Australian football players after a four-year high-intensity interval-training program combined with passive isometric hamstring stretching.
In order to simulate the situation where hamstring strain occurs, athletes were instructed to forward incline the trunk when sprinting.
Conversely, in a study published in 2012 by the journal PLoS One, its authors, with Abdolhamid Daneshjoo as lead author, observed no effect of two typically recommended specific injury prevention programs (FIFA 11 + and Harmo Knee) to enhance hamstring strength in professional football players.
More recently, in a study published in 2013 by the journal Research in Sports Medicine, its authors, with Fernando Naclerio as lead author, reported that four weeks (12 sessions) of an injury-prevention program comprising three sets of eight repetitions of the following were effective in improving the maximal isometric force at both closed (80°) and open (35°) knee angles:
  • Nordic Curls
  • forward lunges on a BOSU (a registered product name for a balance trainer)
  • eccentric single-leg deadlifts
Before training, all participants tended to produce the maximum torque at 45 degrees.
However, after the training intervention, a plateau from 35 degrees to 80 degrees of knee flexion was observed only for the training group that exhibited a more consistent torque, which was maintained over a greater range of knee flexion angles.
The picture below depicts the torque-angle profiles determined before (for all participants n = 20) and after the training program differentiating the training (n = 10) and control group (n = 10).
A line graph with a full line, a broken line and a dotted line showing knee flexion angle in degrees and torque
A line graph with a full line, a broken line and a dotted line showing knee flexion angle in degrees and torque
The modification depicted in the picture above also indicates a trend to increase the capacity of the hamstring to apply higher levels of force over a larger range of motion.
Possibly, the effects on the knee-angle torque would be manipulated by an appropriate exercise selection.
For example, the benefits of performing Nordic Curls would depend on the range of motion achieved during its execution.
When performing lunges, a more quasi-isometric action of the hamstring will be required and therefore, a predominant strengthening at closer knee angles would be elicited.
The single-leg deadlifts involves both an active stretch and a knee stabilizer co-contraction action of the hamstrings that in turn will be stimulating a strength enhancement of the hamstring at both the closer angle and the open-knee angle positions.

Designing A Hamstring Injury Prevention Program

Four important key points:
1. The positive effects of a low-volume injury prevention program could be obtained over a four-week period, with only two sessions per week involving three sets of six to eight repetitions of three open and closed kinetic chain exercises.
2. The selected exercises should involve eccentric and quasi-isometric hamstring muscle actions.
3. Eccentric muscle actions should be performed at moderate-to-high velocity, using light-to-moderate loads and along the largest possible range of motion.
4. The injury prevention program should be included at the end of warm up or training session.

Conclusion

In order to prevent hamstring injuries, programs must be designed so that they include both eccentric activity and co-contracting knee stabilizer exercises.
No single approach should be considered as the gold standard for hamstring injury prevention.
In addition to the functional improvement recommended in this review, coaches should also consider the important role of correcting good sports specific technique and motor control.
Future research should analyze the specific structural and functional modifications elicited by the regular application of different types of exercise protocols for hamstring injury prevention.
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