Hamstrings and Motor Control | Article by Frans Bosch

For an extended period, the sporting domain has demonstrated considerable intrigue in the domain of hamstrings. The resurgence of interest is particularly pronounced during instances of injury surges in football, where hamstrings occupy a prominent position on the current injury hierarchy. Notably, even in a sport like rugby, hamstrings secure the third position in injury prevalence, despite formidable competition from injuries in alternate anatomical regions, such as the shoulders and neck. What renders the hamstrings distinctive, and by what means might we mitigate the likelihood of a hamstring injury?

In comparison to ball sports, athletics has maintained a more consistent interest in the significance of hamstrings, given their crucial role in determining maximum running speed. Even a minor discomfort, not yet classified as an injury but manifesting as stiffness, can lead to a noticeable decline in performance.

Specificity of the Hamstrings

Based on the notion that an exercise is only meaningful if there are certain similarities with the intended movement (specificity), understanding the functioning of the hamstrings in functional movements is a prerequisite for designing effective training. Literature on hamstring injuries often provides surprisingly careless descriptions of this functioning. Even reputable studies^{1, 7} frequently offer vague and undifferentiated explanations. The functioning is commonly described in broad terms as eccentric-concentric, yet there are no measurements to nuancedly explain how an eccentric-concentric movement occurs within the muscle. The consequence of this imprecise approach is the formulation of various hypotheses regarding mechanisms in the hamstrings that may not actually occur. As a result, the research hardly yields conclusions applicable to the prevention and rehabilitation of injuries.

The issue is typically considered from a too limited perspective, predominantly relying on (simple) biomechanical models as the starting point. Neurophysiological influences, particularly the way motor skills are controlled, are often overlooked. This article elaborates on the impact of motor control. Ultimately, this leads to a hypothesis that has far-reaching implications for how hamstring training should be approached.

Range of Motion of the Hamstrings

Due to several anatomical features (see, for example, 3, 4) of the hamstrings, it can be concluded that the muscle group fundamentally possesses many degrees of freedom. In other words, it can exhibit highly differentiated movement capabilities (see Figure 1):

Figure 1. Hamstrings: spanning over two joints (1), with complex architecture (2), a mono-articular head (3), collaborative movement in the sagittal plane (4), and, in differentiated functioning, contributing to both internal rotation and external rotation (5).

1. The hamstrings (excluding the short head of the biceps femoris) span two joints: the hip and the knee. This implies that the muscle length does not have a one-to-one relationship with the position of either joint. During strong hip anteflexion and a straightened knee, the hamstring can be extremely elongated. However, when the knee is bent, as in a squat position, the hamstrings will be closer to their optimal length. There is, therefore, a dynamic relationship between the movement of the hip and knee and the length changes of the hamstrings.

2. The muscle group provides extension of the hip joint and flexion of the knee (or inhibits, as in functional movement, knee extension). The medial group contributes to internal rotation, while the biceps femoris contributes to external rotation of the thigh. This implies that different parts of the muscle must exert tension at varying lengths when navigating curves during running.

3. The muscle has a pennate structure, involving intricate architecture of active and passive components. This complex structure allows different parts of the muscle to perform scissoring movements relative to each other.

4. The short head of the biceps femoris is innervated by a different nerve than the long head. Differentiated innervation enables independent contractions of these parts, allowing them to execute markedly different muscle contractions simultaneously. Contraction of the entire muscle group, therefore, requires a coordinated interplay between both sources of stimulation.

Within a single movement pattern, there are numerous contraction possibilities for the hamstrings. In rehabilitation following hamstring injuries, it is often assumed that it is wise to go through and "retrain" all these possibilities. The muscle group must be loaded at various lengths, combining the load with internal and external rotation movements of the hip, and so on. However, it raises the question of whether this is indeed a sound conclusion. Based on knowledge of motor control, one could also speculate that it might be more beneficial to train the hamstrings in a specialized manner. More on this will be discussed later.

Do Hamstrings work Eccentric-Concentric, or Different?

As mentioned, there is a notable lack of nuance in the literature, both on injuries and muscle function, regarding what happens when a muscle lengthens and shortens. There are two ways of lengthening and shortening (refer to Hill's model⁵ and Figure 2):

1. The lengthening and shortening primarily occur in the muscle fibers. When the muscle fibers elongate against an opposing external force, there is limited stretching in the (series) elastic elements of the muscle.

2. The muscle fibers "brace themselves" and do not undergo significant changes in length. The elongation and shortening of the muscle-tendon complex primarily occur in the (series) elastic elements of the muscle: tendons, elastic elements in the muscle belly, etc.

Figure 2. On the left, an eccentric-concentric action (from top to bottom), where the muscle fibers lengthen and shorten. On the right, the same muscle action in which the elastic elements change in length while the muscle fibres remain constant.

During high-speed running, the largest external opposing forces on the hamstrings occur when the lower leg forcefully swings forward. This action results in an elongation of the entire muscle. Does this elongation occur through the lengthening of the muscle fibers or the elongation of the elastic elements? And perhaps an even more crucial question: is there a substantial difference between these two eccentric forms? If the difference is marginal, then both forms of contraction are specific to running, and exercise selection may not need to consider it extensively. However, if the difference is crucial, it becomes evident that precise adherence to the correct functioning of the hamstrings is essential in training.

Motor Control and Specificity of the Hamstrings

To establish positive transfer between an exercise and the target movement, it is necessary to identify the similarities between the two. The concept of specificity is typically described by enumerating the following sub-features:

• Similarity in the internal structure of the movement (intra- and intermuscular coordination).

• Similarity in the external structure of the movement (comparable movement pattern and range of motion); potentially, similarity in the use of energy systems.

This mainly concerns the biomechanical and physiological aspects of the movement. This perspective is obvious for those observing and trying to understand the movement from the outside. However, the adaptive system itself, the learning and training body, does not establish transfer between movement forms by comparing processes. The learning system is not particularly interested in processes ("the brain knows nothing about muscles"). It is primarily interested in the consequence of the movement. Therefore, the system establishes transfer between two movements mainly when the results of the movements are comparable. For more information on this topic, refer to Schmidt & Wrisberg⁶, Wulf⁹, and Wulf & Prinz⁸. While it might seem obvious that our body is result-oriented, the implications of this perspective have barely permeated the thinking about training. Therefore, it is meaningful to further expand the sub-features of specificity with aspects of how the learning system organizes transfer itself. Two characteristics can be added to the description of specificity:

• Similarity in the sensory aspects linked to the movement.

• Similarity in the consequence/result of the movement.

Considering the ways in which a muscle can work "eccentrically-concentrically" from the last characteristic of specificity (the result of the movement), there is a significant difference between the two forms of contraction. When the muscle fibers lengthen in the eccentric phase, the energy from the opposing force is absorbed and dissipates, mainly as heat. However, when the elongation in the eccentric phase occurs in the elastic elements, the absorbed energy can be used in the muscle's rebound. For instance, if someone lands from a height of 30 cm, eccentric action in the muscle fibers will bring them to a stop, while eccentric action in the elastic elements will make them bounce back up.

The same principle applies to the functioning of the hamstrings during the swing phase in running. If eccentric action occurs in the fibers, it will only slow down the swing. This is evident, for example, when a soccer player decelerates after kicking a ball. However, if the muscle fibers maintain their length and the elastic elements are stretched, the subsequent release of elastic energy will move the leg backward.

 Figure 3. The swinging motion of the lower leg imposes a resisting force on the hamstrings. Additionally, tilting the pelvis forward can stretch the hamstrings.

In both landing after a jump or fall and the swing of the lower leg during running, the results of the two variants of muscle elongation are very different. Therefore, it can be assumed that two possible forms of elongation (in the muscle fibers or in the elastic elements) are not very specific to each other. Hence, it is meaningful to clearly delineate this distinction and investigate what truly happens in the hamstrings during high-speed running.

It seems plausible that the functioning of the hamstrings during running is elastic. The utilization of elastic loading and unloading in the hamstrings is indeed an ideal way to reuse the kinetic energy stored during the swing phase in the scissor movement. This process becomes beneficial when initiating the reversal of the scissor movement after the swing phase of the lower leg.

Further speculation leads to the thought that footballers and rugby players may have an increased risk of hamstring injuries because they alternately use both types of eccentric contractions: the elongation of muscle fibers after kicking a ball and the isometric contraction of fibers during high-speed running. This alternation might occasionally "confuse" the body, potentially resulting in injuries. However, it's essential to emphasize that this is currently a purely speculative notion.

Hamstring function in Soccer and Athletics

The conversion of kinetic energy present in the scissor movement into (storage of) elastic energy in the hamstrings, to execute the reversal of the scissor movement economically, is conceived from an ideal model of movement. This model might be highly applicable in a 100m sprint in athletics, which can be considered a so-called closed skill: a skill where environmental factors are so fixed that the ideal execution of the movement is predetermined. However, soccer, rugby, and other team sports are open skills. The environment (especially the opponent) varies so much that the execution of the movement is not predetermined, and adjustments must constantly be made according to the demands of the moment. Running in the woods on uneven terrain can also be classified as a more or less open skill. The question now arises whether the hamstrings function the same way in a closed skill as in an open setting. Should the functioning of the hamstrings during a high-speed rush in a rugby match perhaps be adjusted to the field and opponents' conditions, making the operation not purely elastic and involving significant eccentric action in the muscle fibers as well?

Attractors and Fluctuations

The dynamic pattern theory, a significant paradigm in the field of motor control (see, e.g.²), demonstrates that an open skill is not solely composed of movement elements that vary in execution (so-called "fluctuations"). Instead, an open skill is constructed from elements that are variable and elements that are fixed and not varied. These latter elements are referred to as "attractors."

This organization of open skills makes sense because a movement becomes uncontrollable if it consists of only many variable elements. Consider controlling movement like steering a car. If there were a separate steering wheel for each wheel, the car's maneuverability would be maximal (it could rotate around its axis), but practically steering the car would become impossible. The number of degrees of freedom would be too great. Therefore, the degrees of freedom of a car have been reduced to one: the single steering wheel. The same principle applies to the control of movements. A movement in an open setting can only be controlled when the number of degrees of freedom is reduced to a manageable amount. In a well-executed improvised movement (running on uneven terrain in soccer, hitting a forehand in tennis, etc.), there are only a limited number of fluctuations that are adjusted to the environment. Other elements of the movement are fixed and not varied. Skilled baseball hitters, for example, do not adjust the arm movement (as an attractor) based on the ball's trajectory. All necessary adjustments are made solely through the trunk movement (as a fluctuation: more or less lateral tilting). Less skilled baseball hitters modify both the trunk movement and the arm movement in their attempts to hit the ball. Another example: novice table tennis players try to hit the ball in the desired location by varying both the moment of hitting and the speed of the swing. Top players no longer vary the speed of the swing and hit the ball in the right place by only varying their timing (the moment when the swing movement is initiated). Limiting the number of controllable movement elements is thus a crucial characteristic of a well-controlled and environment-adapted movement.

Is the functioning of the hamstrings during high-speed running a fluctuation or an attractor? For two reasons, it is plausible that the hamstrings work as an attractor in well-executed open skills:

1. The functioning of the hamstrings is deeply embedded in the running cycle. It is nearly impossible for a runner to consciously control or alter the timing of hamstring activity during running. Only by consciously keeping the knees lower at the end of the swing phase can the functioning be influenced. Other sub-movements, such as those of the ankles or the trunk, can be much more easily consciously controlled. It is evident that those elements, which are somewhat more detached from the running cycle, are more suitable for the task of fluctuation.

2. The opposing forces acting on the hamstrings during high-speed running are perhaps the largest they encounter in any movement. Because the hamstrings reach the limits of their load capacity when running at maximum speed, there is little room for variation. Other loads, such as opposing forces on the calf muscles, are less constrained by their capabilities and thus allow more room for variation.

Training of Hamstrings

The considerations about specificity and degrees of freedom lead to a model in which the functioning of the hamstrings, even in open skills like running, is a fixed and unchanging attractor. Elsewhere in the body, fluctuations ensure adaptation to environmental demands. The hamstrings function as a purely elastic attractor to optimize the return of movement energy. If the technique is not yet optimized, the hamstrings may function as fluctuations during high-speed running. In this case, they work outside isometric conditions, involving eccentric-concentric actions in the fibers. This is detrimental to running performance and increases the risk of injuries.

This is still just a model because science, unfortunately, is not yet capable of measuring it accurately. Measurement errors are typically too large, and the property of muscle "rise time" and “muscle slack” complicates the accurate interpretation of what happens in a muscle when its attachment points move apart. However, the model is interesting because it leads to several implications for rehabilitation and training. One interesting thought arising from the organization of movements into fluctuations and attractors is that it better explains why injuries often don't occur in isolation but seem to move through the body in chains. For example, if there's an injury to the Achilles tendon, the fluctuating movement of the ankle becomes less effective, potentially compromising the necessary adaptation to the environment. Consequently, the hamstring may need to abandon its fixed role as an attractor and function more as a fluctuator, making it more vulnerable. This notion has significant implications for hamstring injury rehabilitation, suggesting the need for a comprehensive recovery program for the coordination of the overall movement pattern.

The model also has implications for performance training. The hamstrings need specialized training for isometric contractions and for the elastic return of stretch. This means that the core of hamstring training should consist of maximal strength training at optimum length. Range of motion training, such as using a leg curl machine, should be avoided. Maximal strength training should be combined with training the elastic functioning of the hamstrings through load-bearing exercises (also at optimum length).

To ensure that the hamstrings are loaded at optimum length, the exercise must meet two criteria (see Figure 5):

• The hamstring should aim to extend the hip and resist knee extension during the exercise. Exercises that involve bending the knee (such as leg curls) are counterproductive to developing the correct coordination pattern.

• Pelvic tilting forward and backward should be possible during the exercise. Tilting the pelvis allows the optimum length of the hamstrings to be sought and found.

 Figure 5. Criteria for hamstring training; sufficient overload (one leg fixed). (1): hamstring wants to move the hip (2), at optimum length (3), allowing the pelvis to move to find the optimum length (4)

In addition to these intramuscular-focused patterns, the hamstrings should also be trained in complex intermuscular patterns. In this context, the control of pelvic tilting (read: core stability) and the collaboration between hamstrings and calf muscles during knee extension are crucial. This can be achieved through complex strength training forms as well as in running drills. Running on varied surfaces, for instance, can contribute to further differentiation between attractor and fluctuation functions.

Several conclusions

• Hamstrings are complex structures and play a crucial role in intricate coordinative patterns.

• During high-speed running, hamstrings likely operate at their optimum length, combining elasticity with isometric conditions in muscle fibers.

• It is wise to train hamstrings in a manner consistent with the demands of high-speed running, involving maximal strength training and load-bearing exercises.

• Additionally, the quality of fluctuating elements in the running cycle should be continually monitored.

• Before coaches and rehabilitation trainers can effectively work on preventing hamstring injuries, a precise description of how hamstrings function in the running movement is essential.

• For the advancement of training principles, it is necessary to incorporate scientific knowledge about motor control into conceptual models.

Literature
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