András Hegyi is a final-stage Ph.D. student working at the Neuromuscular Research Center at the University of Jyväskylä in Finland. His Ph.D. work focuses on regional and intermuscular hamstrings EMG activity in different hamstring exercises and in running. One of the four studies included in this project was awarded a Young Investigators Award by the European College of Sport Science in 2017. Hegyi is interested in improving biomechanical methodologies to assess hamstrings to further understand hamstring muscle function and injury mechanisms.
Freelap USA: EMG is very important to sport science, but it requires careful interpretation. Can you explain why your technique differs from conventional surface EMG?
András Hegyi: As I tend to say, friends do not let friends believe in conventional EMG. When using conventional surface EMG on hamstring muscles, usually one pair of electrodes is placed over the mid-region of each muscle to estimate the activity of each hamstring muscle. However, this approach ignores regional (i.e., proximal-distal) differences in muscle activation.
To overcome this limitation, we initiated the use of linear arrays on hamstring muscles (one type of high-density surface EMG), which cover 16 centimeters proximal-distally over each muscle to capture proximal-distal differences in muscle activation. Our results show large differences between the EMG activity of distinct muscle regions in several exercises and in running. For example, in the Nordic hamstring exercise, biceps femoris long head (BFlh) EMG activity is the largest in the distal region, while semitendinosus EMG activity is the largest in the mid-region.
As I mentioned, conventional EMG measures only from the mid-region of each muscle, which means that comparisons of the activity of these muscles are biased toward finding selective slow twitch (ST) activation when using conventional EMG. It is important to mention that each hamstring exercise shows distinct regional EMG activity patterns, and in running, the regional activation of these muscles is highly individual. For example, some athletes activate proximal BFlh selectively, while others activate distal BFlh selectively. Therefore, it is inevitable to use spatially robust methods to reduce region bias when estimating the activation of hamstring muscles.Another important advantage of high-density EMG over conventional EMG is that it effectively minimizes cross-talk, and is therefore more accurate, says @And_Hegyi. Click To Tweet
Another important advantage is that high-density EMG effectively minimizes cross-talk. It is, therefore, more accurate. I should also emphasize that using high-density EMG is not the only way to get spatially robust recordings. Placing conventional EMG electrodes over different muscle regions can also be used to define regional hamstrings activity, similar to the study of Schoenfeld et al. published in 2015.
Due to the anatomy of hamstring muscles, it is important to minimize cross-talk. You can do this by placing electrodes as far from muscle borders as possible, decreasing inter-electrode distance, recording double-differential signals, and using electrodes with a small pick-up area. These methodological issues are really important to accurately define hamstrings activity.
Freelap USA: Coaches can get confused about the role of the adductor magnus muscle in sporting activities. While not a hamstring, it sometimes has duties that appear to have similar contributions. Can you break down any nuances of that muscle group that coaches should know about?
András Hegyi: The adductor magnus is the second biggest muscle in the leg, after vastus lateralis, with a slightly larger physiological cross-sectional area (~20.5 cm2, indicating high force production capacity) compared to that of the semimembranosus (~18.4 cm2), which is the strongest of all hamstring muscles (Ward et al., 2009). The adductor magnus is challenging to examine due to its deep location; therefore, we have very little experimental evidence on the link between the function of this muscle and hamstring injuries.
Despite what the name “adductor magnus” suggests, a recent study by Benn et al. (2018) using intramuscular EMG found that this muscle is a hip extensor rather than a hip adductor. In this sense, it shares a common function with the biarticular heads of the hamstrings. Additionally, this study has also shown that the adductor magnus’ proximal portion contributes to external rotation, sharing a common function with the BF long head, and its distal region contributes to internal rotation, sharing a common function with the semitendinosus and semimembranosus muscles.Despite what the name “adductor magnus” suggests, a recent study using intramuscular EMG found that this muscle is a hip extensor rather than a hip adductor, says @And_Hegyi. Click To Tweet
To put this in a practical perspective, we may speculate that superior contribution of the adductor magnus to hip extension in sprinting reduces the load from the hamstrings, which may be protective against hamstring strain injuries. A similar scenario has been shown for the gluteus maximus by Schuermans et al. in 2017 and Edouard et al. in 2019. Intermuscular coordination likely has a large effect on hamstring injuries, and the adductor magnus is definitely an important muscle to consider when we put hamstrings function into the whole picture.
Freelap USA: The hamstring attachments may help with other needs that are not flexion of the knee and extension of the hip. Please illustrate the hamstring function beyond gross motions. Some interest in rotational forces is gaining traction because of the attachment points.
András Hegyi: Hip extension and knee flexion are common functions of the biarticular hamstrings; however, emerging evidence suggests that the relative contribution of these muscles change when hip, knee, and ankle joint angles are manipulated. For example, if the Nordic hamstring exercise (NHE) is performed with extended hips, the semitendinosus and BFlh show very similar EMG activity (provided that the athlete performs the exercise through the full range of motion). On the contrary, if you perform Nordics with hips flexed to 90 degrees (the exercise we call NHE90), the semitendinosus shows higher EMG activity than the BFlh. This means that the relative activity of these muscles can be manipulated by changing hip position.
Nonetheless, if the training goal is to emphasize fascicle lengthening of the BFlh, the athlete should perform Nordics with flexed hips rather than with extended hips due to higher passive forces in NHE90. This implies that coaches who apply Nordics to monitor athletes should control (or at least monitor) hip angle to ensure that the same physiological mechanisms are tested across the season within the same athlete, and between athletes.
Aside from hip angle, knee range of motion seems to be important, too. When the Nordics are performed with extended hips, knee range of motion affects the relative activity of BFlh and ST; in the early phase of the exercise, ST is more active than the BFlh, while at near-extended knee, the BFlh is more active than the ST. This implies that if you want to activate BFlh selectively, then it seems important that the athlete is able to control the movement close to the terminal phase of the Nordics while keeping the hips extended.
It should be mentioned here that, due to the contribution of the gastrocnemii muscles, ankle plantar flexion (and not dorsiflexion) should also be applied to increase BFlh activity in the terminal phase of the Nordic hamstring exercise. These collectively suggest that hip, knee, and ankle angles determine which of the hamstring muscles are activated the most and that these should be controlled/monitored when testing athletes.
Rotational functions of hamstring muscles are also important to mention. BFlh is an external rotator of the femur and the tibia, while semitendinosus and semimembranosus are internal rotators. Some experimental data shows that performing exercises with internal or external rotations can increase the relative activity of the medial or lateral hamstrings, respectively (e.g., Lynn and Costigan, 2009). This provides coaches with additional ways to selectively activate medial and lateral hamstrings.Besides muscle activity, you should also consider muscle mechanics at the level of muscle fibers (i.e., eccentric vs. concentric) when selecting exercises, @And_Hegyi. Click To Tweet
Collectively, because hamstrings function is very sensitive to joint angles in all planes of the movements, the performance technique of the exercise will determine which of the hamstring muscle is targeted the most. However, we should always remember that, besides muscle activity, muscle mechanics at the level of muscle fibers (i.e., eccentric vs. concentric) should also be considered when selecting exercises. Unfortunately, this remains a challenge until some improvements are made with current real-time imaging methods to follow the complex movement of BFlh fascicles.
Freelap USA: Hamstring injuries are common in sprinting and soccer, but not all athletes like to do Nordic exercises. Can a holistic program with multiple approaches satisfy the needs of preparing the athlete for high-speed running? Coaches of some athletes who find severe irritation in the tendons behind the knee may benefit from an alternative approach.
András Hegyi: NHE seems effective to reduce hamstring injury risk in many athletes, but it does not seem to be effective for other athletes. The main reason is unclear; however, I assume that it has a lot to do with individual variations in the relative importance of risk factors such as hamstrings anatomy and function, intermuscular coordination, running technique, etc. Because the mechanisms of hamstring injuries are not fully understood, it is difficult to understand which exercises are the most appropriate to decrease hamstring injury risk.
The idea behind the good efficacy of NHE is that the exercise increases fascicle lengths; therefore, the muscle can elongate more before rupture in the late swing phase of sprinting—the amount of fiber strain is a good predictor of strain injury. However, the amount of strain in BFlh in sprinting is affected by the intermuscular coordination of several muscles in the lumbopelvic region (Chumanov et al., 2007). This suggests that an alternative (or complementary) approach to NHE may be to focus on the neuromuscular control of those muscles, such as improving running technique by minimizing anterior pelvic tilt.The fact that most research is on the Nordic hamstring exercise doesn’t mean it’s the only exercise that could work to reduce hamstring reinjuries, says @And_Hegyi. Click To Tweet
To reduce hamstring reinjuries, Askling’s exercises requiring long hamstrings operating lengths also seem effective. The fact that most research is on NHE does not mean that it is the only exercise that could work. With athletes who have severe pain behind the knee, I would try hip-dominant exercises and a progressive increase in load in NHE, for example, with the use of resistance bands. NHE is a high-intensity exercise that places high loads on the hamstrings muscle-tendon units, which may be too aggressive for some athletes who are not used to doing such exercises.
When applying a holistic approach, more of the risk factors are targeted. Therefore, it may be more effective for a group of athletes than using one exercise only. However, it can be very time-consuming and unrealistic to implement. The most effective method would be to identify the risk factors for each individual and personalize the training program. Of course, this is very challenging because the relative importance of risk factors is not fully understood and is likely different between athletes. More research on the actual mechanisms of hamstring injuries is needed to get closer to this.
Freelap USA: Eccentric lengthening is popular now, but what about the need for fast rates of force and relaxation? Can coaches use other approaches outside of sprinting and weights to prepare the hamstrings for sport—maybe drills or other exercises outside of conventional movements? Or are we still stuck with the mainstays?
András Hegyi: Eccentric lengthening is popular because it seems to increase the fascicle length of the BFlh and increase peak eccentric knee flexor strength, which seem to be associated with hamstring injuries. But, as you mention, these are not the only factors that determine hamstrings function in running. Rate of force development (RFD) and rate of relaxation are likely important, too.
This may be because high-speed running requires a cyclic activation-relaxation from the hamstrings at very high rates. In the swing phase, the hamstring muscle-tendon units are rapidly stretched, and in this phase, the muscles must increase force rapidly to stiffen the fascicles. This increases the capacity of the hamstrings to absorb energy while being elongated, which seems to be protective against strain injury (Garrett et al., 1987). Then, in the early stance, the ability to increase hamstrings force rapidly improves the capacity of the hamstrings to accelerate the center of mass in running. NHE likely increases the stiffness of the tendinous structures of the hamstrings, which, in turn, contributes to an increased RFD. This may partly explain why NHE seems to be effective to improve acceleration/horizontal force in sprinting.
Another point here is that correlation between increased fascicle length and decreased injury rate after NHE intervention does not necessarily mean that the risk decreases because of the increased fascicle length—correlation and causation are not always the same. It may be that positive changes in other factors such as increased RFD also contribute to decreasing injury risk. We cannot be entirely sure that increased fascicle length is the only main driver of decreased injury rate after NHE interventions.We cannot be entirely sure that increased fascicle length is the only main driver of decreased injury rate after Nordic hamstring exercise interventions, says @And_Hegyi. Click To Tweet
Askling’s exercises, which require long muscle lengths but low muscle activity, are very unlikely to induce meaningful increases in fascicle length, but they still seem to be effective to prevent hamstring reinjuries. As I mentioned before, as long as we are not fully aware of the relative importance of risk factors for each individual, it is very challenging to prevent hamstring injuries. More studies focusing on other exercises than NHE, and other factors than fascicle length and strength, are needed to broaden evidence-based exercise recommendations for hamstring injury risk reduction.
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