Muscle Activation Differences During Eccentric Hamstring Exercises
by Benjamin Bunting BA(Hons) PGCert
Written by Ben Bunting: BA, PGCert. (Sport & Exercise Nutrition) // British Army Physical Training Instructor // S&C Coach.
The hamstrings are muscles that bend your knee and help you move your leg backward. They also work to add stability to your lower body movements, such as running.
Eccentric hamstring exercises increase strength at longer muscle lengths (the descending limb of the length-tension curve). This is thought to protect the hamstring from high forces during high-speed running.
Biceps femoris is a muscle of the posterior thigh that forms part of the group known as the hamstrings. It is a long muscle that runs from the ischial tuberosity to the proximal part of the fibula and crosses both the knee and hip joints. It is composed of two heads; the long and the short head.
The long head originates from the medial facet (inferomedial impression) of the ischial tuberosity, medial to the origin of semimembranosus and superior to the origin of adductor magnus. Its tendon runs for a distance and then separates into two distinct muscles, biceps femoris and semitendinosus.
It is innervated by the nerves tibial nerve and dorsal rami of sciatic nerve. It is also connected to the sacrotuberous ligament of the thigh.
Activation of the biceps femoris during eccentric exercises is a critical issue for clinicians and coaches. They need to know which exercise will activate the biceps femoris more and what exercise is less effective for this muscle.
A systematic review was conducted to evaluate the biceps femoris long head activation in different types of hamstring strength exercises. The review included 29 studies that compared the biceps femoris activity with semitendinosus during 114 different hamstring exercises.
The authors grouped the exercises into several categories: Nordics, isokinetic, lunges, squats, deadlifts, good mornings, hip thrusts, bridges, leg curls, swings, hip and back extensions, and others. They analyzed each of the exercises using a combination of EMG and MVIC.
Results showed that Nordic and isokinetic hamstring exercises achieved the highest biceps femoris activation (>60% of Maximal Voluntary Isometric Contraction) than the other exercises. In addition, the Nordic hamstring exercise ankle dorsiflexion exhibited the highest biceps femoris short head activation (128.1% of Maximal Voluntary Isometric contraction).
The biceps femoris muscle is one of the most important muscles in the human body. Its primary function is to extend the thigh at hip and flex the knee when the knee is flexed.
In one study, the research team investigated the muscle activation differences between the biceps femoris (BF), semitendinosus (ST), and semimembranosus (SM) muscles during eccentric hamstring exercises. Muscle functional magnetic resonance imaging (mfMRI) was used to assess hamstring force production and muscle activity. The study consisted of six exercises and two maximal sprints performed by adult male and female athletes.
The mfMRI data revealed that the BF and ST muscle bellies were highly interdependent in terms of the magnitude of tissue loading and the adequacy of muscle functioning. They had synergistic activation and fibre recruitment patterns which led to a complex inter-relationship in both timing as well as spatial distribution of fibre recruitment. The BF and ST are the most commonly injured muscles in running-related hamstring strains, 18-20, 41,42 suggesting that muscle dysfunction could lead to injury. The mfMRI data also revealed that the SM muscle belly had the lowest tensile shear stress near its proximal myotendinous aponeurosis, while the BF had the highest.
Both the biceps femoris and semitendinosus are hamstring prime movers which have a variety of functions, including hip extension, thigh internal rotation, knee flexion and flexing, as well as stabilising the pelvic girdle. Both these muscles share the same origin, which is the posteromedial impression on the superior part of the ischial tuberosity.
Our mfMRI data revealed that the biceps femoris was more activated than the semitendinosus during the NHE exercise. This was due to a greater loading of the short head of the biceps femoris close to its origin as compared to the long head (Mendiguchia et al., 2013).
While the biceps femoris is more active than the semitendinosus during NHE, this difference was not statistically significant. Moreover, the mfMRI data showed that both the BF and the semitendinosus were able to generate a T2 increase during NHE at a similar level.
This was because both the biceps femoris as well as the semitendinosus were undergoing high levels of tensile shear stress during NHE. This was mainly because of the tensile shear stress produced by the slide leg bridge exercise as well as the Nordic hamstring exercise.
The semimembranosus muscle is located on the back of the thigh and is the largest muscle in the hamstring group. It is a fusiform muscle with long fibre lengths and numerous sarcomeres in series. It is primarily involved in knee extension and flexion, but also prevents valgus and external rotation during hip movement. The muscle receives its arterial blood supply from the deep femoral, inferior gluteal and popliteal arteries.
It also has a branch of the internal iliac artery. It originates from the superolateral aspect of the ischial tuberosity, forming a thick tendon that extends over the upper part of the posterior surface of the leg. It then converges to an aponeurosis that covers the lower portion of the posterior surface of the leg, and muscular fibers arise from this aponeurosis and contract into the tendon of insertion.
Although the semimembranosus muscles have similar functions to the other hamstrings, they have different physiological characteristics and musculoskeletal adaptations. They are particularly important for power during sprinting, as they bend the knee at the knee and extend the hip joint.
There is evidence that sEMG can be used to differentiate the activation of these muscles during different exercises, although this method does not have the spatial resolution of fMRI (Bourne et al., 2016). This is a critical factor for identifying and targeting muscle groups during exercise that will increase power, speed and acceleration.
However, sEMG may also be biased because it is not sensitive to the presence of glycolytic metabolites. Therefore, fMRI is likely to be the better method to determine the effects of training on the hamstrings and other muscles during sprinting.
To investigate the hamstring muscle activation differences during eccentric hamstring exercises, participants performed a range of eccentric hamstring exercise trials. The standing kick, slide leg bridge and Nordic hamstring exercise were used to measure the EMG activity of medial and lateral hamstrings. The EMGs were measured before (T2 Pre), after (T2 Post) and immediately following the execution of a 45deg hip extension or Nordic hamstring exercise.
The T2 changes induced by each of the exercise sessions were compared and the results indicated that the 45deg hip extension and Nordic hamstring exercises significantly stimulated the BFLongHead, BFShortHead and semimembranosus muscles differently. The T2 increase for the semimembranosus muscle was significantly larger than that observed for the other hamstring muscles.
In healthy individuals, there are muscle activation differences during eccentric hamstring exercises between the biceps femoris (BF), semitendinosus (ST), and semimembranosus (SM) muscles. These differences may be important in establishing a suitable pre- and post-habilitation program for lower limb injury prevention.
In this study, a group of healthy participants were asked to perform one of the following eccentric hamstring exercises randomly (3 repetitions each): stiff-leg deadlift (SLDL), unilateral stiff-leg deadlift (USLDL), Nordic hamstring exercise (NHE), and ball leg curl (BLC). Surface electromyographic activity was recorded at different knee joint angles during OKC and CKC exercises, and the EMG activities were compared for all 3 muscles.
The ST muscle was found to have the highest nEMG activity at deep knee flexion angles during leg curl exercises, whereas the SM and BFl muscles were more active at initial knee flexion angles. This suggests that the ST is working at deeper knee flexion angles to complement the reduction in the SM and BFl muscles during OKC exercises.
These findings are in agreement with previous research findings that hamstring muscle activity has been shown to be different during hip extension compared to knee flexion. This difference may result in a lateral/medial imbalance, which has been linked to a variety of lower limb injuries such as ACL tears and Achilles tendinitis4.
This could be because the hamstrings have different functional and architectural properties, which influence their load sharing behaviour during knee flexion tasks. The ST is known to exhibit greater neural, metabolic, and stiffness responses during knee flexion activities as compared to the BFl and SM muscles5,6,16,21. The ST is also known to have a greater functional and structural role during the propulsion phase of locomotion such as running and sprinting16,21,23.
However, there is limited evidence about which muscle performs a superior function during the propulsion phase of locomotion. This could be due to differences in hamstring morphology, fascicle architecture, and fiber type content that influence their ability to generate propulsion forces.
The present study examined the effect of an isometric biceps femoris long head (BFlh) and semitendinosus (ST) knee flexion task on the BFlh and ST active stiffness in a non-fatigued condition, using ultrasound-based shear wave elastography (MVIC). Twelve participants performed 2 sessions separated by 7 days. The time to exhaustion was similar in both sessions (day 1: 443.8 +- 192.5 s; day 2: 474.6 +- 131.7 s; p = 0.323).
The hamstring muscles are multi-directional biarticular muscle groups involved in the hip extension and knee flexion movements, as well as providing multi-directional stability of the tibia and pelvis. They are subjected to large mechanical forces during upper limb, trunk and lower limb locomotion, which is likely the cause of their high injury frequency.
Several studies report positive adaptations to eccentric training regarding both muscle strength and architecture. These changes can lead to reductions in HSI risk and improved sports performance (Askling et al., 2003; Brooks et al., 2006; Arnason et al., 2008).
However, the effect of eccentric training on the hamstring muscle’s strength and architecture is controversial, with some studies reporting decreased hamstring strength during the NHE (Brockett et al., 2001). A literature review was performed to identify studies that assessed the impact of eccentric hamstring training on hamstring muscle strength and/or injury risk.
Eccentric hamstring exercises are commonly prescribed to reduce HSI risk and improve sport performance in professional athletes (Brooks et al., 2006; Petersen et al., 2011). However, compliance rates are poor, possibly due to the high volume of training prescribed during the early stages of the intervention (Brooks et al., 2005).
The Nordic hamstring exercise is one of the most commonly used eccentric exercises. It has been shown to reduce first time and recurrent HSIs in elite soccer players (Petersen et al., 2011). Despite this, the NHE has been poorly adopted in professional soccer teams, potentially due to the high volume of training prescribed in the early stages of the intervention. Consequently, we sought to evaluate the effect of reduced NHE volumes on eccentric strength and muscle architecture, with an aim to increase intervention compliance and potentially reduce HSI risk in professional soccer players.