Superimposing hip extension on knee flexion evokes higher activation in biceps femoris than knee flexion alone

Differences in activation amplitude between semitendinosus and biceps femoris during hamstring exercises: A systematic and critical review with meta-analysis

The aim of this systematic review and meta-analysis was to compare the amplitude of electromyographic (EMG) recordings for the semitendinosus (ST) and biceps femoris (BF) muscles during selected exercises. Searches were conducted in PubMed, Web of Science, and Scopus databases up to 31 August 2024. Included studies focused on healthy adults, reported ST and BF muscle activation during hamstring strengthening exercises, and presented outcomes as a percentage of maximal voluntary contraction. The analysis included 619 participants. No overall significant difference in activation was observed between ST and BF across knee- and hip-based exercises. However, EMG amplitude was greater in ST during the Nordic hamstring exercise (Standardized mean difference, SMD: -0.33, p = 0.01), whereas EMG amplitude was greater in BF during prone-lying curls (SMD: 0.94, p = 0.0076). Among hip-based exercises, EMG amplitude was greater in BF during back extensions (SMD: 0.34, p = 0.0092), whereas EMG amplitude was greater in ST during kettlebell swings (SMD: -0.59, p = 0.0007). Nonetheless, the variability in the methods used to record and analyze EMG as well as the properties of the signal itself make it difficult to reach firm conclusions on whether specific exercises can preferentially recruit ether ST or BF.

Different morphology and function of hip extensor muscles between sprint runners and sprint cyclists

During a specific task, the whole-body motion favors those with muscles suited for its execution; in addition, it leads to adaptations in muscle morphology and function of muscles. Thus, the well suited and/or adapted muscles are particularly crucial for enhancing performance in each competitive sport. Here, we compared muscle morphology, strength, and EMG activity during isometric maximal voluntary contraction (MVC) at multiple hip angles in runners and cyclists with similar sagittal motion (but different ranges of motion) during competition. Ten sprint track runners and ten sprint track cyclists performed the MVC test of hip extension, and magnetic resonance images of the buttocks and thighs were acquired. Maximum hip extension torque and surface electromyography (EMG) signals from the gluteus maximus (GM) biceps femoris long head (BFlh) and semitendinosus (ST) muscles were assessed during the MVC test at six hip flexion angles (45°, 60°, 75°, 90°, 105°, and 120°). Muscle volumes were determined using magnetic resonance images. Cyclists had a significantly higher MVC torque normalized by the muscle volume than that observed in runners at a hip flexion angle of 120°. At hip flexion angles of 105° and 120°, the normalized root mean square of the EMG signal in cyclists was significantly higher than in runners. Torque-angle relationship in hip extension differed between runners and cyclists, with cyclists generating greater extension torque by increasing GM muscle EMG activity in the flexed position. This may differ in the neuromuscular adaptation to and/or suitability for specific tasks of hip extensor morphology and function in runners and cyclists.

Changes in hamstrings' active stiffness during fatigue tasks are modulated by contraction duration rather than intensity

Despite the increase in research of hamstring stiffness through the use of ultrasound-based shear wave elastography, the active stiffness of biceps femoris long head (BFlh) and semitendinosus (ST) muscles under fatigue conditions at various contraction intensities has not been sufficiently explored. This study aimed to compare the effects of knee flexor's isometric contraction until exhaustion performed at 20% vs. 40% of maximal voluntary isometric contraction (MVIC), on the active stiffness responses of BFlh and ST. Eighteen recreationally active males performed two experimental sessions. The knee flexors' MVIC was assessed before the fatiguing task, which involved a submaximal isometric contraction until failure at 20% or 40% of MVIC. Active muscle stiffness of the BFlh and ST was assessed using shear wave elastography. BFlh active stiffness remained relatively unaltered at 20% of MVIC, while ST active stiffness decreased from ≅ 91% contraction time (55.79 to 44.52 kPa; p < 0.001). No intramuscular stiffness changes were noted in BFlh (36.02 to 41.36 kPa; p > 0.05) or ST (63.62 to 53.54 kPa; p > 0.05) at 40% of MVIC session. Intermuscular active stiffness at 20% of MVIC differed until 64% contraction time (p < 0.05) whereas, at 40% of MVIC, differences were observed until 33% contraction time (p < 0.05). BFlh/ST ratios were not different between intensities (20%=0.75 ± 0.24 ratio vs. 40%=0.72 ± 0.32 ratio; p > 0.05), but a steeper increase in BFlh/ST ratio was found for 20% (0.004 ± 0.003 ratio/%) compared to 40% (0.001 ± 0.003 ratio/%) of MVIC (p = 0.003). These results suggest that contraction duration could play a major role in inducing changes in hamstrings' mechanical properties during fatigue tasks compared to contraction intensity.

Hamstring Activity Before and After Break-Point Angle Calculated By Smartphone Application During the Nordic Hamstring Exercise

INTRODUCTION

Previous studies have reported a gradual decrease in biceps femoris (BF) electromyography (EMG) activity after the break-point angle (BPA) during the Nordic hamstring exercise (NHE). However, no investigation has been conducted on BF EMG activity before and after BPA as calculated using a smartphone application (Nordic Angle app).

HYPOTHESIS/PURPOSE

The aim of this study was to investigate the BF EMG activity before and after BPA, as calculated using the Nordic Angle app. The hypotheses were that BF EMG activity would peak near the BPA and gradually diminish afterward.

METHODS

After a warm-up, participants performed three repetitions of prone leg curls to discern maximum voluntary isometric contraction (MVIC) of the hamstrings. The peak value of the BF EMG activity during the prone leg curl was used to convert BF EMG activity during NHE to %MVIC. BPA during NHE was calculated using the Nordic Angle app by analyzing a movie recorded with an iPhone camera. Additionally, the knee flexion angle during NHE was determined using two-dimensional motion analysis software based on video data. To compare EMG activity before and after BPA calculated by the Nordic Angle app, the knee flexion angle was divided into seven phases: 10-15° before BPA, 5-10° before BPA, BPA ± 5°, 5-10° after BPA, 10-15° after BPA, 15-20° after BPA, and 20-25° after BPA.

RESULTS

There was no significant difference between the BPA of the Nordic angle and the knee flexion angle at peak BF EMG activity (d = 0.13, p = 0.678). The BF EMG activity at 20-25° after BPA was significantly lower than the BF EMG activity at BPA ± 5° (d = 0.87, p = 0.011).

CONCLUSIONS

To prevent the recurrence of hamstring injuries, it is important to incorporate knee flexion exercises that enhance BF EMG activity at 15-35° of knee flexion (0° indicates a fully extended knee). Thus, it is recommended to keep the BPA of the Nordic Angle within 35° to effectively prevent recurrent hamstring injuries during NHE.

Level of evidence

3b.

Modulating the Nordic Hamstring Exercise From "Zero to Hero": A Stepwise Progression Explored in a High-Performance Athlete

BACKGROUND

The Nordic hamstring exercise (NHE) is commonly implemented to selectively improve eccentric knee-flexor strength. However, the standard version of the exercise (leveled shanks, extended hip joint) is too strenuous for most individuals, whose muscle activity rapidly decreases at extended knee angles. Hitherto, a gradual approach to the exercise has been missing. In this exploratory case study, we investigated elite performance to introduce a stepwise progression to the NHE.

OBJECTIVE

To determine the extent to which exercise modifications (shank inclination, additional load, hip flexion) altered NHE mechanics.

DATA COLLECTION AND ANALYSIS

One male long jumper (age = 33 years, height = 171 cm, mass = 69 kg) with high-level expertise in the NHE performed 20 exercise variations. The corresponding kinematics, kinetics, and electromyographic activity of the biceps femoris long head (BFlh) and semitendinosus (ST) muscles were evaluated.

RESULTS

Exercise variations demonstrated gradually increased peak moments from 69% (zigzag pose) to 154% (inclined bent single-legged version) versus a standard NHE. Shank inclination and additional load elicited small to moderate effects on peak moments, BFlh, and ST (0.24 ≤ d ≤ 0.72), whereas hip flexion largely affected all tested variables (2.80 ≤ d ≤ 6.66), especially muscle activity (BFlh = -63%; ST = -55% of maximal voluntary isometric contraction).

COMMENTARY

These insights will help practitioners and scientists design multifaceted stepwise NHE progressions by creating differentiated stimuli that best match the strength capacities of individuals and address their specific needs.

[Importance and application of injury prevention in Austrian football - a survey among 687 coaches]

BACKGROUND

Soccer is one of the most popular sports in the world but associated with a high risk of injury. For this reason, the study of the etiology of injuries is of great interest and numerous prevention programs have been developed in recent years. Since these prevention programs must be embedded into training routine, trainers are primarily responsible for their implementation. The present study aimed to survey Austrian soccer coaches engaged in professional, amateur or youth teams with respect to their opinions on injuries as well as the application of respective prevention programs.

METHODS

An online questionnaire was developed and sent to all coaches registered in the Austrian Football Association.In addition to personal data, the trainers' attitudes to the subject of injury prevention were also asked. It was also asked which preventive measures the trainers consider important and which they also use in their training and to what extent.

RESULTS

A total of 687 trainers took part in the survey. 2,3%, 37,5% and 43,6% of trainers were engaged in professional, amateur and youth clubs, respectively. The rest gave no information. The majority of respondents (56%) considered injuries a major concern in soccer. Inadequate fitness (75,7%) as well as insufficient preparation (60,7%) and poor regeneration (59,2%) were seen as the greatest risk factors for injuries. Appropriate warm-up (66,8%), regeneration (59,4%) and core stabilization training (58,2%) were considered the most effective preventative measures. More than 50% of the participants were not familiar with any of the most widely applied injury prevention programs, and only 15.4% actually implemented those programs in their training.The substantial interest in injury prevention notwithstanding, the respective standard of knowledge amongst Austrian coaches is poor. In light of the high prevalence of injuries, it is most important to inform trainers about injury prevention programs and possibilities to implement them in training practice.

Activity distribution among the hamstring muscles during high-speed running: A descriptive multichannel surface EMG study

PURPOSE

This study assessed activity distribution among the hamstring muscles during high-speed running. The objective was to compare within and between muscle activity, relative contribution and hip and knee joint angles at peak muscle activity during high-speed running.

METHODS

Through multichannel electromyography, we measured muscle activity in male basketball players during high-speed running on a treadmill at 15 locations: five for biceps femoris long head, four for semitendinosus, and six for semimembranosus. Muscle activity was calculated for each location within each hamstring muscle individually for each percent of a stride cycle.

RESULTS

Twenty-nine non-injured basketball players were included (mean age: 17 ± 1 years; mass, 85 ± 9 kg; height, 193 ± 9 cm). Heterogeneous activity was found for all individual hamstring muscles across multiple events of the stride cycle. In the late-swing phase, muscle activity and relative contribution of the semimembranosus was significantly higher than of the semitendinosus. There was no significant difference in hip and knee joint angles at instant of peak muscle activity, assessed locally within individual hamstring muscles, as well as in general over the whole hamstring muscle.

CONCLUSION

Hamstring muscles were most active in the late-swing phase during high-speed running. In this phase, the semimembranosus was most active and the semitendinosus was least active. Within the biceps femoris long head, the most proximal region was significantly more active in the late-swing phase, compared to other muscle regions. For each muscle and location, peak muscle activity occurred at similar hip and knee joint angles.

Semitendinosus muscle morphology in relation to surface electrode placement in anterior cruciate ligament reconstructed and contralateral legs

The semitendinosus tendon is commonly harvested as graft tissue for anterior cruciate ligament reconstruction (ACLR). Although the semitendinosus tendon can regenerate following harvesting, ACLR results in substantial reductions in semitendinosus muscle size and length, potentially complicating electrode placement for electromyography. The purpose of this study was to assess whether the most commonly used electrode placement [recommended by the "Surface Electromyography for Non-Invasive Assessment of Muscles" (SENIAM) project] is appropriate for measuring semitendinosus electromyograms after ACLR. In nine participants (unilateral ACLR with a semitendinosus graft), B-mode ultrasonography was used to bilaterally determine (i) the semitendinosus muscle-tendon junction position and the state of tendon regeneration (latter for the ACLR leg only) and (ii) the anatomical cross-sectional area (ACSA) of the semitendinosus muscle at the SENIAM-recommended electrode placement site at rest and during isometric maximal voluntary contraction (MVC) at two knee joint angles. Depending on the contraction state and joint angle, the semitendinosus muscle had retracted past the recommended placement site in 33-78% of ACLR legs, but not in any contralateral legs. The ACSA of semitendinosus was smaller both at rest and MVC in the ACLR compared to contralateral leg. The ACSA for both legs decreased at MVC compared to rest and at deep compared to shallow knee flexion angles, likely due to sliding of the muscle under the skin. These results suggest SENIAM guidelines are likely unsuitable for recording surface electromyograms from the semitendinosus muscle after tendon harvesting for ACLR as the muscle of interest may not be within the electrode detection volume.

Hamstrings force-length relationships and their implications for angle-specific joint torques: a narrative review

Temporal biomechanical and physiological responses to physical activity vary between individual hamstrings components as well as between exercises, suggesting that hamstring muscles operate differently, and over different lengths, between tasks. Nevertheless, the force-length properties of these muscles have not been thoroughly investigated. The present review examines the factors influencing the hamstrings’ force-length properties and relates them to in vivo function. A search in four databases was performed for studies that examined relations between muscle length and force, torque, activation, or moment arm of hamstring muscles. Evidence was collated in relation to force-length relationships at a sarcomere/fiber level and then moment arm-length, activation-length, and torque-joint angle relations. Five forward simulation models were also used to predict force-length and torque-length relations of hamstring muscles. The results show that, due to architectural differences alone, semitendinosus (ST) produces less peak force and has a flatter active (contractile) fiber force-length relation than both biceps femoris long head (BFlh) and semimembranosus (SM), however BFlh and SM contribute greater forces through much of the hip and knee joint ranges of motion. The hamstrings’ maximum moment arms are greater at the hip than knee, so the muscles tend to act more as force producers at the hip but generate greater joint rotation and angular velocity at the knee for a given muscle shortening length and speed. However, SM moment arm is longer than SM and BFlh, partially alleviating its reduced force capacity but also reducing its otherwise substantial excursion potential. The current evidence, bound by the limitations of electromyography techniques, suggests that joint angle-dependent activation variations have minimal impact on force-length or torque-angle relations. During daily activities such as walking or sitting down, the hamstrings appear to operate on the ascending limbs of their force-length relations while knee flexion exercises performed with hip angles 45–90° promote more optimal force generation. Exercises requiring hip flexion at 45–120° and knee extension 45–0° (e.g. sprint running) may therefore evoke greater muscle forces and, speculatively, provide a more optimum adaptive stimulus. Finally, increases in resistance to stretch during hip flexion beyond 45° result mainly from SM and BFlh muscles.

Muscle synergies for the control of single-limb stance with and without visual information in young individuals

Purpose

Single-limb stance is a demanding postural task featuring a high number of daily living and sporting activities. Thus, it is widely used for training and rehabilitation, as well as for balance assessment. Muscle activations around single joints have been previously described, however, it is not known which are the muscle synergies used to control posture and how they change between conditions of normal and lack of visual information.

Methods

Twenty-two healthy young participants were asked to perform a 30 s single-limb stance task in open-eyes and closed-eyes condition while standing on a force platform with the dominant limb. Muscle synergies were extracted from the electromyographical recordings of 13 muscles of the lower limb, hip, and back. The optimal number of synergies, together with the average recruitment level and balance control strategies were analyzed and compared between the open- and the closed-eyes condition.

Results

Four major muscle synergies, two ankle-dominant synergies, one knee-dominant synergy, and one hip/back-dominant synergy were found. No differences between open- and closed-eyes conditions were found for the recruitment level, except for the hip/back synergy, which significantly decreased (p = 0.02) in the closed-eyes compared to the open-eyes condition. A significant increase (p = 0.03) of the ankle balance strategy was found in the closed-eyes compared to the open-eyes condition.

Conclusion

In healthy young individuals, single-limb stance is featured by four major synergies, both in open- and closed-eyes condition. Future studies should investigate muscle synergies in participants with other age groups, as well as pathological conditions.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13102-021-00392-z.