Impact of exercise selection on hamstring muscle activation

Measuring Muscle Activity in Sprinters Using T2-Weighted Magnetic Resonance Imaging

PURPOSE

This study aimed to investigate the level of muscle activity during sprint running using T2-weighted magnetic resonance imaging.

METHODS

Fourteen male sprinters (age 21.2 [4.0] y; height 171.8 [4.2] cm, weight 65.5 [5.3] kg, 100-m personal record 11.01 [0.41] s; mean [SD]) performed 3 sets of three 60-m round-trip sprints. Before and after the round-trip sprints, 3 T magnetic resonance imaging scans were performed to obtain the T2 values of the 14 athletes' lower-extremity muscles.

RESULTS

After the 60-m round-trip sprints, the T2 value of the gluteus maximus, long head of biceps femoris, semitendinosus, semimembranosus, adductor brevis, adductor longus, adductor magnus, and gracilis increased significantly. The rate of change in the T2 values before and after the 60-m round-trip sprints was notably higher in the semitendinosus and gluteus maximus than in the other muscles.

CONCLUSIONS

These findings demonstrate the specific physiological metabolism of the lower-extremity muscles during fast sprinting. There are particularly high levels of muscle activity in the gluteus maximus and semitendinosus during sprint performance.

Poor Reporting of Exercise Interventions for Hamstring Strain Injury Rehabilitation: A Scoping Review of Reporting Quality and Content in Contemporary Applied Research

OBJECTIVE

To review the quality of reporting and identify the content of exercise interventions prescribed for hamstring strain injury (HSI) rehabilitation in the scientific literature from 2010 to 2020.

DESIGN

Scoping review.

LITERATURE SEARCH

We searched the bibliometric databases Web of Science, CINAHL, SPORTDiscus, Scopus, Cochrane Library, MEDLINE, and Embase.

STUDY SELECTION CRITERIA

Original research articles (randomized controlled trials and cohort studies) published from 2010 to 2020 that described an exercise rehabilitation intervention for participants with acute HSIs were included. Injuries must have been confirmed within 7 days of occurrence via clinical assessment and/or diagnostic imaging.

DATA SYNTHESIS

The quality of reporting, in terms of completeness of exercise intervention description, was evaluated using the Consensus on Exercise Reporting Template (CERT), and the content of interventions was categorized into exercise types.

RESULTS

Fourteen studies were included; exercise intervention quality of reporting was moderate in 3 studies and low in 11 studies. Using the 19-item CERT, an average of 8.8 items (range, 4-14) were reported across all studies. Two studies reported sufficient exercise content and progression information to allow replication. Exercises categorized as hamstring flexibility, hamstring strength, running related, and non-hamstring specific were prescribed in 13, 11, 10, and 10 studies, respectively. Half of the included studies incorporated all 4 exercise types in their exercise interventions.

CONCLUSION

There is a wide variety of exercise interventions applied in published research that has addressed HSI rehabilitation. Researchers must improve reporting quality to support other professionals in replicating exercise interventions and help practitioners to effectively implement research in practice. J Orthop Sports Phys Ther 2022;52(3):130-141. Epub 21 Sep 2021. doi:10.2519/jospt.2022.10641.

An Electromyographic Analysis of Romanian, Step-Romanian, and Stiff-Leg Deadlift: Implication for Resistance Training

The present study examined the posterior chain muscle excitation in different deadlift variations. Ten competitive bodybuilders (training seniority of 10.6 ± 1.8 years) performed the Romanian (RD), Romanian standing on a step (step-RD), and stiff-leg deadlift (SD) with an 80% 1-RM. The excitation of the gluteus maximus, gluteus medius, biceps femoris, semitendinosus, erector spinae longissimus, and iliocostalis was assessed during both the ascending and descending phases. During the ascending phase, the RMS of the gluteus maximus was greater in the step-RD than in the RD (effect size (ES): 1.70, 0.55/2.84) and SD (ES: 1.18, 0.11/2.24). Moreover, a greater RMS was found in the SD than in the RD (ES: 0.99, 0.04/1.95). The RMS of the semitendinosus was greater in the step-RD than in the RD (ES: 0.82, 0.20/1.44) and SD (ES: 3.13, 1.67/4.59). Moreover, a greater RMS was found in the RD than in the SD (ES: 1.38, 0.29/2.48). The RMS of the longissimus was greater in the step-RD than in the RD (ES: 2.12, 0.89/3.34) and SD (ES: 3.28, 1.78/4.78). The descending phase had fewer differences between the exercises. No further differences between the exercises were found. The step-RD increased the overall excitation of the posterior chain muscles, possibly because of the greater range of movement and posterior muscle elongation during the anterior flexion. Moreover, the RD appeared to target the semitendinosus more than the SD, while the latter excited the gluteus maximus more.

Muscle Activation During Single-Legged Squat Is Affected by Position of the Nonstance Limb

CONTEXT

The single-legged squat (SLS) is appropriate for targeting activation, strengthening, and neuromuscular retraining of the gluteus maximus, gluteus medius, and quadriceps. However, the effects of different nonstance-limb positions on muscle activity have not been fully evaluated.

OBJECTIVE

To compare the muscle activity of selected stance-limb hip muscles during the SLS using 3 nonstance-limb positions: in front (SLS-Front), in the middle (SLS-Middle), and in back (SLS-Back).

DESIGN

Controlled laboratory study.

SETTING

Biomechanics laboratory.

PATIENTS OR OTHER PARTICIPANTS

A total of 17 healthy adults (8 men, 9 women; age = 22.6 ± 3.6 years, height = 173.3 ± 10.7 cm, mass = 71.2 ± 11.0 kg) participated.

MAIN OUTCOME MEASURE(S)

Surface electromyographic data of the gluteus maximus, gluteus medius, lateral hamstrings, medial hamstrings, rectus femoris, and tensor fascia lata (TFL) as well as kinetic data of the hip and knee were collected while participants performed the 3 SLS tasks. Mean muscle-activation levels during the descent and ascent phases for the selected hip muscles and hip and knee kinetics in all 3 planes were compared for the 3 tasks. Each variable of interest was analyzed using a separate linear regression model with a generalized estimating equations correction.

RESULTS

Muscle-activation levels of the gluteus maximus, gluteus medius, medial hamstrings, rectus femoris, and TFL for the stance limb during descent (P ≤ .04) and the medial hamstrings and TFL during ascent (P ≤ .002) were different among the SLS tasks. The greatest number of differences occurred between SLS-Front and SLS-Back. During descent, gluteal muscle activity was greater in SLS-Front (P ≤ .03) and SLS-Middle (P = .03) than in SLS-Back. For both phases, TFL activity was greater during SLS-Front than during both SLS-Middle and SLS-Back (P ≤ .006). Kinetic differences at the hip and knee between SLS tasks were also observed (P values ≤ .02).

CONCLUSIONS

The 3 SLS tasks had different muscle-activation and kinetic profiles. Clinicians and researchers can vary nonstance-limb position during the SLS to manipulate muscle-activation levels and tailor the exercise to assist with goals at different stages of rehabilitation.

The Efficacy of Flywheel Inertia Training to Enhance Hamstring Strength

The purpose of this narrative review is to examine the efficacy of flywheel inertia training to increase hamstring strength. Hamstring strain injury is common in many sports, and baseline strength deficits have been associated with a higher risk of hamstring strain injury. As a result, strength and conditioning professionals actively seek additional techniques to improve hamstring strength with the aim of minimising the incidence of hamstring strain injury. One method of strength training gaining popularity in hamstring strength development is flywheel inertia training. In this review, we provide a brief overview of flywheel inertia training and its supposed adaptions. Next, we discuss important determinants of flywheel inertia training such as familiarisation, volume prescription, inertia load, technique and specific exercise used. Thereafter, we investigate its effects on hamstring strength, fascicle length and hamstring strain injury reduction. This article proposes that hamstring specific flywheel inertia training can be utilised for strength development, but due to the low number of studies and contrary evidence, more research is needed before a definite conclusion can be made. In addition, as with any training modality, careful consideration should be given to flywheel inertia training determinants. This review provides general recommendations of flywheel inertia training determinants that have value when integrating flywheel inertia training into a hamstring strengthening program.

Longing for a Longitudinal Proxy: Acutely Measured Surface EMG Amplitude is not a Validated Predictor of Muscle Hypertrophy

Surface electromyography amplitudes are commonly measured in acute sports and exercise science studies to make inferences about muscular strength, performance, and hypertrophic adaptations that may result from different exercises or exercise-related variables. Here, we discuss the presumptive logic and assumptions underlying these inferences, focusing on hypertrophic adaptations for simplicity's sake. We present counter-evidence for each of its premises and discuss evidence both for and against the logical conclusion. Given the limited evidence validating the amplitude of surface electromyograms as a predictor of longitudinal hypertrophic adaptations, coupled with its weak mechanistic foundation, we suggest that acute comparative studies that wish to assess stimulus potency be met with scrutiny.

New setup for multi-parametric MRI in young and old rat gastrocnemius at 4.7 and 7 T during muscle stimulation

T₁ and T₂ relaxation times combined with ³¹ P spectroscopy have been proven efficient for muscular disease characterization as well as for pre- and post-muscle stimulation measurements. Even though ³¹ P spectroscopy can already be performed during muscle exercise, no method for T₁ and T₂ measurement enables this possibility. In this project, a complete setup and protocol for multi-parametrical MRI of the rat gastrocnemius before, during and after muscle stimulation at 4.7 and 7 T is presented. The setup is fully MRI compatible and is composed of a cradle, an electro-stimulator and an electronic card in order to synchronize MRI sequences with muscle stimulation. A 2D triggered radial-encoded Look-Locker sequence was developed, and enabled T₁ measurements in less than 2 min on stimulated muscle. Also, a multi-slice multi-echo sequence was adapted and synchronized for T₂ measurements as well as ³¹ P spectroscopy acquisitions in less than 4 min in both cases on stimulated muscle. Methods were validated on young rats using different stimulation paradigms. Then they were applied on older rats to compare quantification results, using the different stimulation paradigms, and allowed observation of metabolic changes related to aging with good reproducibility. The robustness of the whole setup shows wide application opportunities.

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.

The Effect of Hip Extension and Nordic Hamstring Exercise Protocols on Hamstring Strength: A Randomized Controlled Trial

ABSTRACT

Whyte, EF, Heneghan, B, Feely, K, Moran, KA, and O'Connor, S. The effect of hip extension and Nordic hamstring exercise protocols on hamstring strength: A randomized controlled trial. J Strength Cond Res 35(10): 2682-2689, 2021-Lower and between-limb asymmetrical eccentric hamstring isokinetic strength may be associated with hamstring injuries, which mainly affect the biceps femoris. The 45° hip extension exercise (HEE) strengthens the hamstrings in the lengthened position where the biceps femoris experiences greatest loading during high-speed running. However, the effect of an HEE program on eccentric hamstring isokinetic strength has not been investigated or compared with the Nordic Hamstring Exercise (NHE) program. Twenty-four male, varsity, Gaelic footballers (22.4 ± 2.7 years; 182.1 ± 6.0 cm; 84.4 ± 7.9 kg) were randomly allocated to an HEE or NHE 4-week program. Isokinetic (60°·s-1) eccentric peak torque, functional hamstring-to-quadriceps ratio, and between-limb eccentric strength (absolute and percentage) asymmetries were recorded before and after intervention. Muscle soreness scores were reported after exercise using a visual analogue scale. A mixed between-within analysis of variance investigated group (HEE vs. NHE) by time (pre vs. post) interaction effects, and any main effects. An independent-samples t-test compared muscle soreness scores between the NHE and HEE groups. No interaction or group main effects were found. Main effects for time were observed on dominant and nondominant limbs for eccentric peak torque (p < 0.001, η2 = 0.21, p = 0.01, η2 = 0.51, respectively) and functional hamstring quadriceps ratio (p = 0.03, η2 = 0.59, p < 0.0001, η2 = 0.26, respectively). There were no main effects for time between-limb eccentric peak torque or percentage asymmetries (p = 0.41, η2 = 0.03, p = 0.20, η2 = 0.07, respectively), or differences in muscle soreness scores (p > 0.05). A 4-week HEE program increases eccentric hamstring isokinetic strength similar to a NHE program, which may be useful as part of a hamstring injury prevention program.

Knee flexor strength and symmetry vary by device, body position and angle of assessment following ACL reconstruction with hamstring grafts at long-term follow-up

PURPOSE

Persistent deficits in knee flexor strength following harvest of semitendinosus and gracilis for anterior cruciate ligament reconstruction are inconsistent in the literature. Variation in methodology, including measuring torque at higher knee flexion angles may partially explain these discrepant findings. The objective of this study was to determine whether positioning (seated vs supine), consideration of peak or joint-angle-specific torque or device (Isokinetic Dynamometer vs NordBord Hamstring Dynamometer) impact the magnitude of knee flexor strength differences between limbs.

METHODS

Participants (n = 31, 44.2 ± 10.7 years,) who were at 14 ± 4.4 years follow-up for unilateral ACL reconstruction with semitendinosus/gracilis grafts completed the ACL Quality of Life outcome and an assessment including isokinetic concentric knee extensor and flexor strength in seated and supine with peak torque and torque at 60° (T60) and 75° (T75) knee flexion measured, followed by an eccentric Nordic Hamstring Curl.

RESULTS

Isokinetic concentric knee flexor torque was reduced in supine relative to seated, on the reconstructed limb against the unaffected, and at higher degrees of knee flexion relative to peak torque (T60 and T75 against peak torque). Limb symmetry varied by methodology (F_(6,204) = 8.506, p = 0.001) with reduced symmetry in supine T75 against all measures (71.1 ± 16.5%, p < 0.05), supine T60 against seated peak torque (82.7 ± 14.2%, p < 0.05), and the NordBord was lower than seated peak torque that was not statistically significant (83.9 ± 12.8%, n.s.). Knee extensor peak (r² = 0.167 (F_(1,27) = 5.3, p = 0.03) and Nordic curl eccentric torque (r² = 0.267, F_(2,26) = 4.736, p = 0.02) were predictors of ACL-QoL score, although a combined model did not improve over Nordic torque alone.

CONCLUSION

Limb symmetry cannot be assumed in clinical practice across differing assessment methods for knee flexor strength as deficits are greatest in the supine position with torque measured at 75° knee flexion. Isokinetic knee extensor and eccentric knee flexor torque during the Nordic hamstring curl were predictors of ACL-QoL scoring and should be considered alongside patient-reported outcomes for patients following ACL reconstruction with hamstring grafts.

LEVEL OF EVIDENCE

Level IV.

sEMG Activity in Superimposed Vibration on Suspended Supine Bridge and Hamstring Curl

Traditionally in strength and conditioning environments, vibration has been transmitted using platforms, barbells, dumbbells, or cables but not suspension devices. This study aimed to examine the effects on the lower limb of applying superimposed vibration on a suspension device. Twenty-one physically active men and women performed supine bridge and hamstring curl exercises in three suspended conditions (non-vibration, vibration at 25 Hz, and vibration at 40 Hz). In each exercise condition, the perceived exertion scale for resistance exercise (OMNI-Res) was registered, and the electromyographic signal was assessed for gastrocnemius (medialis and lateralis), biceps femoris, semitendinosus, gluteus maximus, and rectus femoris. A linear mixed model indicated a significant fixed effect for vibration at 25 Hz and 40 Hz on muscle activity in suspended supine bridge (p < 0.05), but no effect for suspended hamstring curl (p > 0.05). Likewise, the Friedman test showed a significant main effect for vibration at 25 Hz and 40 Hz in suspended supine bridge (p < 0.05) but not for suspended hamstring curl (p > 0.05) on OMNI-Res. Post hoc analysis for suspended supine bridge with vibration at 25 Hz showed a significant activation increase in gastrocnemius lateralis (p = 0.008), gastrocnemius medialis (p = 0.000), semitendinosus (p = 0.003) activity, and for semitendinosus under 40 Hz condition (p = 0.001) compared to the non-vibration condition. Furthermore, OMNI-Res was significantly higher for the suspended supine bridge at 25 Hz (p = 0.003) and 40 Hz (p = 0.000) than for the non-vibration condition. Superimposed vibration at 25 Hz elicits a higher neuromuscular response during the suspended supine bridge, and the increase in vibration frequency also raises the OMNI-Res value.

Biceps Femoris Activation during Hamstring Strength Exercises: A Systematic Review

BACKGROUND

The aim of the study was to systematically evaluate the biceps femoris long head activation across cross-sectional hamstring strength exercise studies.

METHODS

A systematic review design was followed. The search strategy conducted in PubMed, Cochrane Library, and Web of Sciences databases found a total of 3643 studies. Once inclusion and exclusion criteria were applied, 29 studies were finally included in this systematic review. A total of 507 participants and 114 different exercises were analyzed. Exercises were evaluated individually and grouped into several categories: Nordics, isokinetic exercises, lunges, squats, deadlifts, good mornings, hip thrusts, bridges, leg curls, swings, hip and back extensions, and others.

RESULTS

Results showed the isokinetic and Nordic exercises as the categories with highest biceps femoris activation (>60% of Maximal Voluntary Isometric Contraction). Nordic hamstring exercise ankle dorsiflexion was the exercise that achieved the highest biceps femoris long head activation (128.1% of its Maximal Voluntary Isometric Contraction).

CONCLUSIONS

The results from this systematic review suggest that isokinetic and Nordic exercises seem to be the best option to activate biceps femoris long head. Future studies evaluating the implementation of these exercises in prevention programs are needed.

Specificity of eccentric hamstring training and the lack of consistency between strength assessments using conventional test devices

Hamstring injuries are endemic, but influences of test-specific training and the application of different test methods on decision making remain elusive. Sport-students were randomised to isokinetic (IG) or Nordic hamstring (NG) exercise or a control group (CG) for six weeks. Training and testing procedures were matched to biomechanical parameters. Hamstring strength (EPT), work, muscle soreness (visual analogue scale (VAS)), biceps femoris (BF_lh) muscle size and architecture were assessed. Anthropometrics and strength parameters did not differ at baseline. Yet, body mass normalised EPT, and work revealed a significant group × time × device effect, with a significant main effect for devices. Experimental conditions triggered meaningful increases in EPT compared to the control group, but the effects were higher when recorded on the training device. Despite significant group × time interactions, normalised average work on the NHD was only higher in the NG compared to CG of the left leg (+ 35%). No effects were found for BF_lh parameters. Hamstrings showed a high training specificity, but adaptations likely remain undetected owing to the low sensitivity of conventional test devices. Moreover, strength increase of ~ 15% does not necessarily have to be reflected in BF_lh parameters.

Electromyographical Differences Between the Hyperextension and Reverse-Hyperextension

ABSTRACT

Cuthbert, M, Ripley, NJ, Suchomel, TJ, Alejo, R, McMahon, JJ, and Comfort, P. Electromyographical differences between the hyperextension and reverse-hyperextension. J Strength Cond Res 35(6): 1477-1483, 2021-The aims of this study were to compare muscle activation of the erector spinae (ES), gluteus maximus (GMax), and biceps femoris (BF) during the hyperextension (HE) and reverse-HE (RHE) exercises. Ten subjects (age, 23 ± 4 years; height, 175.9 ± 6.9 cm; mass, 75.2 ± 9.7 kg) had electromyography (EMG) electrodes placed on the ES, GMax, and BF muscles in accordance with SENIAM (Surface EMG for Non-Invasive Assessment of Muscles) guidelines. Subjects performed 3 maximum voluntary isometric contraction trials of lumbar extension and hip extension using a handheld and isokinetic dynamometer, respectively, to normalize the EMG during the HE and RHE exercises. Three repetitions of each exercise were executed in a randomized order. High reliability (intraclass correlation coefficient ≥0.925) was observed with low variability (coefficient of variation [CV] < 10%) in all but the GMax during the extension phase of the HE (CV = 10.64%). During the extension and flexion phases, the RHE exhibited significantly greater (p ≤ 0.024; 34.1-70.7% difference) peak EMG compared with the HE in all muscles tested. Similarly, the RHE resulted in significantly greater mean EMG compared with the HE (p ≤ 0.036; 28.2-65.0% difference) in all muscles except the BF during the flexion phase (p = 9.960). Therefore, the RHE could be considered as a higher-intensity exercise for the posterior chain muscles compared with the HE, potentially eliciting greater increases in strength of the posterior chain muscles.

Hamstrings load bearing in different contraction types and intensities: A shear-wave and B-mode ultrasonographic study

The main aim was to examine the load bearing of individual hamstring muscles in different contraction types and intensities, through local stiffness measurement by shear wave elastography (SWE). A secondary aim was to examine the relationship between the SWE stiffness measure and hamstrings morphology. Ten healthy males (age 22.1±4.1 years; height 173.7±5.9 cm; body mass 68.6±12.4 kg; mean ± SD) performed knee flexions on an isokinetic dynamometer at different intensities (20–70%MVC, random order) in three separate, randomized conditions: isometric (ISO), concentric (CON) and eccentric (ECC). SWE was used to measure muscle shear wave velocity (SWV) in biceps femoris long head (BFlh), semitendinosus (ST) and semimembranosus (SM) during contraction. Muscle anatomical cross-sectional area (ACSA) was measured with magnetic resonance imaging and muscle architecture with B-mode ultrasonography. Muscle SWV increased linearly with contraction intensity, but at a varying rate among muscles and contraction types. ST exhibited greater SWV than BFlh and SM in all contraction types, however, there was an upward shift in the SM SWV–torque relationship in ECC compared to ISO and CON. Strong negative correlations were found between peak ISO SWV and ST ACSA (r = -0.81, p = 0.005) and BFlh pennation angle (r = -0.75, p = 0.012). These results suggest that ST has a primary role in hamstrings load bearing in all contraction types, likely due to its morphology; however, there is evidence of increased contribution from SM in eccentric muscle actions.

Hamstring and gluteal activation during high-speed overground running: Impact of prior strain injury

This study examined the spatial patterns of hamstring and gluteal muscle activation during high-speed overground running in limbs with and without aprior hamstring strain injury. Ten active males with arecent (<18 month) unilateral biceps femoris long head (BFLH) strain injury underwent functional magnetic resonance imaging before and immediately after arepeat-sprint running protocol. Transverse relaxation (T2) time, an index of muscle activation, of the BFLH and short head (BFSH), semitendinosus (ST), semimembranosus (SM), gluteus maximus (GMAX) and medius (GMED) was assessed pre-post exercise. No significant between-limb differences in running-induced mean T2 changes were observed (p = 0.949), however, decision tree induction revealed that previously injured limbs were characterised by highly variable intramuscular activation of the ST (SD5.3). T2 times increased more for GMAX than all other muscles (all p< 0.001, d= 0.5-2.5). Further, T2 changes were greater for ST than BFSH, SM, GMED, and BFLH (all p≤ 0.001, d= 0.5-2.9); and were greater for BFLH than BFSH, SM, and GMED (all p< 0.001, d= 1.2-1.6). Athletes display heterogenous patterns of posterior thigh activation when sprinting (G_MAX>ST>BF_LH>G_MED>SM>BF_SH) and may exhibit altered intramuscular hamstring activation after returning to sport from BFLH strain injury.

Biceps Femoris Compensates for Semitendinosus After Anterior Cruciate Ligament Reconstruction With a Hamstring Autograft: A Muscle Functional Magnetic Resonance Imaging Study in Male Soccer Players

BACKGROUND

Rates of reinjury, return to play (RTP) at the preinjury level, and hamstring strain injuries in male soccer players after anterior cruciate ligament reconstruction (ACLR) remain unsatisfactory, due to multifactorial causes. Recent insights on intramuscular hamstring coordination revealed the semitendinosus (ST) to be of crucial importance for hamstring functioning, especially during heavy eccentric hamstring loading. Scientific evidence on the consequences of ST tendon harvest for ACLR is scarce and inconsistent. This study intended to investigate the repercussions of ST harvest for ACLR on hamstring muscle function.

HYPOTHESIS

Harvest of the ST tendon for ACLR was expected to have a significant influence on hamstring muscle activation patterns during eccentric exercises, evaluated at RTP in a population of male soccer athletes.

STUDY DESIGN

Controlled laboratory study.

METHODS

A total of 30 male soccer players with a history of ACLR who were cleared for RTP and 30 healthy controls were allocated to this study during the 2018-2019 soccer season. The influence of ACLR on hamstring muscle activation patterns was assessed by comparing the change in T2 relaxation times [ΔT2 (%) = post-exercise-T2pre-exerciseT2pre-exercise] of the hamstring muscle tissue before and after an eccentric hamstring loading task between athletes with and without a recent history of ACLR through use of muscle functional magnetic resonance imaging, induced by an eccentric hamstring loading task between scans.

RESULTS

Significantly higher exercise-related activity was observed in the biceps femoris (BF) of athletes after ACLR compared with uninjured control athletes (13.92% vs 8.48%; P = .003), whereas the ST had significantly lower activity (19.97% vs 25.32%; P = .049). Significant differences were also established in a within-group comparison of the operated versus the contralateral leg in the ACLR group (operated vs nonoperated leg: 14.54% vs 11.63% for BF [P = .000], 17.31% vs 22.37% for ST [P = .000], and 15.64% vs 13.54% for semimembranosus [SM] [P = .014]). Neither the muscle activity of SM and gracilis muscles nor total posterior thigh muscle activity (sum of exercise-related ΔT2 of the BF, ST, and SM muscles) presented any differences in individuals who had undergone ACLR with an ST tendon autograft compared with healthy controls.

CONCLUSION

These findings indicate that ACLR with a ST tendon autograft might notably influence the function of the hamstring muscles and, in particular, their hierarchic dimensions under fatiguing loading circumstances, with increases in relative BF activity contribution and decreases in relative ST activity after ACLR. This between-group difference in hamstring muscle activation pattern suggests that the BF partly compensates for deficient ST function in eccentric loading. These alterations might have implications for athletic performance and injury risk and should probably be considered in rehabilitation and hamstring injury prevention after ACLR with a ST tendon autograft.

Comparison of Electromyographic Activity During Hip Extension Exercises Under Gravitational or Inertial Loading Conditions

BACKGROUND

Hamstring injury prevention programs include strengthening, especially eccentric exercises using both gravitational and inertial loading. Inertial exercises are characterized by eccentric contractions of high intensity and velocity. This study aimed to analyze the muscular activation of the biceps femoris (BF), semitendinosus (ST), gluteus maximus (GM), and gracilis (GC) muscles during hip extension (HE) exercises performed under both gravitational and inertial loading conditions.

HYPOTHESIS

Inertial training would generate a greater activation of HE muscles than gravitational training.

STUDY DESIGN

Cross-sectional study.

LEVEL OF EVIDENCE

Level 4.

METHODS

Fifteen resistance-trained men performed the unilateral straight knee bridge (SKB), 45° of HE, and stiff-leg deadlift (SDL) exercises under gravitational and inertial loading conditions. Concentric and eccentric phases were identified with a linear encoder. Differences between load types, exercises, and their interaction were examined to establish the electromyographic (EMG) activity of each muscle and BF/ST ratio.

RESULTS

In the concentric phase, inertial loading showed a higher normalized EMG than gravitational loading for BF, ST, and GM. SKB and HE activated BF and ST between 9.6% and 24.3% more than SDL. In the eccentric phase, the inertial modality achieved greater GM activation than the gravitational form (18.1%). BF activation was increased with HE and SKB as compared with SDL (24.4% and 16.4%, respectively), while ST activation was likewise enhanced with HE as compared with SDL (15.1%).

CONCLUSION

Inertial training is more effective than gravitational training for the concentric activation of the hamstring muscles while SDL showed lower hamstring activation than HE and SKB. Therefore, HE and SKB with inertial loading should be taken into account in hamstring training programs.

CLINICAL RELEVANCE

Inertial training is more effective than gravitational training for the concentric activation of the hamstring muscles. HE and SKB with inertial loading should be taken into account in hamstring training programs.

Sprinting, Strength, and Architectural Adaptations Following Hamstring Training in Australian Footballers

The aim of this study was to determine the sprinting, strength, and architectural adaptations following a hip-dominant flywheel (FLY) or Nordic hamstring exercise (NHE) intervention in Australian footballers. Twenty-seven male athletes were randomized to FLY (n = 13) or NHE (n = 14) training across a 39-week period (inclusive of pre-season and in-season). Biceps femoris long head (BFlh) architecture was assessed throughout. Eccentric hamstring strength and 40 m sprint times (with force-velocity profiling) were assessed at baseline, end of pre-season, and following the intervention. After the intervention, BFlh fascicle length was longer in both groups compared to baseline (FLY: 1.16 cm, 95%CI: 0.66 to 1.66 cm, d = 1.99, p < 0.001; NHE: 1.08 cm, 95%CI: 95%CI 0.54 to 1.61 cm, d = 1.73, p < 0.001). Both groups also increased their eccentric strength (FLY: mean change 82 N, 95%CI 12 to 152 N, d = 1.34, p = 0.026; NHE: mean change 97 N, 95%CI 47 to 146 N, d = 1.77, p = 0.001). After pre-season, the NHE group improved their 5 m sprint time by 3.5% (±1.2%) and were 3.7% (±1.4%) and 2.0% (±0.5%) faster than the FLY group across 5 m and 10 m, respectively. At the end of pre-season, the FLY group improved maximal velocity by 3.4% (±1.4%) and improved horizontal force production by 9.7% in-season (±2.2%). Both a FLY and NHE intervention increase BFlh fascicle length and eccentric strength in Australian Footballers. An NHE intervention led to enhanced acceleration capacity. A FLY intervention was suggested to improve maximal sprint velocity and horizontal force production, without changes in sprint times. These findings have implications for hamstring injury prevention but also programs aimed at improving sprint performance.

Optimising the 'Mid-Stage' Training and Testing Process After ACL Reconstruction

Outcomes following anterior cruciate ligament (ACL) reconstruction need improving, with poor return-to-sport rates and high risk of secondary re-injury. There is a need to improve rehabilitation strategies after ACL reconstruction, if we can support enhanced patient outcomes. This paper discusses how to optimise the mid-stage rehabilitation process after ACL reconstruction. Mid-stage is a difficult and vitally important stage of the functional recovery process and provides the foundation on which to commence late-stage rehabilitation training. Often many aspects of mid-stage rehabilitation (e.g. knee extensors isolated muscle strength) are not actually restored prior to return-to-sport. In addition, if we are to allow time for optimal late-stage rehabilitation and return-to-sport training, we need to optimise the mid-stage rehabilitation approach and complete it in a timely manner. This paper forms a key part of a strategy to optimise the ACL rehabilitation approach and considers factors more specific to mid-stage rehabilitation characterised in 3 areas: (1) muscle strength: muscle and joint specific, in particular at the knee level, with the knee extensors and flexors and distally with the triceps surae and proximally with the lumbo-pelvic-hip complex, as well as closed kinetic chain strength; (2) altered basic motor patterning (movement quality) and (3) fitness re-conditioning. In addition, the paper provides recommendations on how to implement these into practice, discussing training planning and programming and suggests specific screening to monitor work and when the athlete is able to progress to the next stage (e.g. late-stage rehabilitation criteria).

Biceps Femoris Muscle is Activated by Performing Nordic Hamstring Exercise at a Shallow Knee Flexion Angle

The semitendinosus (ST) muscle is primarily used during Nordic hamstring exercise (NHE), which is often prescribed for preventing hamstring injury, though the biceps femoris long head (BFlh) muscle that is more susceptible to injuries. Thus, this study aimed to identify the modulation of BFlh muscle activity with different knee flexion angles during NHE using an inclined platform. Fourteen male athletes performed NHE and maintained their position at maximum inclination (NH). Subjects also performed isometric NHE using a platform inclined to 50° (ICL) and 40° (ICH), and the knee flexion angle was controlled to 50° and 30°. The electromyography (EMG) activity of the BFlh, ST, semimembranosus, gluteus maximus, elector spinae, and rectus abdominus muscles was determined during each exercise. The EMG of the ST was higher than that of the BFlh during NHE and the highest of all muscles in all exercises (p < 0.05). Moreover, the activity of the BFlh tended to be higher than that of the ST for ICH than for ICL, regardless of the knee joint angle. The activity of the BFlh becomes equivalent to that of the ST during NHE at a knee flexion angle of less than 50°. These results indicate that performing NHE at a shallow knee flexion angle will enhance the activity of the BFlh muscle.

Individual differences in the distribution of activation among the hamstring muscle heads during stiff-leg Deadlift and Nordic hamstring exercises

The aim of this study was to compare the distribution of activation among the three heads of the hamstring between a knee flexion-oriented exercise (Nordic hamstring) and a hip extension-oriented exercise (stiff-leg Deadlift) at the group and individual level. Data were collected for 20 participants. Muscle activation of the semimembranosus (SM), semitendinosus (ST), and biceps femoris (BF) was estimated using surface electromyography (EMG) during Nordic hamstring and stiff-leg Deadlift exercises. Although Nordic hamstring exercise induced a higher normalized RMS EMG value for BF (64.5 ± 17.4%) compared to SM (48.6 ± 14.6%; P<0.001) and ST (55.9 ± 17.4%; P < 0.001), the greatest active muscle varied between individuals. Similar interindividual differences in the greatest active muscle were found for the stiff-leg Deadlift exercise. Regarding the distribution of activation, the stiff-leg Deadlift favoured the contribution of the SM compared to ST (P < 0.001, 18/20 participants) whereas the Nordic hamstring exercise favoured the contribution of the ST compared to SM (P < 0.001, 19/20 participants). Importantly, these tasks affected the contribution of the activation of BF in different ways between individuals. The distribution of activation across the three muscles was well correlated between the two exercises (r values ≥ 0.42).

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

Hamstring muscle function during knee flexion has been linked to hamstring injury and performance. However, it is unclear whether knee flexion alone (KF) requires similar hamstring electromyography (EMG) activity pattern to simultaneous hip extension and knee flexion (HE-KF), a combination that occurs in the late swing phase of sprinting. This study examined whether HE-KF maximal voluntary isometric contraction (MVIC) evokes higher (EMG) activity in biceps femoris long head (BFlh) and semitendinosus (ST) than KF alone. Effects of shank rotation angles were also tested. Twenty-one males performed the above-mentioned MVICs while EMG activity was measured along ST and BFlh. Conditions were compared using a one-way mixed functional ANOVA model under a fully Bayesian framework. Higher EMG activity was found in HE-KF in all shank rotation positions than in KF in the middle region of BFlh (highest in the 9th channel, by 0.022 mV [95%CrI 0.014 to 0.030] in neutral shank position). For ST, this was only observed in the neutral shank position and in the most proximal channel (by 0.013 mV [95%CrI 0.001 to 0.025]). We observed muscle- and region-specific responses to HE-KF. Future studies should examine whether hamstring activation in this task is related to injury risk and sprint performance.

Hamstrings Muscle Morphology After Anterior Cruciate Ligament Reconstruction: A Systematic Review and Meta-Analysis

BACKGROUND

Hamstrings muscle morphology is determinant of muscle function (i.e. strength). Among individuals with ACL reconstruction (ACLR), less cross-sectional area (CSA) and volume in the ACLR-limb are associated with muscle weakness, and may contribute to lower rates of return to preinjury activity level and an increased risk for long-term sequelae.

OBJECTIVES

To effectively treat muscular impairments, an accurate understanding of differences in hamstrings morphology following ACLR is needed. A systematic review and meta-analysis were undertaken to describe the morphology of the hamstring muscle complex after ACLR.

METHODS

We searched five databases for studies evaluating the difference between hamstrings size and architecture in individuals with ACLR. Two independent reviewers assessed each paper for inclusion and quality. Means and standard deviations were extracted from each included study to allow fixed-effect size meta-analysis calculations for comparison of results.

RESULTS

Twenty-four studies were included for final review. Eight categories of morphological outcomes were identified, and studies were grouped accordingly: (1) volume, (2) cross-sectional area (CSA), (3) muscle length, (4) muscle thickness, (5) fascicle length, (6) pennation angle, (7) fiber area, and (8) fiber type. Meta-analysis demonstrated lower hamstrings volume in the ACLR-limb in both contralateral and control group comparisons, and lower CSA, length, and thickness in the ACLR-limb in contralateral comparisons. The semitendinosus and gracilis were most profoundly impacted. Limited moderate evidence demonstrated greater biceps femoris pennation angle in the ACLR-limb.

CONCLUSIONS

Individuals with ACLR demonstrated large deficits in semitendinosus and gracilis muscle CSA and volume in the ACLR-limb compared contralaterally, with no differences observed in the biceps femoris or semimembranosus. Clinical implications regarding assessment and treatment of individuals with ACLR are discussed.

The task dependent differences in electromyography activity of hamstring muscles during leg curls and hip extensions

This study aimed to investigate the influence of the task type on the relative electromyography (EMG) activity of biceps femoris long head (BFlh) to semitendinosus (ST) muscles, and of proximal to distal regions during isometric leg-curl (LC) and hip-extension (HE). Twenty male volunteers performed isometric LC with the knee flexed to 30° (LC30) and 90° (LC90), as well as isometric HE with the knee extended (HE0) and flexed to 90° (HE90), at 40% and 100% maximal voluntary contraction (MVIC). Hip position was neutral in all conditions. EMG activity was recorded from the proximal and distal region of the BFlh and ST muscles. BFlh/ST was calculated from the raw root-mean-square (RMS) amplitudes. The RMS of 40% MVIC was normalized using MVIC data and the proximal/distal (P/D) ratio of normalized EMG (NEMG) was calculated. The BFlh/ST ratio was higher in HE0 than in LC90 during MVIC and 40% MVIC (p<0.05), and was higher in HE90 than in LC90 (p<0.05) during 40% MVIC at the proximal region, whereas no difference was observed between HE0 and LC30. There was no inter-task difference in BFlh/ST ratio in the distal region. Furthermore, the P/D ratio was higher in LC90 than in LC30 and HE0 (p<0.05) in BFlh and ST muscles, and was higher in HE90 than in LC30 and HE0 (p<0.05) in BFlh during 40% MVIC. However, there was no difference in P/D ratio between LC30 and LC90, and HE0 and HE90. This showed that there was no task-dependent difference in the EMG activity of the BFlh muscle relative to the ST muscle between prone hip extension and prone knee flexion when the knee joint was set at an equivalent angle. Similarly, there was no task-dependent difference in the NEMG of the proximal region relative to the distal region in BFlh and ST muscles during 40% MVIC.

Regional Differences in Biceps Femoris Long Head Stiffness during Isometric Knee Flexion

This study sought to investigate whether the stiffness of the biceps femoris long head differs between proximal and distal regions during isometric knee flexion at different contraction intensities and muscle lengths. Twelve healthy individuals performed knee flexion isometric contractions at 20% and 60% of maximum voluntary isometric contraction, with the knee flexed at 15 and 45 degrees. Muscle stiffness assessment was performed using ultrasound-based shear wave elastography. Proximal and distal regions of the biceps femoris long head were assessed. Biceps femoris long head muscle showed a greater stiffness (i) in the distal region, (ii) at higher contraction intensity, and (iii) at longer muscle length. The proximal-to-distal stiffness ratio was significantly lower than 1 (i.e., heterogenous) at lower contraction intensity regardless of the muscle length. However, this was not observed at higher contraction intensity. This study is the first to show heterogeneity in the active stiffness of the biceps femoris long head. Given the greater incidence of injury at the proximal region of biceps femoris long head, this study opens new directions for future research. Additionally, the present study results indicate that studies assessing muscle stiffness at one single muscle region should be interpreted with caution.

Sprint Specificity of Isolated Hamstring-Strengthening Exercises in Terms of Muscle Activity and Force Production

To train hamstring muscle specifically to sprint, strengthening programs should target exercises associated with horizontal force production and high levels of hamstring activity. Therefore, the objectives of this study were to analyze the correlation between force production capacities during sprinting and hamstring strengthening exercises, and to compare hamstring muscle activity during sprinting and these exercises. Fourteen track and field regional level athletes performed two maximal 50-m sprints and six strengthening exercises: Nordic hamstring exercises without and with hip flexion, Upright-hip-extension in isometric and concentric modalities, Standing kick, and Slide-leg-bridge. The sprinting horizontal force production capacity at low (F0) and high (V0) speeds was computed from running velocity data. Hamstring muscle performances were assessed directly or indirectly during isolated exercises. Hamstring muscle electromyographic activity was recorded during all tasks. Our results demonstrate substantially large to very large correlations between V0 and performances in the Upright-hip-extension in isometric (r_s = 0.56; p = 0.040), Nordic hamstring exercise without hip flexion (r_s = 0.66; p = 0.012) and with 90° hip flexion (r_s = 0.73; p = 0.003), and between F0 and Upright-hip-extension in isometric (r_s = 0.60; p = 0.028) and the Nordic hamstring exercise without hip flexion (r_s = 0.59; p = 0.030). However, none of the test exercises activated hamstring muscles more than an average of 60% of the maximal activation during top-speed sprinting. In conclusion, training programs aiming to be sprint-specific in terms of horizontal force production could include exercises such as the Upright-hip-extension and the Nordic hamstring exercise, in addition to maximal sprinting activity, which is the only exercise leading to high levels of hamstring muscle activity.

Applying a holistic hamstring injury prevention approach in elite football: 12 seasons, single club study

The aim was to investigate the preventive effect of a complex training program based on holistic hamstring health understanding in elite professional soccer players. This study involved an elite club in Europe and was conducted over 12 seasons. The last 2 seasons were the intervention period, and the others were the control seasons. During the intervention period, players performed a complex program organized into different interventions throughout the week having as a priority the player health. Hamstring injuries, absenteeism, injury rates, and injury burden between the control and intervention seasons were compared using a rate ratio (RR) with 95% CI. Players had a mean exposure of 333.5 ± 18.6 hours per season with no significant differences between the intervention and control seasons. The overall injury rate was 3 times lower during the two intervention seasons than during the previous seasons (P < .01); the match injury rate was 2.7 times lower (P < .01) and the training rate 4.3 times (P < .01). Injury burden was almost 4 times lower during the two intervention seasons than during the previous seasons (P < .01), and recurrences in the control group were 10% vs 0% in the intervention group. Hamstring injuries were reduced ~3 times during the seasons in which elite football players were exposed to multicomponent, complex prevention training with individual approaches based on player needs, management of training load, individualized physiotherapy treatment, and planned staff communication, in comparison to the control seasons without a clearly defined and structured injury prevention intervention.

Recommendations for Hamstring Function Recovery After ACL Reconstruction

It is important to optimise the functional recovery process to enhance patient outcomes after major injury such as anterior cruciate ligament reconstruction (ACLR). This requires in part more high-quality original research, but also an approach to translate existing research into practice to overcome the research to implementation barriers. This includes research on ACLR athletes, but also research on other pathologies, which with some modification can be valuable to the ACLR patient. One important consideration after ACLR is the recovery of hamstring muscle function, particularly when using ipsilateral hamstring autograft. Deficits in knee flexor function after ACLR are associated with increased risk of knee osteoarthritis, altered gait and sport-type movement quality, and elevated risk of re-injury upon return to sport. After ACLR and the early post-operative period, there are often considerable deficits in hamstring function which need to be overcome as part of the functional recovery process. To achieve this requires consideration of many factors including the types of strength to recover (e.g., maximal and explosive, multiplanar not just uniplanar), specific programming principles (e.g., periodised resistance programme) and exercise selection. There is a need to know how to train the hamstrings, but also apply this to the ACLR athlete. In this paper, the authors discuss the deficits in hamstring function after ACLR, the considerations on how to restore these deficits and align this information to the ACLR functional recovery process, providing recommendation on how to recover hamstring function after ACLR.

Hamstring muscle injury in the athlete: state of the art

Hamstring injuries (HSI) are the source of significant impairment and disability for both professional and recreational athletes. The incidence and prevalence of HSIs has been well documented in the literature, as they are among the most common soft tissue injuries reported. The significant time loss due to injury and the inherent risk of reinjury pose a significant issue to the athlete, their career longevity and the success of their respective team. This review will deal predominantly with describing the prevalence and incidence of HSI in athletes, discuss risk factors and the mechanisms of injury for HSI, how to properly diagnose, image and prognosticate appropriate return to sport (RTS) for individuals who have sustained an HSI, prescribe treatment and prevention strategies and to discuss relevant options to decrease overall risk of primary and secondary recurrence of HSI.Current treatments of acute HSI necessitate a thorough understanding of the mechanism of injury, identifying muscle imbalances and/or weakness, inclusion of eccentric and concentric hamstring (HS) and hip extension (HE) exercises, evaluation of pathokinematic movement patterns and use non-surgical methods to promote healing and RTS. This methodology can be used prospectively to mitigate the overall risk of HSI. Injection therapies for HSI, including ultrasound-guided platelet-rich plasma and corticosteroids, may impart some short-term benefit, but the existing literature is largely inconclusive with respect to long-term functional outcomes. Future directions should prioritise injury prevention, early diagnosis and targeted interventions that combine both non-surgical and minimally invasive orthobiological approaches and identifying biomechanical risk factors prospectively to mitigate risk.

Acute Hamstring Muscle Tears in Climbers-Current Rehabilitation Concepts

Acute hamstring injuries are often caused by the heel hook technique. This technique is unique to climbing and causes injury to muscular and inert tissues of the posterior thigh. The heel hook is used by climbers during strenuous ascent on overhanging walls and when crossing difficult terrain. The technique reduces the amount of upper body strength required during strenuous climbing because the climber's center of mass is retained within the base of support. The heel hook is stressful collectively for the hamstring muscle group and musculotendinous junction. Depending on injury severity, both conservative and surgical methods exist for the management of hamstring injuries. Contemporary approaches to rehabilitation primarily advocate the use of eccentric muscle strengthening strategies because of high rates of elongation stress associated with sprinting and team sports. However, there is reason to doubt whether this alone is sufficient to rehabilitate the climbing athlete in light of the high degree of concentric muscle strength required in the heel hook maneuver. This review examines the contemporary rehabilitation and strength and conditioning literature in relation to the management of acute hamstring musculotendinous injuries for the climbing athlete. The review provides a comprehensive approach for the rehabilitation and athletic preparation of the climbing athlete from the initial injury to full return to sports participation.

Validity of an inertial system for measuring velocity, force, and power during hamstring exercises performed on a flywheel resistance training device

Background

Inertial hamstring exercises promote functional changes leading to lower rates of hamstring injuries. However, variable training measurement systems have not been specifically validated for hamstring exercises. Accordingly, this study aimed to evaluate the validity of the Inertial Measurement System (IMS) to measure the velocity, force, and power during the performance of different hamstring exercises on a flywheel resistance training device.

Methods

Fifteen males (average age: 22.4 ± 2.5 years; body mass: 77.3 ± 9.8 kg; height: 179.5 ± 7.4 cm; weekly physical activity: 434.0 ± 169.2 min; years of strength training: 4.3 ± 2.2 years) performed the bilateral stiff-leg deadlift (SDL), 45° hip extension (HE), and unilateral straight knee bridge (SKB) in two sessions (familiarization and evaluation) with a 1-week interval between them. The velocity, force, and power (average and peak values) in the concentric and eccentric phases for each of the exercises were recorded simultaneously with IMS and MuscleLab.

Results

Consistency between IMS and MuscleLab was good to excellent for all variables, with r ranges from 0.824 to 0.966 in SDL, from 0.822 to 0.971 in HE, and from 0.806 to 0.969 in SKB. Acceptable levels of agreement between devices were observed in general for all exercises, the “bias” ranging from 1.1% to 13.2%. Although MuscleLab showed higher values than IMS for peak velocity, force and power values, the effect size was only relevant for 5 of the 36 parameters. IMS is a new and valid system to monitor inertial hamstring exercises on a new flywheel device. In this way, IMS could have potential practical applications for any professional or athlete who wants to monitor inertial hamstring exercises.

Spatial muscle activation patterns during different leg exercise protocols in physically active adults using muscle functional MRI: a systematic review

An emerging method to measure muscle activation patterns is muscle functional magnetic resonance imaging (mfMRI), where preexercise and postexercise muscle metabolism differences indicate spatial muscle activation patterns. We evaluated studies employing mfMRI to determine activation patterns of lumbar or lower limb muscles following exercise in physically active adults. Electronic systematic searches were conducted until March 2020. All studies employing ≥1.5 Tesla MRI scanners to compare spatial muscle activation patterns at the level of or inferior to the first lumbar vertebra in healthy, active adults. Two authors independently assessed study eligibility before appraising methodological quality using a National Institutes of Health assessment tool. Because of heterogeneity, findings were synthesized without meta-analysis. Of the 1,946 studies identified, seven qualified for inclusion and pertained to hamstring (n = 5), quadriceps (n = 1) or extrinsic foot (n = 1) muscles. All included studies controlled for internal validity, with one employing assessor blinding. MRI physics and differing research questions explain study methodology heterogeneity. Significant mfMRI findings were: following Nordic exercise, hamstrings with previous trauma (strain or surgical autograft harvest) demonstrated reduced activation compared with unharmed contralateral muscles, and asymptomatic individuals preferentially activated semitendinosus; greater biceps femoris long head to semitendinosus ratios reported following 45° hip extension over Nordic exercise; greater rectus femoris activation occurred in "flywheel" over barbell squats. mfMRI parameters differ on the basis of individual research questions. Individual muscles show greater activation following specific exercises, suggesting exercise specificity may be important for rehabilitation, although evidence is limited to single cohort studies comparing interlimb differences preexercise versus postexercise.

Muscle Activation Differences During Eccentric Hamstring Exercises

BACKGROUND

The hamstring muscles play a critical role in the prevention of lower limb injuries. However, it is still unclear which exercises are more effective in terms of muscle activation.

HYPOTHESIS

In healthy individuals, there are differences between muscular activations of the biceps femoris (BF), semitendinosus (ST), and semimembranosus (SM) muscles during eccentric hamstring exercises.

STUDY DESIGN

Cross-sectional.

LEVEL OF EVIDENCE

Level 2.

METHODS

A total of 31 healthy participants (18 male; mean age, 22.5 years; SD, 3.1) were included in this study. The maximum voluntary isometric contraction of the hamstring muscles was measured using an isokinetic dynamometer. The participants were asked to perform one of the following exercises randomly (3 repetitions each): stiff-leg deadlift (SLDL), unilateral stiff-leg deadlift (USLDL), Nordic hamstring exercise (NHE), and ball leg curl (BLC). Activation of the BF, ST, and SM muscles was measured using surface electromyography during the exercises. In the statistical analysis of this study, factorial analysis of variance was used to compare the effects of each exercise on the muscle groups and to analyze which exercise type was more effective for each muscle group.

RESULTS

The NHE led to higher muscle activation than the other exercises (P < 0.001). When exercise type and muscle interaction were examined, SM activation was lower than BF (P = 0.04) and ST (P = 0.001) during NHE (P < 0.05). The highest level of muscular activation was recorded during the NHE in both male and female participants.

CONCLUSION

The NHE may be the most effective exercise for the hamstring muscles as it leads to greater muscle activation. SLDL, USLDL, and BLC exercises may be preferred at the beginning of strength training programs since they lead to lower muscular activation compared with the NHE.

CLINICAL RELEVANCE

To select the optimum hamstring exercise, it is important to know the activation levels of the hamstring muscles during different eccentric exercises.

The Effect of Nordic Hamstring Exercise Intervention Volume on Eccentric Strength and Muscle Architecture Adaptations: A Systematic Review and Meta-analyses

Background

Although performance of the Nordic hamstring exercise (NHE) has been shown to elicit adaptations that may reduce hamstring strain injury (HSI) risk and occurrence, compliance in NHE interventions in professional soccer teams is low despite a high occurrence of HSI in soccer. A possible reason for low compliance is the high dosages prescribed within the recommended interventions. The aim of this review was to investigate the effect of NHE-training volume on eccentric hamstring strength and biceps femoris fascicle length adaptations.

Methods

A literature search was conducted using the SPORTDiscus, Ovid, and PubMed databases. A total of 293 studies were identified prior to application of the following inclusion criteria: (1) a minimum of 4 weeks of NHE training was completed; (2) mean ± standard deviation (SD) pre- and post-intervention were provided for the measured variables to allow for secondary analysis; and (3) biceps femoris muscle architecture was measured, which resulted in 13 studies identified for further analysis. The TESTEX criteria were used to assess the quality of studies with risk of bias assessment assessed using a fail-safe N (Rosenthal method). Consistency of studies was analysed using I² as a test of heterogeneity and secondary analysis of studies included Hedges’ g effect sizes for strength and muscle architecture variables to provide comparison within studies, between-study differences were estimated using a random-effects model.

Results

A range of scores (3–11 out of 15) from the TESTEX criteria were reported, showing variation in study quality. A ‘low risk of bias’ was observed in the randomized controlled trials included, with no study bias shown for both strength or architecture (N = 250 and 663, respectively; p < 0.001). Study consistency was moderate to high for strength (I² = 62.49%) and muscle architecture (I² = 88.03%). Within-study differences showed that following interventions of ≥ 6 weeks, very large positive effect sizes were seen in eccentric strength following both high volume (g = 2.12) and low volume (g = 2.28) NHE interventions. Similar results were reported for changes in fascicle length (g ≥ 2.58) and a large-to-very large positive reduction in pennation angle (g ≥ 1.31). Between-study differences were estimated to be at a magnitude of 0.374 (p = 0.009) for strength and 0.793 (p < 0.001) for architecture.

Conclusions

Reducing NHE volume prescription does not negatively affect adaptations in eccentric strength and muscle architecture when compared with high dose interventions. These findings suggest that lower volumes of NHE may be more appropriate for athletes, with an aim to increase intervention compliance, potentially reducing the risk of HSI.

Is Fatigue a Risk Factor for Anterior Cruciate Ligament Rupture?

Neuromuscular fatigue is a commonly accepted risk factor for anterior cruciate ligament (ACL) injury. It has been proposed that fatigue leads to transient reductions in muscle strength, and deleterious changes in lower limb kinematics and kinetics, during potentially hazardous tasks such as cutting or landing. The purpose of this clinical commentary is to (1) highlight the complexity of fatigue; (2) discuss the theoretical basis by which it is thought to contribute to ACL injury; and (3) critically discuss the evidence underpinning this hypothesis. Despite a significant amount of research, none of the published fatigue protocols appear to have any consistent effect on any lower limb kinematic or kinetic variables known to increase ACL injury risk. On the contrary, fatigued athletes appear to land with greater peak knee and hip flexion angles, and lower landing forces than unfatigued athletes-all of which are considered favourable movement strategies for reducing ACL loading. These data support recent analyses demonstrating no relationship between player workload in training and competition and the occurrence of ACL injury in sport.

Hamstring muscle activation and morphology are significantly altered 1-6 years after anterior cruciate ligament reconstruction with semitendinosus graft

PURPOSE

Harvest of the semitendinosus (ST) tendon for anterior cruciate ligament reconstruction (ACLR) causes persistent hypotrophy of this muscle even after a return to sport, although it is unclear if hamstring activation patterns are altered during eccentric exercise. It was hypothesised that in comparison with contralateral control limbs, limbs with previous ACLR involving ST grafts would display (i) deficits in ST activation during maximal eccentric exercise; (ii) smaller ST muscle volumes and anatomical cross-sectional areas (ACSAs); and (iii) lower eccentric knee flexor strength.

METHODS

Fourteen athletes who had successfully returned to sport after unilateral ACLR involving ST tendon graft were recruited. Median time since surgery was 49 months (range 12-78 months). Participants underwent functional magnetic resonance imaging (MRI) of their thighs before and after the Nordic hamstring exercise (NHE) and percentage change in transverse (T2) relaxation time was used as an index of hamstring activation. Muscle volumes and ACSAs were determined from MRI and distal ST tendons were evaluated via ultrasound. Eccentric knee flexor strength was determined during the NHE.

RESULTS

Exercise-induced T2 change was lower for ST muscles in surgical than control limbs (95% CI - 3.8 to - 16.0%). Both ST muscle volume (95% CI - 57.1 to - 104.7 cm³) and ACSA (95% CI - 1.9 to - 5.0 cm²) were markedly lower in surgical limbs. Semimembranosus (95% CI 5.5-14.0 cm³) and biceps femoris short head (95% CI 0.6-11.0 cm³) volumes were slightly higher in surgical limbs. No between-limb difference in eccentric knee flexor strength was observed (95% CI 33 N to - 74 N).

CONCLUSION

ST activation is significantly lower in surgical than control limbs during eccentric knee flexor exercise 1-6 years after ACLR with ST graft. Lower levels of ST activation may partially explain this muscle's persistent hypotrophy post ACLR and have implications for the design of more effective rehabilitation programs.

LEVEL OF EVIDENCE

IV.

Electromyographic activity in deadlift exercise and its variants. A systematic review

The main purpose of this review was to systematically analyze the literature concerning studies which have investigated muscle activation when performing the Deadlift exercise and its variants. This study was conducted according to the Preferred Reporting Items for Systematic Reviews and Meta-Analysis Statement (PRISMA). Original studies from inception until March 2019 were sourced from four electronic databases including PubMed, OVID, Scopus and Web of Science. Inclusion criteria were as follows: (a) a cross-sectional or longitudinal study design; (b) evaluation of neuromuscular activation during Deadlift exercise or variants; (c) inclusion of healthy and trained participants, with no injury issues at least for six months before measurements; and (d) analyzed "sEMG amplitude", "muscle activation" or "muscular activity" with surface electromyography (sEMG) devices. Major findings indicate that the biceps femoris is the most studied muscle, followed by gluteus maximus, vastus lateralis and erector spinae. Erector spinae and quadriceps muscles reported greater activation than gluteus maximus and biceps femoris muscles during Deadlift exercise and its variants. However, the Romanian Deadlift is associated with lower activation for erector spinae than for biceps femoris and semitendinosus. Deadlift also showed greater activation of the quadriceps muscles than the gluteus maximus and hamstring muscles. In general, semitendinosus muscle activation predominates over that of biceps femoris within hamstring muscles complex. In conclusion 1) Biceps femoris is the most evaluated muscle, followed by gluteus maximus, vastus lateralis and erector spinae during Deadlift exercises; 2) Erector spinae and quadriceps muscles are more activated than gluteus maximus and biceps femoris muscles within Deadlift exercises; 3) Within the hamstring muscles complex, semitendinosus elicits slightly greater muscle activation than biceps femoris during Deadlift exercises; and 4) A unified criterion upon methodology is necessary in order to report reliable outcomes when using surface electromyography recordings.

Pain-Free Versus Pain-Threshold Rehabilitation Following Acute Hamstring Strain Injury: A Randomized Controlled Trial

OBJECTIVE

The primary aim was to compare time from acute hamstring strain injury (HSI) to return-to-play (RTP) clearance following a standardized rehabilitation protocol performed within either pain-free or pain-threshold limits. Secondary aims were to compare isometric knee flexor strength, biceps femoris long head (BFLH) fascicle length, fear of movement, and reinjury occurrence at the 6-month follow-up between pain-free and pain-threshold groups.

DESIGN

Randomized controlled trial.

METHODS

Forty-three men with acute HSIs were randomly allocated to a pain-free (n = 22) or pain-threshold (n = 21) rehabilitation group. Days from HSI to RTP clearance, isometric knee flexor strength, BFLH fascicle length, fear of movement, and reinjury occurrence at the 6-month follow-up were reported.

RESULTS

Median time from HSI to RTP clearance was 15 days (95% confidence interval [CI]: 13, 17) in the pain-free group and 17 days (95% CI: 11, 24) in the pain-threshold group, which was not significantly different (P = .37). Isometric knee flexor strength recovery at 90° of hip and 90° of knee flexion was greater in the pain-threshold group at RTP clearance by 15% (95% CI: 1%, 28%) and by 15% (95% CI: 1%, 29%) at 2-month follow-up, respectively. Improvement in BFLH fascicle length from baseline was 0.91 cm (95% CI: 0.34, 1.48) greater at 2-month follow-up in the pain-threshold group. Two reinjuries occurred in both the pain-free and pain-threshold groups between RTP clearance and the 6-month follow-up.

CONCLUSION

Pain-threshold rehabilitation did not accelerate RTP clearance, but resulted in greater recovery of isometric knee flexor strength and better maintenance of BFLH fascicle length, compared to pain-free rehabilitation. J Orthop Sports Phys Ther 2020;50(2):91-103. Epub 28 Jun 2019. doi:10.2519/jospt.2020.8895.

Eccentric Resistance Training in Youth: Perspectives for Long-Term Athletic Development

The purpose of this narrative review is to discuss the role of eccentric resistance training in youth and how this training modality can be utilized within long-term physical development. Current literature on responses to eccentric exercise in youth has demonstrated that potential concerns, such as fatigue and muscle damage, compared to adults are not supported. Considering the importance of resistance training for youth athletes and the benefits of eccentric training in enhancing strength, power, speed, and resistance to injury, its inclusion throughout youth may be warranted. In this review we provide a brief overview of the physiological responses to exercise in youth with specific reference to the different responses to eccentric resistance training between children, adolescents, and adults. Thereafter, we discuss the importance of ensuring that force absorption qualities are trained throughout youth and how these may be influenced by growth and maturation. In particular, we propose practical methods on how eccentric resistance training methods can be implemented in youth via the inclusion of efficient landing mechanics, eccentric hamstrings strengthening and flywheel inertia training. This article proposes that the use of eccentric resistance training in youth should be considered a necessity to help develop both physical qualities that underpin sporting performance, as well as reducing injury risk. However, as with any other training modality implemented within youth, careful consideration should be given in accordance with an individual's maturity status, training history and technical competency as well as being underpinned by current long-term physical development guidelines.

PRP and BMAC for Musculoskeletal Conditions via Biomaterial Carriers

Platelet-rich plasma (PRP) and bone marrow aspirate concentrate (BMAC) are orthobiologic therapies considered as an alternative to the current therapies for muscle, bone and cartilage. Different formulations of biomaterials have been used as carriers for PRP and BMAC in order to increase regenerative processes. The most common biomaterials utilized in conjunction with PRP and BMAC clinical trials are organic scaffolds and natural or synthetic polymers. This review will cover the combinatorial strategies of biomaterial carriers with PRP and BMAC for musculoskeletal conditions (MsCs) repair and regeneration in clinical trials. The main objective is to review the therapeutic use of PRP and BMAC as a treatment option for muscle, bone and cartilage injuries.

Kinematic and electromyographic analysis of variations in Nordic hamstring exercise

The purpose of this study was to present and biomechanically evaluate several variations of the Nordic hamstring exercise (NHE), achieved by altering the slope of the lower leg support and by asumming different hip flexion angles. Electromyographic and 2D kinematic measurements were conducted to analyse muscle activity (biceps femoris, semitendinosus, gluteus maximus, erector spine and lateral head of the gastrocnemius), knee and hip joint torques during 6 variations of NHE. The study involved 18 adults (24.9 ± 3.7 years) with previous experience in resistance training, but with little or no experience with NHE. Increasing the slope of the lower leg support from 0° (standard NHE) to 20° and 40° enabled the participants to perform the exercise through a larger range of motion, while achieving similar peak knee and hip torques. Instructions for increased hip flexion from 0° (standard NHE) to 25°, 50° and 75° resulted in greater peak knee and hip torque, although the participants were not able to maintain the hip angle at 50° nor 75°. Muscle activity decreased or remained similar in all modified variations compared to the standard NHE for all measured muscles. Our results suggest that using the presented variations of NHE might contribute to optimization of hamstring injury prevention and rehabilitation programs, by providing appropriate difficulty for the individual’s strength level and also allow eccentric strengthening at longer hamstring lengths.

The effects of sprint training and the Nordic hamstring exercise on eccentric hamstring strength and sprint performance in adolescent athletes

BACKGROUND

Hamstring strain injuries (HSI) are among the most common injuries in field-based team sports with a high-speed running component. The implementation of the Nordic hamstring exercise (NHE) is a well-documented method of improving eccentric hamstring strength to mitigate the risk of HSI occurrence. Sprint training is specific to the injury mechanism and is thought to activate the hamstrings through maximal eccentric contractions. The purpose of this study was to compare the effects of sprint training and the NHE on eccentric hamstring strength and sprint performance.

METHODS

Twenty-eight participants (mean±SD age=16.21±1.34 years; height=1.75m±0.10m; body mass=68.5kg±12.1kg) completed an eccentric hamstring strength assessment and 40m sprint to assess acceleration and maximum speed. Participants were randomly allocated to either a NHE training or sprint training group. Two sessions per week for four-weeks of training was performed with baseline testing procedures repeated in the week following the intervention. Perceptions of soreness were recorded following the warm-up in each training session.

RESULTS

Both the NHE (effect size=0.39, P<0.05) and sprint training (effect size=0.29, P<0.05) groups displayed significant gains in eccentric hamstring strength. The NHE group reported trivial improvements in sprint performance, whilst the sprint training group experienced a moderate improvement, specifically in maximum speed (ES=0.83 Moderate). Sprint training also produced greater perceptions of soreness than the NHE following a four-week training intervention, specifically before the start of the last session (P<0.05).

CONCLUSIONS

These findings indicate that sprint training had a beneficial effect for both eccentric hamstring strength and sprint performance, whilst also producing greater soreness than the NHE following the final training session. It was concluded that a four-week block of maximum speed training may have both an injury prevention and performance enhancement benefit.

Impact of Hip Flexion Angle on Unilateral and Bilateral Nordic Hamstring Exercise Torque and High-Density Electromyography Activity

BACKGROUND

In the bilateral Nordic hamstring exercise (NHE), hamstrings operate at relatively short lengths, which may limit the efficacy of the NHE in hamstring injury prevention.

OBJECTIVES

To examine knee flexion torque and biceps femoris long head (BFLH) and semitendinosus (ST) high-density electromyography (EMG) activity during the unilateral and bilateral NHE, performed with either neutral (NHE0) or 90°-flexed (NHE90) hips.

METHODS

In this laboratory study, exercises were performed on a novel device at the eccentric 1-repetition maximum load defined for 90° to 15° of knee range of motion. Torque and EMG signals normalized to maximal voluntary isometric activity were compared in different phases of the exercises with statistical parametric mapping.

RESULTS

The EMG levels were lower in NHE90 than in NHE0, mainly in the second half of the movement. Knee flexor eccentric torque was higher in NHE90 than in NHE0 from the beginning to 87% of the bilateral movement, and over the entire unilateral movement. In NHE0, ST activity compared to BFLH activity was higher during the initial movement phase and lower when the movement was close to knee extension. Torque and EMG activity were generally similar in the bilateral and unilateral modes.

CONCLUSION

If performed with neutral hips, the NHE selectively activates the BFLH near full knee extension. Performing the NHE with hips flexed to 90° is preferable when higher passive torque and ST selectivity are targeted at a longer muscle length. Performing these exercises unilaterally could help train each limb separately, with similar torque and EMG output to those of the bilateral conditions. J Orthop Sports Phys Ther 2019;49(8):584-592. Epub 26 Mar 2019. doi:10.2519/jospt.2019.8801.

Hamstring rehabilitation in elite track and field athletes: applying the British Athletics Muscle Injury Classification in clinical practice

Rationale

Hamstring injuries are common in elite sports. Muscle injury classification systems aim to provide a framework for diagnosis. The British Athletics Muscle Injury Classification (BAMIC) describes an MRI classification system with clearly defined, anatomically focused classes based on the site of injury: (a) myofascial, (b) muscle–tendon junction or (c) intratendinous; and the extent of the injury, graded from 0 to 4. However, there are no clinical guidelines that link the specific diagnosis (as above) with a focused rehabilitation plan.

Objective

We present an overview of the general principles of, and rationale for, exercise-based hamstring injury rehabilitation in British Athletics. We describe how British Athletics clinicians use the BAMIC to help manage elite track and field athletes with hamstring injury. Within each class of injury, we discuss four topics: clinical presentation, healing physiology, how we prescribe and progress rehabilitation and how we make the shared decision to return to full training. We recommend a structured and targeted diagnostic and rehabilitation approach to improve outcomes after hamstring injury.