The role of neuromuscular inhibition in hamstring strain injury recurrence

Hamstring strain injuries are amongst the most common and problematic injuries in a wide range of sports that involve high speed running. The comparatively high rate of hamstring injury recurrence is arguably the most concerning aspect of these injuries. A number of modifiable and nonmodifiable risk factors are proposed to predispose athletes to hamstring strains. Potentially, the persistence of risk factors and the development of maladaptations following injury may explain injury recurrence. Here, the role of neuromuscular inhibition following injury is discussed as a potential mechanism for several maladaptations associated with hamstring re-injury. These maladaptations include eccentric hamstring weakness, selective hamstring atrophy and shifts in the knee flexor torque-joint angle relationship. Current evidence indicates that athletes return to competition after hamstring injury having developed maladaptations that predispose them to further injury. When rehabilitating athletes to return to competition following hamstring strain injury, the role of neuromuscular inhibition in re-injury should be considered.

Knee flexor strength and biceps femoris activation deficits following hamstring strain injury

Background: Hamstring strain injuries (HSI) are prevalent in sport and re-injury rates have been high for many years. Maladaptation following HSI are implicated in injury recurrence however nervous system function following HSI has received little attention. Aim: To determine if recreational athletes with a history of unilateral HSI, who have returned to training and competition, will exhibit lower levels of voluntary activation (VA) and median power frequency (MPF) in the previously injured limb compared to the uninjured limb at long muscle lengths. Methods: Twenty-eight recreational athletes were recruited. Of these, 13 athletes had a history of unilateral HSI and 15 had no history of HSI. Following familiarisation, all athletes undertook isokinetic dynamometry testing and surface electromyography assessment of the biceps femoris long head and medial hamstrings during concentric and eccentric contractions at ± 180 and ± 60deg/s. Results: The previously injured limb was weaker at all contraction speeds compared to the uninjured limb (+180deg/s mean difference(MD) = 9.3Nm, p = 0.0036; +60deg/s MD = 14.0Nm, p = 0.0013; -60deg/s MD = 18.3Nm, p = 0.0007; -180deg/s MD = 20.5Nm, p = 0.0007) whilst VA was only lower in the biceps femoris long head during eccentric contractions (-60deg/s MD = 0.13, p = 0.0025; -180deg/s MD = 0.13, p = 0.0003). There were no between limb differences in medial hamstring VA or MPF from either biceps femoris long head or medial hamstrings in the injured group. The uninjured group showed no between limb differences with any of the tested variables. Conclusion: Previously injured hamstrings were weaker than the contralateral uninjured hamstring at all tested speeds and contraction modes. During eccentric contractions biceps femoris long head VA was lower in the previously injured limb suggesting neural control of biceps femoris long head may be altered following HSI. Current rehabilitation practices have been unsuccessful in restoring strength and VA following HSI. Restoration of these markers should be considered when determining the success of rehabilitation from HSI. Further investigations are required to elucidate the full impact of lower levels of biceps femoris long head VA following HSI on rehabilitation outcomes and re-injury risk.

Impact of previous hamstring strain injury on rate of torque and EMG development during eccentric contraction.

Background: Hamstring strain injuries (HSIs) are prevalent in sport and re-injury rates have been high for many years. Whilst much focus has centred on the impact of previous hamstring strain injury on maximal eccentric strength, high rates of torque development is also of interest, given the important role of the hamstrings during the terminal swing phase of gait. The impact of prior strain injury on neuromuscular function of the hamstrings during tasks requiring high rates of torque development has received little attention. The purpose of this study is to determine if recreational athletes with a history of unilateral hamstring strain injury, who have returned to training and competition, will exhibit lower levels of eccentric muscle activation, rate of torque development and impulse 30, 50 and 100ms after the onset of electromyographical or torque development in the previously injured limb compared to the uninjured limb. Methods: Twenty-six recreational athletes were recruited. Of these, 13 athletes had a history of unilateral hamstring strain injury (all confined to biceps femoris long head) and 13 had no history of hamstring strain injury. Following familiarisation, all athletes undertook isokinetic dynamometry testing and surface electromyography assessment of the biceps femoris long head and medial hamstrings during eccentric contractions at -60 and -1800.s-1. Results: In the injured limb of the injured group, compared to the contralateral uninjured limb rate of torque development and impulse was lower during -600.s-1 eccentric contractions at 50 (RTD, p=0.008; IMP, p=0.005) and 100ms (RTD, p=0.001; IMP p<0.001) after the onset of contraction. There was also a non-significant trend for rate of torque development during -1800.s-1 to be lower 100ms after onset of contraction (p=0.064). Biceps femoris long head muscle activation was lower at 100ms at both contraction speeds (-600.s-1, p=0.009; -1800.s-1, p=0.009). Medial hamstring activation did not differ between limbs in the injured group. Comparisons in the uninjured group showed no significant between limbs difference for any variables. Conclusion: Previously injured hamstrings displayed lower rate of torque development and impulse during eccentric contraction. Lower muscle activation was confined to the biceps femoris long head. Regardless of whether these deficits are the cause of or the result of injury, these findings have important implications for hamstring strain injury and re-injury and suggest greater attention be given to neural function of the knee flexors.

Acute hamstring strain injury in track and field athletes : a 3-year observational study at the Penn Relay Carnival

Purpose To observe the incidence rates of hamstring strain injuries (HSIs) across different competition levels and ages during the Penn Relays Carnival. Methods Over a 3-year period all injuries treated by the medical staff were recorded. The type of injury, anatomic location, event in which the injury occurred, competition level and demographic data were documented. Absolute and relative HSI (per 1000 participants) were determined and odds ratios (OR) were calculated between genders, competition levels and events. Results Throughout the study period 48,473 athletes registered to participate in the Penn Relays Carnival, with 118 HSIs treated by the medical team. High school females displayed lesser risk of HSI than high school males (OR = 0.55, p = 0.021), and masters athletes were more likely than high school (OR = 4.26, p < 0.001) and college (OR = 3.55, p = 0.001) level athletes to suffer a HSI. The 4x400m relay displayed a greater likelihood of HSI compared to the 4x100m relay (OR = 1.77, p = 0.008). Conclusions High school males and masters levels athletes are most likely to suffer HSI, and there is higher risk in 400m events compared to 100m events.

Is there a potential relationship between prior h amstring strain injury and increased risk for future a nterior cruciate ligament injury?

Hamstring strain injuries (HSIs) are the most prevalent injury in a number of sports, and while anterior cruciate ligament (ACL) injuries are less common, they are far more severe and have long-term implications, such as an increased risk of developing osteoarthritis later in life. Given the high incidence and severity of these injuries, they are key targets of injury preventive programs in elite sport. Evidence has shown that a previous severe knee injury (including ACL injury) increases the risk of HSI; however, whether the functional deficits that occur after HSI result in an increased risk of ACL injury has yet to be considered. In this clinical commentary, we present evidence that suggests that the link between previous HSI and increased risk of ACL injury requires further investigation by drawing parallels between deficits in hamstring function after HSI and in women athletes, who are more prone to ACL injury than men athletes. Comparisons between the neuromuscular function of the male and female hamstring has shown that women display lower hamstring-to-quadriceps strength ratios during isokinetic knee flexion and extension, increased activation of the quadriceps compared with the hamstrings during a stop-jump landing task, a greater time required to reach maximal isokinetic hamstring torque, and lower integrated myoelectrical hamstring activity during a sidestep cutting maneuver. Somewhat similarly, in athletes with a history of HSI, the previously injured limb, compared with the uninjured limb, displays lower eccentric knee flexor strength, a lower hamstrings-to-quadriceps strength ratio, lower voluntary myoelectrical activity during maximal knee flexor eccentric contraction, a lower knee flexor eccentric rate of torque development, and lower voluntary myoelectrical activity during the initial portion of eccentric contraction. Given that the medial and lateral hamstrings have different actions at the knee joint in the coronal plane, which hamstring head is previously injured might also be expected to influence the likelihood of future ACL. Whether the deficits in function after HSI, as seen in laboratory-based studies, translate to deficits in hamstring function during typical injurious tasks for ACL injury has yet to be determined but should be a consideration for future work.

Muscle activation patterns in the Nordic hamstring exercise: Impact of prior strain injury

This study aimed to determine: 1) the spatial patterns of hamstring activation during the Nordic hamstring exercise (NHE); 2) whether previously injured hamstrings display activation deficits during the NHE, and; 3) whether previously injured hamstrings exhibit altered cross-sectional area. Ten healthy, recreationally active males with a history of unilateral hamstring strain injury underwent functional magnetic resonance imaging (fMRI) of their thighs before and after 6 sets of 10 repetitions of the NHE. Transverse (T2) relaxation times of all hamstring muscles (biceps femoris long head, (BFlh); biceps femoris short head (BFsh); semitendinosus (ST); semimembranosus (SM)), were measured at rest and immediately after the NHE and cross-sectional area (CSA) was measured at rest. For the uninjured limb, the ST’s percentage increase in T2 with exercise was 16.8, 15.8 and 20.2% greater than the increases exhibited by the BFlh, BFsh and SM, respectively (p<0.002 for all). Previously injured hamstring muscles (n=10) displayed significantly smaller increases in T2 post-exercise than the homonymous muscles in the uninjured contralateral limb (mean difference -7.2%, p=0.001). No muscles displayed significant between limb differences in CSA. During the NHE, the ST is preferentially activated and previously injured hamstring muscles display chronic activation deficits compared to uninjured contralateral muscles.

Neuromuscular adaptations in football athletes with prior history of hamstring strain injury

Background: Hamstring strain injuries (HSI) are one of the most common injuries in a wide variety of running-sports, resulting in a considerable loss of competition and training time. One of the most problematic consequences regarding HSI is the recurrence rate and its non-decrease over the past decades, despite increasing evidence. Recent studies also found several maladaptations post-HSI probably due to neuromuscular inhibition and it has been proposed that these adaptations post-injury may contribute as risk factors for the injury-reinjury cycle and high recurrence rates. Furthermore it has been recently proposed not to disregard the inter-relationship between these adaptations and risk-factors post-injury in order to better understand the mechanisms of this complex injury. Objective: To determine, analyze and correlate neuromuscular adaptations in amateur football players with prior history of HSI per comparison to uninjured athletes in similar conditions. Methodology: Every participant was subjected to isokinetic concentric (60 and 240deg.sec) and eccentric (30 and 120deg.sec¯¹) testing, and peak torque, angle of peak torque and hamstrings to quadriceps (H:Q) conventional ratios were measured, myoelectrical activity of Bicep Femoris (BF) and Medial Hamstrings (MH) were also measured during isokinetic eccentric testing at both velocities and muscle activation percentages were calculated at 30, 50 and 100ms after onset of contraction. Furthermore active and passive knee extension, knee joint position sense (JPS) test, triple-hop distance (THD) test and core stability (flexors and extensors endurance, right and left side bridge test) were used and correlated. Results: Seventeen players have participated in this study: 10 athletes with prior history of HSI, composing the Hamstring injury group (HG) and 7 athletes without prior severe injuries as control group (CG). We found statistical significant differences between HG injured and uninjured sides in the BF myoelectrical activity at almost all times in both velocities and between HG injured and CG non-dominant sides at 100ms in eccentric 120deg.sec¯¹ velocity (p<.05). We found no differences in MH activity. Regarding proprioception we found differences between the HG injured and uninjured sides (p=.027). We found no differences in the rest of used tests. However, significant correlation between myoelectrical activation at 100ms in 120deg.sec¯¹ testing and JPS with initial position at 90º (r-.372; p=0.031) was found, as well as between isokinetic H:Q ratio at 240deg.sec and THD score (r=-.345; p=.045). Conclusion: We found significant differences that support previous research regarding neuromuscular adaptations and BF inhibition post-HSI. Moreover, to our knowledge, this was the first study that found correlation between these adaptations, and may open a door to new perspectives and future studies.

Effect of simvastatin on passive strain-induced skeletal muscle injury in rats

Introduction: HMG-CoA reductase inhibitors are the most frequently prescribed drugs for treatment of lipid imbalance, but they have side effects, such as myopathy. Our aim was to assess the effect of simvastatin on the inflammatory process induced by skeletal muscle injury. Methods: Rats were divided into experimental groups [control group, simvastatin (20 mg/kg) group, group treated with simvastatin (20 mg/kg) and subjected to injury, and group subjected to injury only]. Histological analysis and analyses of creatine kinase activity and C-reactive protein were performed. Results: Animals treated with simvastatin exhibited significantly greater morphological and structural skeletal muscle damage in comparison to the control group and injured animals without treatment. Conclusions: Although simvastatin has a small anti-inflammatory effect in the early stage after a muscle strain injury, the overall picture is negative, as simvastatin increases the extent of damage to muscle morphology. Further studies are needed. Muscle Nerve 46: 908-913, 2012

Infrared (810 nm) Low-Level Laser Therapy in Experimental Model of Strain-Induced Skeletal Muscle Injury in Rats: Effects on Functional Outcomes

Muscle strains are among the most prevalent causes for athletes absence from sport activities. Low-level laser therapy (LLLT) has recently emerged as a potential contender to nonsteroidal anti-inflammatory drugs in muscle strain treatment. In this work we investigated effects of LLLT and diclofenac on functional outcomes in the acute stage after muscle strain injury in rats. Muscle strain was induced by overloading the tibialis anterior muscle of rats during anesthesia. The injured groups received either no treatment, or a single treatment with diclofenac 30 min prior to injury, or LLLT (810 nm, 100 mW) with doses of 1, 3, 6 or 9 J, at 1 h after injury. Functional outcome measures included a walking index and assessment of electrically induced muscle performance. All treatments (except 9 J LLLT) significantly improved the walking index 12 h postinjury compared with the untreated group. The 3 J group also showed a significantly better walking index than the drug group. All treatments significantly improved muscle performance at 6 and 12 h. LLLT dose of 3 J was as effective as the pharmacological agent in improving functional outcomes in the early phase after a muscle strain injury in rats.

A novel device using the Nordic hamstring exercise to assess eccentric knee flexors strength : a reliability and retrospective injury study

STUDY DESIGN: Reliability and case-control injury study. OBJECTIVES: 1) To determine if a novel device, designed to measure eccentric knee flexors strength via the Nordic hamstring exercise (NHE), displays acceptable test-retest reliability; 2) to determine normative values for eccentric knee flexors strength derived from the device in individuals without a history of hamstring strain injury (HSI) and; 3) to determine if the device could detect weakness in elite athletes with a previous history of unilateral HSI. BACKGROUND: HSIs and reinjuries are the most common cause of lost playing time in a number of sports. Eccentric knee flexors weakness is a major modifiable risk factor for future HSIs, however there is a lack of easily accessible equipment to assess this strength quality. METHODS: Thirty recreationally active males without a history of HSI completed NHEs on the device on 2 separate occasions. Intraclass correlation coefficients (ICCs), typical error (TE), typical error as a co-efficient of variation (%TE), and minimum detectable change at a 95% confidence interval (MDC95) were calculated. Normative strength data were determined using the most reliable measurement. An additional 20 elite athletes with a unilateral history of HSI within the previous 12 months performed NHEs on the device to determine if residual eccentric muscle weakness existed in the previously injured limb. RESULTS: The device displayed high to moderate reliability (ICC = 0.83 to 0.90; TE = 21.7 N to 27.5 N; %TE = 5.8 to 8.5; MDC95 = 76.2 to 60.1 N). Mean±SD normative eccentric flexors strength, based on the uninjured group, was 344.7 ± 61.1 N for the left and 361.2 ± 65.1 N for the right side. The previously injured limbs were 15% weaker than the contralateral uninjured limbs (mean difference = 50.3 N; 95% CI = 25.7 to 74.9N; P < .01), 15% weaker than the normative left limb data (mean difference = 50.0 N; 95% CI = 1.4 to 98.5 N; P = .04) and 18% weaker than the normative right limb data (mean difference = 66.5 N; 95% CI = 18.0 to 115.1 N; P < .01). CONCLUSIONS: The experimental device offers a reliable method to determine eccentric knee flexors strength and strength asymmetry and revealed residual weakness in previously injured elite athletes.

Neuromuscular hamstring function in response to intermittent running and previous injury

This thesis examined the impact of previous hamstring injury and fatigue on the function of the hamstring muscles and their neural control. The work established the role of neuromuscular inhibition after hamstring injury and involved the development of a new field testing device for eccentric hamstring strength, which is now in high demand in elite sport worldwide. David has four peer-reviewed publications from this doctoral work.

Eccentric hamstring strength and hamstring injury risk in Australian footballers

Purpose Is eccentric hamstring strength and between limb imbalance in eccentric strength, measured during the Nordic hamstring exercise, a risk factor for hamstring strain injury (HSI)? Methods Elite Australian footballers (n=210) from five different teams participated. Eccentric hamstring strength during the Nordic was taken at the commencement and conclusion of preseason training and in season. Injury history and demographic data were also collected. Reports on prospectively occurring HSIs were completed by team medical staff. Relative risk (RR) was determined for univariate data and logistic regression was employed for multivariate data. Results Twenty-eight HSIs were recorded. Eccentric hamstring strength below 256N at the start of preseason and 279N at the end of preseason increased risk of future HSI 2.7 (relative risk, 2.7; 95% confidence interval, 1.3 to 5.5; p = 0.006) and 4.3 fold (relative risk, 4.3; 95% confidence interval, 1.7 to 11.0; p = 0.002) respectively. Between limb imbalance in strength of greater than 10% did not increase the risk of future HSI. Univariate analysis did not reveal a significantly greater relative risk for future HSI in athletes who had sustained a lower limb injury of any kind within the last 12 months. Logistic regression revealed interactions between both athlete age and history of HSI with eccentric hamstring strength, whereby the likelihood of future HSI in older athletes or athletes with a history of HSI was reduced if an athlete had high levels of eccentric strength. Conclusion Low levels of eccentric hamstring strength increased the risk of future HSI. Interaction effects suggest that the additional risk of future HSI associated with advancing age or previous injury was mitigated by higher levels of eccentric hamstring strength.

Biceps femoris long-head architecture: A reliability and retrospective injury study

Purpose: To determine i) the reliability of two-dimensional ultrasonography for the assessment of biceps femoris long head (BFlh) architectural characteristics; ii) if limbs with a history of strain injury in the BFlh display different architecture and eccentric strength compared to uninjured limbs. Methods: This case-control study (control [n=20], injured group [n=16], males) assessed the BFlh architecture at rest and during graded isometric contractions using two-dimensional ultrasonography. The control group were assessed three times (>24hrs apart) to determine reliability. Previously injured individuals were evaluated once. Results The assessment of BFlh architecture was highly reliable (intraclass correlations >0.90). Fascicle length (p<0.001; d range: 0.67 to 1.34) and fascicle length relative to muscle thickness (p<0.001; d range: 0.58 to 0.85) of the previously injured BFlh were significantly less than the contralateral uninjured BFlh at all intensities. Pennation angle of the previously injured BFlh was significantly greater (p<0.001; d range: 0.62 to 0.88) than the contralateral uninjured BFlh at all intensities. Eccentric strength in the previously injured limb was significantly lower than the contralateral limb (-15.4%; -52.5N; 95% CI=-28.45 to -76.23; p<0.001, d=0.56). Conclusion These data indicate that two-dimensional ultrasonography is reliable for assessing BFlh architecture at rest and during graded isometric contractions. Fascicle length, fascicle length relative to muscle thickness and pennation angle are significantly different in previously injured BFlh compared to an uninjured contralateral BFlh. Eccentric strength of the previously injured limb is also significantly lower than the uninjured contralateral limb. These findings have implications for rehabilitation and injury prevention practices which should consider altered architectural characteristics.

Is there evidence to support the use of the angle of peak torque as a marker of hamstring injury and re-injury risk?

Hamstring strain injuries are the predominant injury in many sports, costing athletes and clubs a significant financial and performance burden. Therefore the ability to identify and intervene with individuals who are considered at a high risk of injury is important. One measure which has grown in popularity as an outcome variable following hamstring intervention/prevention studies and rehabilitation is the angle of peak knee flexor torque. This current opinion article will firstly introduce the measure and the processes behind it. Secondly, this article will summarise how the angle of peak knee flexor torque has been suggested to measure hamstring strain injury risk. Finally various limitations will be presented and outlined as to how they may influence the measure. These include the lack of muscle specificity, the common concentric contraction mode of assessment, reliability of the measure, various neural contributions (such as rate of force development and neuromuscular inhibition) as well as the lack of prospective data showing any predictive value in the measure.