Acute hamstring injuries in Swedish elite sprinters and jumpers: a prospective randomised controlled clinical trial comparing two rehabilitation protocols.

BACKGROUND: Hamstring strain is a common injury in sprinters and jumpers, and therefore time to return to sport and secondary prevention become of particular concern. OBJECTIVE: To compare the effectiveness of two rehabilitation protocols after acute hamstring injury in Swedish elite sprinters and jumpers by evaluating time needed to return to full participation in the training process. STUDY DESIGN: Prospective randomised comparison of two rehabilitation protocols. METHODS: Fifty-six Swedish elite sprinters and jumpers with acute hamstring injury, verified by MRI, were randomly assigned to one of two rehabilitation protocols. Twenty-eight athletes were assigned to a protocol emphasising lengthening exercises, L-protocol, and 28 athletes to a protocol consisting of conventional exercises, C-protocol. The outcome measure was the number of days to return to full training. Re-injuries were registered during a period of 12 months after return. RESULTS: Time to return was significantly shorter for the athletes in the L-protocol, mean 49 days (1SD±26, range 18-107 days), compared with the C-protocol, mean 86 days (1SD±34, range 26-140 days). Irrespective of protocol, hamstring injuries where the proximal free tendon was involved took a significantly longer time to return than injuries that did not involve the free tendon, L-protocol: mean 73 vs 31 days and C-protocol: mean 116 vs 63 days, respectively. Two reinjuries were registered, both in the C-protocol. CONCLUSIONS: A rehabilitation protocol emphasising lengthening type of exercises is more effective than a protocol containing conventional exercises in promoting time to return in Swedish elite sprinters and jumpers.

Acute hamstring injuries in Swedish elite football: a prospective randomised controlled clinical trial comparing two rehabilitation protocols.

BACKGROUND: Hamstring injury is the single most common injury in European professional football and, therefore, time to return and secondary prevention are of particular concern. OBJECTIVE: To compare the effectiveness of two rehabilitation protocols after acute hamstring injury in Swedish elite football players by evaluating time needed to return to full participation in football team-training and availability for match selection. STUDY DESIGN: Prospective randomised comparison of two rehabilitation protocols. METHODS: Seventy-five football players with an acute hamstring injury, verified by MRI, were randomly assigned to one of two rehabilitation protocols. Thirty-seven players were assigned to a protocol emphasising lengthening exercises, L-protocol and 38 players to a protocol consisting of conventional exercises, C-protocol. The outcome measure was the number of days to return to full-team training and availability for match selection. Reinjuries were registered during a period of 12 months after return. RESULTS: Time to return was significantly shorter for the players in the L-protocol, mean 28 days (1SD±15, range 8-58 days), compared with the C-protocol, mean 51 days (1SD±21, range 12-94 days). Irrespective of protocol, stretching-type of hamstring injury took significantly longer time to return than sprinting-type, L-protocol: mean 43 vs 23 days and C-protocol: mean 74 vs 41 days, respectively. The L-protocol was significantly more effective than the C-protocol in both injury types. One reinjury was registered, in the C-protocol. CONCLUSIONS: A rehabilitation protocol emphasising lengthening type of exercises is more effective than a protocol containing conventional exercises in promoting time to return in Swedish elite football.

Non-uniform displacement within the Achilles tendon during passive ankle joint motion.

PURPOSE: An initial step in the understanding of Achilles tendon dynamics is to investigate the effects of passive motion, thereby minimising muscle activation and reducing internal joint forces. Internal tendon dynamics during passive ankle joint motion have direct implications for clinical rehabilitation protocols after Achilles tendon surgery. The aim of this study was to test the hypothesis that tendon tissue displacement is different in different layers of the Achilles tendon during controlled passive ankle joint movements. METHODS: Ultrasound imaging was conducted on the right Achilles tendon of nine healthy recreationally active males. Standardised isokinetic passive dorsi-plantar-flexion movements were performed with a total range of motion of 35°. The tendon was divided into superficial, central and deep layers in the resulting B-mode ultrasound images viewed in the sagittal plane. A block-matching speckle tracking algorithm was applied post-process, with kernels for the measurement of displacement placed in each of the layers. RESULTS: The mean (SD) displacement of the Achilles tendon during passive dorsiflexion was 8.4 (1.9) mm in the superficial layer, 9.4 (1.9) mm in the central portion and 10.4 (2.1) mm in the deep layer, respectively. In all cases, the movement of the deep layer of the tendon was greater than that of the superficial one (P < 0.01). CONCLUSIONS: These results, achieved in vivo with ultrasonographic speckle tracking, indicated complex dynamic differences in different layers of the Achilles tendon, which could have implications for the understanding of healing processes of tendon pathologies and also of normal tendon function.

Elite golfers' kinematic sequence in full-swing and partial-swing shots.

The aim of this study was to investigate whether kinematic proximal-to-distal sequencing (PDS) and speed-summation are common characteristics of both partial and full-swing shots in golf players of different skill levels and genders. A total of 45 golfers participated, 11 male tournament professionals, 21 male and 13 female elite amateurs. They performed partial shots with a wedge to targets at three submaximal distances, 40, 55 and 70 m, and full-swing shots with a 5 iron and a driver for maximal distance. Pelvis, upper torso and hand movements were recorded in 3D with an electromagnetic tracking system (Polhemus Liberty) at 240 Hz and the magnitude of the resultant angular velocity vector of each segment was computed. The results showed a significant proximal-to-distal temporal relationship and a concomitant successive increase in maximum (peak) segment angular speed in every shot condition for both genders and levels of expertise. A proximal-to-distal utilization of interaction torques is indicated. Using a common PDS movement strategy in partial and full-swing golf shots appears beneficial from mechanical and control points of view and could serve the purpose of providing both high speed and accuracy.

Trunk muscle reactions to sudden unexpected and expected perturbations in the absence of upright postural demand.

The aim was to increase the understanding of the multifunctional role of the trunk muscles in spine control, particularly transversus abdominis (TrA). In 11 healthy males, intramuscular fine-wire electromyography (EMG) was obtained bilaterally from TrA, obliquus externus (OE), rectus abdominis (RA) and erector spinae (ES). The subjects lay on their right side on a horizontal swivel-table with immobilized pelvis and lower limbs and the trunk strapped to a movable platform. Unexpected or expected release of loads attached to the table by steel cables produced a perturbation inducing either trunk flexion or extension. The timing and the amplitude of activation of TrA were independent of direction of induced trunk movement. Furthermore, timing of TrA activation was simultaneous to or later than that of the more superficial abdominal muscles. Expectation of the perturbation caused a general shortening of onset latencies. The results indicate a direction independent function of TrA in lumbar spine control. Balancing the trunk vertically appears to add specific demands, since the recruitment of TrA in relation to the other abdominal muscles differed from earlier experiments in standing.

Sway-dependent modulation of the triceps surae H-reflex during standing.

Previous research has shown that changes in spinal excitability occur during the postural sway of quiet standing. In the present study, it was of interest to examine the independent effects of sway position and sway direction on the efficacy of the triceps surae Ia pathway, as reflected by the Hoffman (H)-reflex amplitude, during standing. Eighteen participants, tested under two different experimental protocols, stood quietly on a force platform. Percutaneous electrical stimulation was applied to the posterior tibial nerve when the position and direction of anteroposterior (A-P) center of pressure (COP) signal satisfied the criteria for the various experimental conditions. It was found that, regardless of sway position, a larger amplitude of the triceps surae H-reflex (difference of 9-14%; P = 0.005) occurred when subjects were swaying in the forward compared with the backward direction. The effects of sway position, independent of the sway direction, on spinal excitability exhibited a trend (P = 0.075), with an 8.9 +/- 3.7% increase in the H-reflex amplitude occurring when subjects were in a more forward position. The observed changes to the efficacy of the Ia pathway cannot be attributed to changes in stimulus intensity, as indicated by a constant M-wave amplitude, or to the small changes in the level of background electromyographic activity. One explanation for the changes in reflex excitability with respect to the postural sway of standing is that the neural modulation may be related to the small lengthening and shortening contractions occurring in the muscles of the triceps surae.

Trunk muscle coordination in reaction to load-release in a position without vertical postural demand.

The aim of this study was to investigate the coordination between the innermost muscle layer of the ventro-lateral abdominal wall, the transversus abdominis (TrA), and other trunk muscles, in reaction to a load-release without the postural demand of keeping the trunk upright. Eleven healthy male volunteers participated. Intramuscular fine-wire electromyography (EMG) was obtained bilaterally from the TrA, rectus abdominis (RA), obliquus externus (OE) and erector spinae (ES) muscles. The subjects lay on their right side on a horizontal swivel-table with immobilized pelvis and lower limbs and with the trunk strapped to a movable platform allowing for trunk flexion and extension. Subjects maintained trunk flexion or extension at different force levels against a static resistance, which was suddenly released. They were instructed to resume the start position as fast as possible. EMG signals were analysed with respect to amplitude and timing of muscle activation. Following released static flexion, TrA increased its activity in synergy with ES. Also in released static extension, TrA increased its activity, but now in synergy with RA and OE. The direction-independent activation of TrA indicates a role of this muscle in controlling inter-segmental movements of the lumbar spine. This function was not accompanied by an early activation of TrA as has been shown previously for trunk perturbations in standing, i.e. a situation with an additional demand of maintaining the trunk posture upright against gravity.

Differential control of abdominal muscles during multi-directional support-surface translations in man.

The current study aimed to understand how deep and superficial abdominal muscles are coordinated with respect to activation onset times and amplitudes in response to unpredictable support-surface translations delivered in multiple directions. Electromyographic (EMG) data were recorded intra-muscularly using fine-wire electrodes inserted into the right rectus abdominis (RA), obliquus externus (OE), obliquus internus (OI) and transversus abdominis (TrA) muscles. Twelve young healthy male subjects were instructed to maintain their standing balance during 40 support surface translations (peak acceleration 1.3 m s(-2); total displacement 0.6 m) that were counter-balanced between four different directions (forward, backward, leftward, rightward). Differences between abdominal muscles in EMG onset times were found for specific translation directions. The more superficial RA (backward translations) and OE (forward and leftward translations) muscles had significantly earlier EMG onsets compared to TrA. EMG onset latencies were dependent on translation direction in RA, OE and OI, but independent of direction in TrA. EMG amplitudes in RA and OE were dependent on translation direction within the first 100 ms of activity, whereas responses from the two deeper muscles (TrA and OI) were independent of translation direction during this interval. The current results provide new insights into how abdominal muscles contribute to postural reactions during human stance. Response patterns of deep and superficial abdominal muscles during support surface translations are unlike those previously described during upper-body perturbations or voluntary arm movements, indicating that the neural mechanisms controlling individual abdominal muscles are task-specific to different postural demands.

Acute first-time hamstring strains during high-speed running: a longitudinal study including clinical and magnetic resonance imaging findings.

BACKGROUND: Hamstring muscle strain is one of the most common injuries in sports. Still, knowledge is limited about the progression of clinical and magnetic resonance imaging characteristics and their association with recovery time in athletes. HYPOTHESIS: Knowing the anatomical location and extent of an acute first-time hamstring strain in athletes is critical for the prognosis of recovery time. STUDY DESIGN: Case series (prognosis); Level of evidence, 2. METHODS: Eighteen elite sprinters with acute first-time hamstring strains were prospectively included in the study. All subjects were examined, clinically and with magnetic resonance imaging, on 4 occasions after injury: at day 2 to 4, 10, 21, and 42. The clinical follow-up period was 2 years. RESULTS: All sprinters were injured during competitive sprinting, and the primary injuries were all located in the long head of the biceps femoris muscle. There was an association between the time to return to pre-injury level (median, 16; range, 6-50 weeks) and the extent of the injury, as indicated by the magnetic resonance imaging parameters. Involvement of the proximal free tendon, as estimated by MRI, and proximity to the ischial tuberosity, as estimated both by palpation and magnetic resonance imaging, were associated with longer time to return to pre-injury level. CONCLUSION: Careful palpation during the first 3 weeks after injury and magnetic resonance imaging investigation performed during the first 6 weeks after injury provide valuable information that can be used to predict the time to return to pre-injury level of performance in elite sprinting.

Central and peripheral contributions to fatigue in relation to level of activation during repeated maximal voluntary isometric plantar flexions.

This study aimed to investigate central and peripheral contributions to fatigue during repeated maximal voluntary isometric plantar flexions (MVCs). Changes in joint torque, level of activation (LOA), resting twitch amplitude (RT), electromyographic signals (EMG), and presynaptic inhibition of Ia afferents were investigated during 9 bouts of 10 MVCs. MVCs lasted for 2 s and were separated by 1 s. The interval between bouts was 10 s. Electrical stimulation was applied to the tibial nerve; at rest to evoke RTs, M waves, and two (1.5-s interval) H reflexes; with the soleus EMG at 30% of that during MVC to evoke M waves and two H reflexes; and during MVCs to measure LOA. Over the nine bouts, LOA decreased by 12.6% and RT by 16.2%. EMG root mean square during MVCs remained unchanged for the soleus and tibialis anterior muscles, but it decreased for medial gastrocnemius. Peripheral fatigue (decrease in RT) was positively correlated to LOA, whereas central fatigue (decrease in LOA) was not. Depression of both H reflexes suggests that presynaptic inhibition after the first bout was partly induced by homosynaptic postactivation depression of the Ia terminal. The H-reflex-to-M-wave ratio increased with fatigue in both passive and active states, with no change in the ratio of the second H reflex to the first, thereby indicating a decrease of presynaptic inhibition during fatigue. The results indicate that both central and peripheral mechanisms contributed to the fatigue observed during repeated MVCs and that the development of peripheral fatigue was influenced by the level of voluntary activation and initial plantar flexor torque.

Variations in the soleus H-reflex as a function of activation during controlled lengthening and shortening actions.

The effect of soleus activation on the soleus H-reflex was investigated during controlled lengthening and shortening of the plantar flexor muscles. Maximal H-reflexes and M-waves were evoked at the same muscle length (ankle angle 90 degrees ) during lengthening and shortening (ankle angular velocity 5 degrees s(-1)) with soleus either passive or with its electromyographic activity at 10, 20 and 30% of that during a maximal voluntary isometric plantar flexion. In passive trials, the H(MAX):M(MAX) ratio during lengthening was lower than during shortening. In active trials at 10 and 20%, the H(MAX):M(MAX) ratio tended to be lower during lengthening than shortening. Within the active trials, H(MAX):M(MAX) ratios were not different between the three levels of soleus activation, neither for lengthening nor shortening actions. When all active trials were pooled, the lengthening H(MAX):M(MAX) ratio was significantly lower than the shortening one. In lengthening, the H(MAX):M(MAX) ratio increased in the active with respect to the passive condition, whereas no change occurred in active with respect to the passive shortening. These results indicate action type specificity in the way the Ia-excitatory effect is modulated as the soleus muscle is voluntarily activated.

The mechanics of back-extensor torque production about the lumbar spine.

The purpose of this study was to develop and evaluate a biomechanical model of lumbar back extension over a wide range of positions for the lumbar spine, incorporating the latest information on muscle geometry and intra-abdominal pressure (IAP). Analysis of the Visible Human data was utilised in order to obtain anatomical information unavailable from the literature and magnetic resonance imaging was used to generate subject-specific anatomical descriptions. The model was evaluated by comparisons with measured maximal voluntary static back-extension torques. Predicted maximal specific muscle tensions agreed well with in vitro measurements from the literature. When modelling the maximal static back-extension torque production, it was possible to come fairly close to simultaneous equilibrium about all the lumbar discs simply by a uniform muscle activation of all back-extensor muscles (the caudal part showed, however, less agreement). This indicates that equilibrium in the lumbar spine is mainly regulated by passive mechanical properties, e.g. muscle length changes due to postural changes, rather than due to complex muscle coordination, as earlier proposed. The model showed that IAP (measured during torque exertions) contributes about 10% of the total maximal voluntary back-extensor torque and that it can unload the spine from compression. The spinal unloading effect from the IAP was greatest with the spine held in a flexed position. This is in opposition to the effects of changed muscle lever arm lengths, which for a given load would give the largest spinal unloading in the extended position. These findings have implications for the evaluation of optimal lifting techniques.

Intra-abdominal pressure response to multidirectional support-surface translation.

A complex response of the trunk muscles occurs to restore equilibrium in response to movement of the support surface. Intra-abdominal pressure (IAP) is considered to contribute to control of the trunk. This study investigated the contribution of IAP to the postural response to multidirection support-surface translation. IAP was recorded with a thin-film pressure transducer inserted via the nose into the stomach and trunk motion was recorded with an optoelectronic system with markers over the spinous process of L1. A pattern of trunk movement was recorded in response to the support-surface translations that was consistent with a 'hip' strategy of postural control. The trunk moved in a manner appropriate to move the centre of gravity over the new base of support. IAP was increased with movement in each direction, but varied in timing and amplitude between translation directions. In general, the IAP was greater with translations in the sagittal plane compared to the frontal plane and was initiated earlier for translations in the backward direction. These data indicate that IAP contributes to the postural response associated with support-surface translation and suggest that this is consistent with stiffening the spine.

Sitting balance and effects of kayak training in paraplegics.

OBJECTIVES: The objectives of this study were to evaluate biomechanical variables related to balance control in sitting, and the effects of kayak training, in individuals with spinal cord injury. SUBJECTS: Twelve individuals with spinal cord injury were investigated before and after an 8-week training period in open sea kayaking, and 12 able-bodied subjects, who did not train, served as controls. METHODS: Standard deviation and mean velocity of centre of pressure displacement, and median frequency of centre of pressure acceleration were measured in quiet sitting in a special chair mounted on a force plate. RESULTS: All variables differed between the group with spinal cord injury, before training, and the controls; standard deviation being higher and mean velocity and median frequency lower in individuals with spinal cord injury. A significant training effect was seen only as a lowering of median frequency. CONCLUSION: The results indicate that individuals with spinal cord injury may have acquired and consolidated an alternative strategy for balance control in quiet sitting allowing for only limited further adaptation even with such a vigorous training stimulus as kayaking.

Effects of prolonged vibration on H-reflexes, muscle activation, and dynamic strength.

UNLABELLED: Neural activation is generally lower during maximal voluntary lengthening compared with shortening and isometric muscle actions, but the mechanisms underlying these differences are unclear. In maximal voluntary isometric actions, reduced Ia-afferent input induced by prolonged tendon vibration has been shown to impair neural activation and strength. PURPOSE: This study aimed to investigate whether reducing Ia-afferent input influences neural activation in maximal voluntary dynamic muscle actions and, if so, whether it affects shortening and lengthening muscle actions differently. METHODS: Eight women participated in three familiarization sessions and two randomly ordered experiments. In one experiment, 30-min vibration at 100 Hz was applied to the Achilles tendon to decrease Ia-afferent input as measured by the H-reflex. In the control experiment, rest substituted the vibration. Root mean square EMG from plantar and dorsiflexor muscles and plantar flexor strength were measured during maximal voluntary plantar flexor shortening and lengthening actions (20°·s(-1)) before and after vibration and rest, respectively. Soleus H-reflexes and M-waves were elicited before each set of strength tests. RESULTS: The vibration caused a decrease in H-reflex amplitude by, on the average, 33%, but root mean square EMG and plantar flexor strength remained largely unaffected in both action types. CONCLUSIONS: The findings suggest that Ia-afferent input may not substantially contribute to maximal voluntary dynamic muscle strength of the plantar flexor muscles, as tested here, and thus, the results do not support the notion that Ia-afferent excitation would contribute differently to neural activation in maximal voluntary lengthening and shortening muscle actions.

Deep and superficial abdominal muscle activation during trunk stabilization exercises with and without instruction to hollow.

The deepest muscle of the human ventro-lateral abdominal wall, the Transversus Abdominis (TrA), has been ascribed a specific role in spine stabilization, which has motivated special core stability exercises and hollowing instruction to specifically involve this muscle. The purpose here was to evaluate the levels of activation of the TrA and the superficial Rectus Abdominis (RA) muscles during five common stabilization exercises performed in supine, bridging and four-point kneeling positions, with and without instruction to hollow, i.e. to continuously pull the lower part of the abdomen towards the spine. Nine habitually active women participated and muscle activity was recorded bilaterally from TrA and RA with intramuscular fine-wire electrodes introduced under the guidance of ultrasound. Results showed that subjects were able to selectively increase the activation of the TrA, isolated from the RA, with the specific instruction to hollow and that side differences in the amplitude of TrA activity, related to the asymmetry of the exercises, remained even after the instruction to hollow. The exercises investigated caused levels of TrA activation from 4 to 43% of that during maximal effort and can thus be used clinically to grade the load on the TrA when designing programs aiming at training that muscle.

Age-related changes in postural responses revealed by support-surface translations with a long acceleration-deceleration interval.

OBJECTIVES: The objectives were to examine (a) whether surface translations with a long, compared to a short, acceleration-deceleration interval could reveal more age-related differences in postural control and (b) whether age-related differences were associated with reactive or anticipatory postural mechanisms. METHODS: Ten older (66-81years) and ten young adults (22-39years) stood on a moveable platform that was unexpectedly translated in the backward direction. Subjects' electromyographic (EMG) and kinematic responses were recorded in response to translations with either a SHORT (100ms) or LONG (2s) acceleration-deceleration interval presented in either a predictable or random order. RESULTS: Age-related differences in kinematic postural responses were greater during LONG compared to SHORT translations. However, both LONG and SHORT translations elicited a similar change in EMG latencies and amplitudes between the older and young adults. No age effects on the presentation order (predictable or random) of the translations were observed. CONCLUSIONS: LONG compared to SHORT surface translations magnify the age-related kinematic but not the EMG changes in reactive postural control. The anticipatory component of postural control was not affected by age. SIGNIFICANCE: Translations with longer acceleration-deceleration intervals reveal more age-related differences in postural control, which are otherwise masked by the deceleration effects inherent to shorter translations.

Trunk muscle activation in a person with clinically complete thoracic spinal cord injury.

OBJECTIVE: The aim of this study was to assess if, and how, upper body muscles are activated in a person with high thoracic spinal cord injury, clinically classified as complete, during maximal voluntary contractions and in response to balance perturbations. METHODS: Data from one person with spinal cord injury (T3 level) and one able-bodied person were recorded with electromyography from 4 abdominal muscles using indwelling fine-wire electrodes and from erector spinae and 3 upper trunk muscles with surface electrodes. Balance perturbations were carried out as forward or backward support surface translations. RESULTS: The person with spinal cord injury was able to activate all trunk muscles, even those below the injury level, both in voluntary efforts and in reaction to balance perturbations. Trunk movements were qualitatively similar in both participants, but the pattern and timing of muscle responses differed: upper trunk muscle involvement and occurrence of co-activation of ventral and dorsal muscles were more frequent in the person with spinal cord injury. CONCLUSION: These findings prompt further investigation into trunk muscle function in paraplegics, and highlight the importance of including motor tests for trunk muscles in persons with thoracic spinal cord injury, in relation to injury classification, prognosis and rehabilitation.

Proximal hamstring strains of stretching type in different sports: injury situations, clinical and magnetic resonance imaging characteristics, and return to sport.

BACKGROUND: Hamstring strains can be of at least 2 types, 1 occurring during high-speed running and the other during motions in which the hamstring muscles reach extreme lengths, as documented for sprinters and dancers. HYPOTHESIS: Hamstring strains in different sports, with similar injury situations to dancers, also show similarities in symptoms, injury location, and recovery time. STUDY DESIGN: Case series (prognosis); Level of evidence, 4. METHODS: Thirty subjects from 21 different sports were prospectively included. All subjects were examined clinically and with magnetic resonance imaging (MRI). The follow-up period lasted until the subjects returned to or finished their sport activity. RESULTS: All injuries occurred during movements reaching a position with combined extensive hip flexion and knee extension. They were located proximally in the posterior thigh, close to the ischial tuberosity. The injuries were often complex, but 83% involved the semimembranosus and its proximal free tendon. Fourteen subjects (47%) decided to end their sports activity. For the remaining 16 subjects, the median time for return to sport was 31 weeks (range, 9-104). There were no significant correlations between specific clinical or MRI parameters and time to return to sport. CONCLUSIONS: In different sports, an injury situation in which the hamstring muscles reach extensive length causes a specific injury to the proximal posterior thigh, earlier described in dancers. Because of the prolonged recovery time associated with this type of injury, correct diagnosis, based on history and palpation, and adequate information to the subject are essential.

Acute first-time hamstring strains during slow-speed stretching: clinical, magnetic resonance imaging, and recovery characteristics.

BACKGROUND: Hamstring strains can be of 2 types with different injury mechanisms, 1 occurring during high-speed running and the other during stretching exercises. HYPOTHESIS: A stretching type of injury to the proximal rear thigh may involve specific muscle-tendon structures that could affect recovery time. STUDY DESIGN: Case series (prognosis); Level of evidence, 2. METHODS: Fifteen professional dancers with acute first-time hamstring strains were prospectively included in the study. All subjects were examined, clinically and with magnetic resonance imaging, on 4 occasions after injury: at day 2 to 4, 10, 21, and 42. The clinical follow-up period was 2 years. RESULTS: All dancers were injured during slow hip-flexion movements with extended knee and experienced relatively mild acute symptoms. All injuries were located proximally in the posterior thigh close to the ischial tuberosity. The injury involved the semimembranosus (87%), quadratus femoris (87%), and adductor magnus (33%). All injuries to the semimembranosus involved its proximal free tendon. There were no significant correlations between clinical or magnetic resonance imaging parameters and the time to return to preinjury level (median, 50 weeks; range, 30-76 weeks). CONCLUSION: Stretching exercises can give rise to a specific type of strain injury to the posterior thigh. A precise history and careful palpation provide the clinician enough information to predict a prolonged time until return to preinjury level. One factor underlying prolonged recovery time could be the involvement of the free tendon of the semimembranosus muscle.