Table 9 Summary of included studies stratified by results and methods used to evaluate injury mechanism

Askling et al. [19]^a

15

Investigate the injury mechanism, location and other factors related to acute, first-time hamstring injuries in dancers.

Interview, clinical and MRI examination.

11

Injury occurred while performing a slow-hip flexion with the knee extended in all cases. The location of injuries was close to the ischial tuberosity and most commonly affected the SM (87%), quadratus femoris (87%) and adductor magnus (33%). There were no significant findings in clinical or MRI examinations to determine return to preinjury level.

Stretching movements with simultaneous hip flexion and knee extension can cause a specific type of hamstring injury.

Askling et al. [31]^a

30

Continued investigation of the injury location and recovery time for hamstring injuries in dancers.

Interview, clinical and MRI examination.

10

In all cases, injury occurred close to the ischial tuberosity while the hip was flexed and the knee extended, most commonly in the SM. 47% of the subjects ended their sports activity and there was no significant parameter during clinical or MRI examinations to predict time until return to sport.

Extensive hip flexion with the knee extended can cause a specific type of hamstring injury near the ischial tuberosity.

Sallay et al. [45]

12

Define the injury mechanism and present pathological changes, functional limitations and preventive measures in water skiers.

Interview, clinical examination. In five cases, MRI and in one CT scan.

8

The situation varied although injury occurred due to extensive hip flexion with an extended knee. The injuries were located proximal to the posterior thigh and time until return to sport varied from three months to 1.5 years.

Rapid stretching of the hamstrings can cause a hamstring injury.

Hanley et al. [35]

17

Analyse the work done by the lower limb in world-class race walkers.

Race walking on a 45 m long track, with force plates to measure ground reaction forces, at competition speeds, captured at 100 Hz.

9

Most energy was generated by the extensors and flexors of the hip and during the late stance phase from the ankle plantarflexors. The knee flexors performed the most negative work and absorbed energy during the swing phase.

Injury is most likely to occur during the swing phase due to the negative work performed here which is increased by the straight knee during the first half of the stance phase.

Heiderscheit et al. [37]

1

Identify the time of injury in the gait cycle and the associated biomechanics of a hamstring injury.

Thirty-four reflective markers while running on a treadmill captured at 120 Hz. Toe markers were used to determine ground contact.

8

Based on the first signs of injury, 130 ms of the late swing phase was where the injury occurred. Moreover, during this phase, the biceps femoris reached peak musculotendon length.

The biceps femoris is probably injured during the late swing phase due to eccentric workload.

Fiorentino et al. [32]^a

14

To create and validate a model of the BFlh from MRI-obtained information to predict local tissue strain during sprinting.

A model of the biceps femoris long head was made after measuring dimensions using an MRI camera. The model was validated and then used to perform a forward dynamic simulation of sprinting at different speeds.

8

By comparing in-vivo tissue strain from dynamic MRI experiments, the model used was shown to be working. Sprinting simulations showed the highest tissue strain in the BFlh at the proximal tendinous junctions which increased with increased sprinting speed.

The performed simulations showed non-uniform strain of the local fibres of the Bflh during the late swing phase which was predicted to increase with increased running speed.

Higashihara et al. [39]

8

To investigate differences in hamstring muscle kinematics during sprinting with different positions of the trunk

Thirty-four reflective markers captured at 200 Hz while the subjects ran two maximum-effort sprints, one with forward trunk lean and the other with an upright posture.

8

The forward trunk lean showed higher musculotendon length during the stance phase than upright running. Moreover, the late stance phase showed the highest positive musculotendon lengthening velocity with significantly higher values during the forward trunk lean.

Sprinting with a forward trunk lean causes the hamstrings to be more susceptible to injury during the stance phase.

Mann et al. [28]^a

15

To help increase the knowledge of the kinematics during the ground phase of running.

Subjects were marked at anatomical landmarks and then had 40 m to reach maximum speed before being filmed at 150 frames/second. At least three trials/person.

8

During the stance phase, hip extensors performed concentric work from touchdown into the mid-support phase where activity shifted to the hip flexors which performed eccentric work through take-off. Muscles around the knee were dominated by flexors from touchdown to mid-support where dominance shifted to extensors, both performing eccentric work followed by concentric. At take-off, the flexors again performed eccentric work. Through the stance phase, plantar flexors were active and performed eccentric followed by concentric work.

Injury may occur because of the large forces working on the hamstrings when the foot touches the ground.

Schache et al. [46]

1

Compare the work performed by the different hip extensors and knee flexors during sprinting, as well as investigating asymmetries. Moreover, to compare the load on the hamstrings in different movements and before and after an injury.

Thirty-six reflective markers captured at 120 Hz while running at different speeds on a track containing force plates before suffering a hamstring injury on the 10th sprint.

8

During the terminal swing phase, the hamstring contributed to hip extension and knee flexion and peak force was shown to be greatest there while sprinting. After the hamstring injury occurred, the hamstring was unable to perform eccentric actions.

Because of the eccentric work performed during terminal swing, the hamstrings are most probably injured in this phase.

Schache et al. [47]

1

Investigate asymmetries before, the biomechanical response to and timing of an injury.

A previously injured athlete ran nine 30 m sprints with reflective markers mounted on him, while captured at 120 Hz, on a running track with two force plates before suffering a hamstring injury on the tenth sprint.

8

The first sign of injury was seen during the stance phase, but, due to neuromuscular latency, the calculated time of injury is prior to foot strike. Biomechanical asymmetries were seen in trials prior to the injury.

When sprinting, the hamstrings are most susceptible to injury during the terminal swing phase because of the eccentric work performed there.

Sun et al. [51]

8

Investigate hamstring kinematics and load in sprinting.

Isokinetic strength was measured before sprint trials. Fifty-seven reflective markers on anatomical landmarks. Captured at 300 Hz during three to four maximum-effort sprints on a track. GRF through force plates.

8

During both the initial stance and late swing phase, the hamstrings were subject to increased loading through forces working in opposite directions when the hip was extending and the knee flexing at the same time.

Sprinting or high-speed locomotion forces work on the hamstrings at the knee and hip during both the initial stance and the late swing phase which may cause an injury.

Thelen et al. [52]

14

Help understand the hamstring injury mechanism by investigating the work of the hamstrings in sprinting.

Forty-seven reflective markers on anatomical landmarks. Running on a treadmill at different speeds recorded at 200 Hz.

7

The peak length of the hamstrings was measured during the late swing phase with the biceps femoris being significantly higher and occurring later than the other muscles in the hamstring muscle group. However, no significant difference was found depending on sprinting speeds.

The greatest peak length is found in the biceps femoris during the late swing while sprinting, which is why hamstring injuries are most likely to occur there.

Wan et al. [53]

20

To investigate whether hamstring flexibility relates to peak hamstring muscle strain during sprinting.

Flexibility was measured with a passive straight leg raise after a sufficient warm-up. Sprinting kinematics were measured with reflective markers on anatomical landmarks and filmed at 200 frames/second while performing 20-25 m sprints. Bilateral isokinetic strength tests were also performed.

9

Peak muscle strain of all the hamstring muscles were recorded during the late swing phase and correlated negatively to hamstring flexibility. No gender differences were recorded. The strain in the BFlh and ST was higher than in the SM.

In sprinting, the hamstrings exhibit injury potential during the late swing phase.

Chumanov et al. [34]

12

Compare the hamstring mechanics in the swing and stance phase during sprinting.

Forty-five reflective markers along with surface electrodes, the latter placed on seven muscles of the lower right extremity, were mounted on the subjects before running on a treadmill at different speeds.

9

Eccentric contraction was measured in the hamstring during the swing phase before switching to concentric contraction during late swing which lasted through the stance phase. Increased sprinting speed meant an increased load for the biceps femoris.

The late swing phase is more injury prone than the stance phase during sprinting.

Hanley et al. [36]

20

To investigate the lower extremity during race walking.

Race walking on a 45 m track at competitive speed while filmed at 100 Hz and walking over force plates with surface electrodes on seven muscles of the lower right extremity.

9

Hip extensors during late swing and early stance along with ankle plantarflexors during late stance were the most important in producing energy. Great negative work was seen by knee flexors during the swing phase.

The risk of injury to the hamstrings is highest during the swing phase, due to the negative work performed there.

Higashihara et al. [38]

13

Investigate the hamstring injury mechanism by analysing peak musculotendon length and EMG activity during sprinting.

Forty  m acceleration was allowed on a synthetic track. Thirty-four reflective markers captured at 200 Hz. Surface electrodes on the muscle bellies of BFlh and ST with one on the fibular head for reference.

8

For the biceps femoris, the maximum length and peak EMG activity occurred at the same time during the late swing phase. For the ST, the highest EMG activity was measured before it reached its maximum length.

The hamstrings are most likely to be injured during the late swing phase while sprinting.

Montgomery III et al. [33]^a

30

Investigate EMG activity of muscles around the hip and knee while running at different speeds.

Needle electrodes were placed in three to eight muscles before performing runs at self-determined speeds in front of a high-speed camera.

8

The quadriceps had its major activity during the early stance as knee extensors, hamstrings were active in both knee flexion and hip extension during two to three periods of the gait cycle. Hip flexion was mainly performed by the rectus femoris during stance and iliacus during early-middle swing.

The hamstrings are injured during the swing phase due to eccentric contraction, but the different muscles of the hamstring muscle group are not susceptible at exactly the same time.

Ono et al. [41]

12

Investigate when a hamstring injury occurs by estimating tensile force during sprinting.

Reflective markers, high-speed cameras, force plates and surface electrodes were used to sample data from the subjects while running at maximum speeds on a 50 m track. A maximum voluntary contraction was used as an EMG reference.

8

Peak values for strain of the hamstring were shown during late swing with the highest values in the ST. The BFlh peak EMG activity took place directly after the foot touched the ground.

The BFlh is most likely to be injured during the early stance phase.

Padulo et al. [42]

12

Investigate the hamstring during movements with different types of muscle contraction.

Biceps femoris EMG activity was measured by surface electrodes and subjects were filmed with a high-speed camera while performing a counter-movement jump, squat jump and landing from a 45 cm high box. A maximum voluntary contraction was used as a reference value for the EMG.

8

When comparing a counter-movement jump with a squat jump and the braking phase of a landing, the biceps femoris showed lower activation, in both the concentric and eccentric phases of the counter-movement jump.

A pure eccentric or concentric movement gives rise to higher neuromuscular activity than a stretch-shortening exercise.

Prior et al. [43]

22

Investigate how trunk and pelvis positions affect the muscles of the thigh and hip while standing on one leg.

Markers, high-speed cameras and surface EMG of eight different muscles on both body halves was measured with the subject standing on one leg in different posture and pelvic positions.

14

When comparing anterior with posterior trunk sway during a one-legged stance, the muscles situated in a posterior position in the sagittal plane increased their activity as the anterior muscles decreased their activity. When swaying to the opposite side compared with the same side as the stance leg, the lateral hip abductor activity increased. A lateral drop of the pelvis, compared with a rise, reduced hip abductor activity while the hamstring, adductor longus and vastus lateralis increased their activity.

Trunk and pelvic positions affect the activation of the muscles around the hip and may increase the risk of injury.

Ruan et al. [44]

12

Investigate the effect of static stretching on hamstring injury risk.

Surface EMG, reflective markers, high-speed cameras and force plates collected data to compare parameters before and after a passive static stretch of the hamstrings.

8

The static stretch increased maximum BFlh length without affecting knee flexion torque. It also reduced peak GRF during the early stance phase and hamstring activation during the late swing phase.

The effects of static stretching during both the late swing and early stance phase may help reduce hamstring injuries.

Schache et al. [48]

7

Investigate the loading of the different muscles of the hamstring muscle group during sprinting.

A 110 m running track with embedded force plates was used. Subjects ran maximum sprints with 50 reflective markers captured at 250 Hz while having surface electrodes mounted on the hamstrings with a reference one on the tibial shaft.

8

All hamstring muscles reached their peak values regarding strain and force produced during terminal swing where they also performed negative work. The highest strain was found in the BFlh, the greatest lengthening velocity was found in ST and the highest force was found to be produced by SM which also performed the most work, both negative and positive.

The hamstrings are most likely to be injured during the terminal swing phase.

Yu et al. [54]

20

Investigate hamstring kinematics and activation to obtain knowledge of the hamstring injury mechanism.

Surface electrodes were placed on the dominant semimembranosus and biceps femoris along with bilateral reflective markers before maximum sprints were performed on an indoor track.

9

During both the late stance and late swing phase, the hamstring contracted eccentrically. The eccentric contraction speed showed a significantly higher peak value during the late swing. However, the peak value musculotendon lengths were significantly higher during the late stance.

The hamstrings may suffer an injury because of an eccentric contraction during both the late swing and late stance phases.

Jones et al. [40]

20

Investigate how fatigue affects muscle strength in football players from Africa.

Athletes performed a maximum concentric knee extension and maximum eccentric knee flexion before, during and after a workout protocol that simulates the fatigue of a football game.

9

The workout protocol generated significantly lower concentric quadriceps and eccentric hamstring strength. Moreover, the ratio between them (eccentric hamstring:concentric quadriceps) decreased significantly.

Fatigue-induced eccentric strength deficiency may be the reason for hamstring injuries in football players.

Schuermans et al. [49]

54

Investigate how synergistic work by muscles of the hamstring muscle group affects hamstring injuries.

Twenty-seven uninjured and 27 previously injured athletes underwent an MRI scan of the hamstring before and after an eccentric exercise.

10

The formerly injured group had a significantly more symmetrical muscle recruitment which corresponds to less economic muscle activation. Moreover, the group that had previously suffered an injury showed lower eccentric strength endurance.

Hamstring injuries may be related to the synergistic recruitment of the different muscles.

Schuermans et al. [50]

54

To identify the risk of future hamstring injuries with the help of mfMRI.

All players underwent an mfMRI scan before and after an eccentric hamstring exercise. Hamstring injuries were then registered for the following 1.5 seasons.

11

A first-time hamstring injury was associated with a high metabolic response and a proportionally higher biceps femoris contribution. A re-injury was associated with lower eccentric hamstring endurance.

An increase in the metabolic activity of the BF is predictive of sustaining an index hamstring injury. Concerning re-injuries, lower eccentric endurance capacity is able to predict a recurrent hamstring injury.