Estimation of Tensile Force in the Hamstring Muscles
during Overground Sprinting
Authors T. Ono1, A. Higashihara2, J. Shinohara3, N. Hirose4, T. Fukubayashi5 Affiliations Affiliation addresses are listed at the end of the article
Introduction
▼
Hamstring muscles are frequently injured during various sports activities that involve sprinting, such as football, rugby, and track and field events [1, 2, 21, 29]. It has been reported that the most acute cases of the injuries were involved with the biceps femoris long head (BFlh) muscle [2, 30] during the late swing or early stance phase of sprinting gait cycle [16, 23, 27]. Since these reports are based on injury histories from clinical interviews or two-dimensional video analysis of injury-related situations, it is not enough to have detailed data about the magnitude of voluntary contraction or musculotendon length of the hamstring muscles during sprinting. Previous studies have conducted three-dimensional (3D) motion analysis of treadmill sprinting [3, 7, 25] and revealed that the BFlh muscle was more sus-ceptible to a lengthening contraction injury dur-ing the late swdur-ing phase than the semitendinosus (ST) and semimembranosus (SM) muscle. While these reports provide beneficial information for interpreting hamstring strain injury mecha-nisms, only a limited number of existing studies
have examined the kinematics of overground sprinting.
Hamstring muscles form a multi-articular mus-cle group that crosses the hip and knee joints. These muscles work synergistically to produce hip extension and/or knee flexion torque. During the late swing phase and early stance phase of sprinting, the hamstring muscles contract eccen-trically to decelerate the forward swing of the leg and concentrically to push off the ground. How-ever, it has been demonstrated that each ham-string muscle has inherent morphological features [4, 29], leading to different functional properties even in the case of a single-joint movement [12, 15, 18]. From the results of previ-ous study using electromyography (EMG) and magnetic resonance imaging (MRI) [19, 20], it has been found that the BFlh and SM muscles were selectively recruited to deal with the hip joint movements during standing, bending forward and extending backward from the hip, and the ST muscle with lengthening knee flexion move-ment. It could thus be speculated that most acute cases of hamstring strains involving the BFlh would occur during the stance phase of sprinting,
accepted after revision June 24, 2014
Bibliography
DOI http://dx.doi.org/ 10.1055/s-0034-1385865 Published online: 2014 Int J Sports Med © Georg Thieme
Verlag KG Stuttgart · New York ISSN 0172-4622
Correspondence
Dr. Takashi Ono
Waseda University Faculty of Sports Sciences 2-579-15 Mikajima Tokorozawa 359-1192 Japan Tel.: + 81/42/451 1022 Fax: + 81/42/451 1022 ono-t@aoni.waseda.jp Key words ●▶ muscle strain ●▶ voluntary contraction ●▶ musculotendon length ●▶ ground reaction force ●▶ early stance phase
Abstract
▼
The purpose of this study was to identify the period of the gait cycle during which the ham-string muscles were likely injured by estimating the magnitude of tensile force in each muscle during overground sprinting. We conducted three-dimensional motion analysis of 12 male athletes performing overground sprinting at their maximal speed and calculated the ham-string muscle-tendon length and joint angles of the right limb throughout a gait cycle during which the ground reaction force was measured. Electromyographic activity during sprinting was recorded for the biceps femoris long head,
sem-itendinosus, and semimembranosus muscles of ipsilateral limb. We estimated the magnitude of tensile force in each muscle by using the length change occurred in the musculotendon and nor-malized electromyographic activity value. The study found a quick increase of estimated tensile force in the biceps femoris long head during the early stance phase of the gait cycle during which the increased hip flexion angle and ground reac-tion force occurred at the same time. This study provides quantitative data of tensile force in the hamstring muscles suggesting that the biceps femoris long head muscle is susceptible to a strain injury during the early stance phase of the sprinting gait cycle.
during which the hamstring muscles contract to push off the ground, thereby generating hip extension torque.
Muscle strain injuries occur when excessive tensile force is exerted on the voluntarily contracting muscle [9], i. e., they never occur only with a lengthening force without active muscle contraction or only with an intensive muscle contraction with-out a lengthening force [5]. In such situation, loading of stimu-lated muscle begins at a sarcomere length shorter than the length at which loading of the unstimulated muscle begins, thereby requiring greater total strain to be injured compared to the unstimulated muscle [26]. It has also been reported that the injuries occur at the muscle-tendon junctions within the lamina lucida, which is the serial component of myotendinous prepara-tions with the lowest breaking strength in stimulated muscle. In this study, we hypothesized that the magnitude of tensile force in the hamstring muscles during sprinting could be estimated by combining the magnitude of MTLs change and the quantity of EMG activities.
The purpose of this study was, first, to estimate the magnitude of tensile force in the hamstring muscles during overground sprinting by 3D motion analysis and EMG, and second, to iden-tify the period of the gait cycle during which the hamstring muscles were likely injured, as well as to characterize the biome-chanical conditions associated with the injurious situation.
Material and Methods
▼
Participants
A total of 12 healthy male athletes (mean ± SD; age = 21.5 ± 3.6 years; stature = 173.1 ± 5.2 cm; body mass = 70.3 ± 7.5 kg) were recruited from running-based sports, such as track and field, soccer, and rugby. The subjects were screened for medical and orthopedic conditions that would exclude them from sprinting procedures. Additionally, we excluded individuals with a history of hamstring injury. This study was approved by the Human Research Ethics Committee of Kyushu Kyoritsu University and was conducted in accordance with their guidelines for human experimentation. This study was also performed in accordance with the ethical standards of the International Journal of Sports Medicine [6].
Instrumentation
All testing was conducted on an indoor 50 m running track. Kin-ematic data were acquired using a 3D motion analysis system (Hawk Digital Real Time System; Motion Analysis Corporation, Santa Rosa, CA, USA) with 12 cameras sampling at a rate of 200 Hz. The measurement volume had a length, width and height of 6, 2.4, 2.4 m, respectively, and was situated approxi-mately 40 m along the 50 m running track, allowing ample dis-tance for acceleration and deceleration. The calibration error for the measurement volume was estimated to be no greater than 1 mm for all cameras. 4 large (900 × 600-mm2) Kistler force plates (Kistler Instrument Corporation, Amherst, NY, USA) sam-pling at a rate of 1 000 Hz were used to capture ground reaction force (GRF) data. The 4 force plates were situated immediately adjacent to each other (thereby extending a total length of 3.6 m) and were located at the center of the calibrated measurement area. All force plates were embedded in the floor of the labora-tory and covered with a piece of the running track.
Testing procedures
Prior to the test session, the subjects performed a 10–15-min warm-up that included a few minutes of jogging at a voluntary speed and static stretching of the lower extremity muscles. The static stretching load was less than 10 s per muscle group to avoid compromising muscle length-tension relationship. After the warm-up, each athlete completed a 40 m speed testing trial to establish maximum sprinting speed. Sprinting speed was recorded using timing gates (TC-Timing System; BROWER Tim-ing Systems., Draper, UT, USA). The subjects were then prepared for EMG measurement (the skin was shaved and then abraded and cleaned with sandpaper and ethanol to reduce the skin-electrode impedance). EMG values during 5-s isometric maxi-mal voluntary contraction (MVC) of the hamstring muscles were then collected with the subject in a prone position. Before start-ing the sprintstart-ing session, the participants practiced runnstart-ing at an easy speed until they were acclimated to running with pas-sive markers and attached electrodes. After a few minutes of their individual practice session, the participants were instructed to perform sprinting at their maximal speed. A gait cycle was defined as the time from maximum knee flexion of the right limb to the next maximum knee flexion of the ipsilateral limb. If the gait cycle was not within the measurement volume of motion analysis, the participants were instructed to perform the sprinting session at maximal speed again.
Data Collection
▼
Motion analysis
The participants were fitted with 19 reflective markers that were located on palpable anatomical landmarks. An initial recording of marker positions during quiet upright standing was performed to determine the position of the ischial tuberosity (IT), with one leg extended and the other leg placed on a stool flexing the hip joint to render the IT palpable subcutaneously. The relative coordinate position of the determined IT marker to a plane defined by the 3 markers placed on the rigid landmarks of the pelvis (right and left anterior superior iliac spine, and the middle of right and left posterior superior iliac spine) was calcu-lated to join the virtual marker of the IT in measurement volume throughout the trial. MTLs of the BFlh, ST and SM were com-puted by determining the distance from the virtual marker of the IT to each insertion, accounting for the wrapping of the mus-cles about the hip and knee joints. MTLs were normalized to the respective MTL in an upright standing posture with all lower extremity joint angles set to zero (L0). The kinematic data were synchronized with the EMG data.
Electromyography
Electromyographic (EMG) activities in the right limb during sprinting were recorded using a portable telemetry EMG system (MQ16; KISSEI COMTEC Corporation, Ltd, Matsumoto, Nagano, Japan) at a sampling rate of 1 000 Hz. The surface EMG was recorded from the mid-bellies of the BFlh, ST, and SM muscles with bipolar surface electrodes with an interelectrode distance of 30 mm. The positions of the surface electrodes were deter-mined through palpation of each muscle belly during isometric contraction. In addition, electrode cables were secured with an elastic tape to minimize motion artifacts. A computer software program (Kineanalyzer; KISSEI COMTEC Corporation, Ltd,
Matsumoto, Nagano, Japan) was used for the EMG analysis. The EMG data were band-pass filtered through 50–500 Hz. The digi-tal filtered EMG signals were full-wave rectified and normalized with the integrated EMG values collected during a 5-s isometric MVC for each subject (nEMG = %MVC).
Data analysis
The time (percentage of the gait cycle: %GC) and magnitude of maximum MTL and nEMG value were determined for each ham-string muscle. The time ( %GC) and magnitude of hip and knee flexion/extension angles at peak were also computed. Captured GRF data were normalized with each subject’s body weight (N/ kg) and the time ( %GC) and magnitude of peak GRF values being investigated. We calculated the product of normalized MTL by nEMG value and defined the calculated value as tensile force index (TFI = normalized MTL (/L0) × nEMG ( %MVC)) for each mus-cle. The time ( %GC) and magnitude of peaked TFI were also determined. All of the descriptive data were expressed as mean ± SD, and the differences between the values in the case of each muscle were evaluated using one-way analysis of variance (ANOVA) with repeated measures. When the differences were significant according to the ANOVA, Bonferroni’s post-hoc test-ing was performed. The level of significance was set at P < 0.05 for all comparison.
Results
▼
Maximal sprinting speed in testing trial for the subjects aver-aged 8.1 ± 3.4 m/s. The time ( %GC) course change of the hip and knee flexion angles, MTL ratios, nEMG values, TFIs, and the GRF are shown in ●▶ Fig. 1. ●▶ Table 1 shows time ( %GC) and
magni-tude of maximum MTL ratio, nEMG value and TFI of each muscle along with the hip and knee joint angles at that moment. The stance phase of sprinting was from 35.3 ± 2.1 to 62.9 ± 12.5 %GC (shaded in ●▶ Fig. 1). During the late swing phase, some typical
changes of kinematic parameters were found in the time course. First, the maximum flexion of the hip joint (57.7 ± 22.9 °) occurred at the time of 16.5 %GC. At this time, all of the ham-string muscles were lengthening and the knee joint was extend-ing. The hip joint then started to extend and the nEMG values of each hamstring muscle started to increase gradually and exceeded over 20 %MVC. The second typical change was found in the MTL ratio of the SM and it peaked (1.38 ± 0.09) at 23.0 %GC. Just after this timing, the maximum MTL of the ST (1.43 ± 0.09) was found at 23.7 %GC. The MTL ratio last peaked in the BFlh (1.33 ± 0.11) at 27.0 %GC. The nEMG values of each muscle peaked by the same order as the length ratios did: the SM (34.6 ± 24.5 %MVC) at 27.0 %GC, ST (40.9 ± 33.6 %MVC) at 31.6 %GC, and BFlh (42.8 ± 32.1 %MVC) at 35.6 %GC. The peak time of nEMG in the BFlh was immediately after that of foot contact. The time difference at peak between the MTL ratio and the nEMG value in each muscle was the smallest in the SM (3.9 %), medium in the ST (8.2 %), and greatest in the BFlh (8.8 %).
Fig. 1 The time (percentage of the gait cycle) course change of biomechanical parameters associated with hamstring muscle kinematics during a gait cycle of sprinting. Shaded region in each graph represent stance phase. Upper left: hip flexion angle (degree); Middle left: knee flexion angle (degree); Lower left: ground reaction force; Upper middle: normalized EMG ( %MVC) and muscle-tendon length ratio (/L0) of the biceps femoris long head (BFlh); Center: nEMG
and length ratio of the semitendinosus (ST); Lower middle: nEMG and length ratio of the semimembranosus (SM); Upper right: tensile force index (TFI) of the BFlh; Middle right: TFI of the ST; Lower right: TFI of the SM.
70 60 50 40 30 20 10 160 140 120 100 80 60 40 20 30 25 20 15 10 5 0 0 0 50 1.5 1.4 1.3 1.2 1.1 1 0.9 0.8 0.7 0.6 0.5 1.5 1.4 1.3 1.2 1.1 1 0.9 0.8 0.7 0.6 0.5 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 40 45 30 35 20 25 10 15 5 0 50 40 45 30 35 20 25 10 15 5 0 50 40 45 30 35 20 25 10 15 5 0 60 50 40 30 20 10 0 60 50 40 30 20 10 0 60 50 40 30 20 10 0 0 10 20 30 40
Hip Flexion (deg
)
nEMG (%
MVC)
Tensile Force Inde
x
Tensile Force Inde
x
Tensile Force Inde
x
Muscle Length Ratio (/
L0
)
Muscle Length Ratio (/
L0
)
Muscle Length Ratio (/
L0 ) nEMG (% MVC) nEMG (% MVC)
% Gait Cycle % Gait Cycle % Gait Cycle
SM SM
ST ST
BF1h BF1h
Knee Flexion (deg
)
Ground Reaction Force (N/kg
) 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100
Immediately following foot contact (35.3 %GC), GRF steeply increased and peaked (25.2 ± 14.8 N/kg) at 37.9 %GC ( ●▶ Table 2).
At this moment, which is called early-stance phase, the nEMG value of the BFlh peaked (42.8 ± 32.1 %MVC) at 35.6 %GC, and the knee joint maximally extended (30.0 ± 6.6 °) at 36.3 %GC. After this steep peak, the GRF increased again to the magnitude of 10.2 ± 6.0 N/kg at 43.0 %GC ( ●▶ Table 2) and then decreased to the
minimum (2.4 ± 1.7 N/kg) at 47.7 %GC. During this second peaked phase of GRF, called mid-stance phase, the knee joint kept flex-ing to the peak of 55.3 ± 8.7 ° at 49.5 %GC. However, the hip joint stopped extending at 33.3 ± 6.9 ° at 41.2 %GC and switched to flexing to the peak of 35.1 ± 6.9 ° at 45.3 %GC. The nEMG value showed a steep increase in the BFlh at 39.1 %GC (32.6 ± 29.4 %MVC) and 43.3 %GC (31.0 ± 22.9 %MVC), and also in the ST at 42.8 %GC (28.1 ± 20.8 %MVC) during the mid-stance phase. The GRF showed the third and last peak at 54.4 %GC (8.5 ± 7.6 N/kg,
●▶ Table 2), and nEMG of the ST steeply increased (33.3 ±
35.7 %MVC) just before this time at 53.7 %GC. This period is called late-stance phase, during which both the hip and knee joints are extending to push off the ground.
●▶ Table 1 shows the times ( %GC) and magnitudes of peak TFI for
each hamstring muscle along with the hip and knee joint angles at that moment. The time ( %GC) of maximum TFI in the BFlh (52.4 ± 43.1) and the ST (56.4 ± 47.3) completely corresponded at 31.6 %GC, which was the time in the middle between the maxi-mum MTL ratio in the BFlh (27.0 %GC) and foot contact (35.3 %GC). Both TFI values were significantly higher than the value of SM (37.5 ± 26.9, P < 0.05). The occurrence of peak TFI of the SM was obviously earlier (27.0 %GC) than those of BFlh and ST, and that time corresponded to that of the maximum MTL ratio in the BFlh.
During the stance phase, TFI values at first peak in BFlh (51.8 ± 10.3) and ST (50.0 ± 10.5) were completely corresponded
at 35.6 %GC ( ●▶ Table 3), which was immediately following foot
contact. The second peak of the ST (38.2 ± 28.5) and BFlh (40.3 ± 37.6) during stance phase occurred at 37.7 and 38.8 %GC ( ●▶ Table 3), which were immediately before and immediately
after the time first peak of GRF (37.9 %GC, ●▶ Table 2),
respec-tively. The third peak of the BFlh (31.7 ± 38.3) occurred at 44.7 %GC ( ●▶ Table 3), the phase during which hip flexion
occurred (41.2–45.3 %GC). The third peak of the ST (34.3 ± 30.1) occurred at 54.2 %GC ( ●▶ Table 3), which is almost the same time
of the third peak of GRF (54.4 %GC, ●▶ Table 2), the phase during
which the hip and knee joints were extending to push off the ground.
Discussion
▼
We estimated the magnitude of tensile force in the hamstring muscles during overground sprinting by 3D motion analysis, measuring the change of MTL in hamstring muscles and angles of the hip and knee joint, and EMG activity to identify the period of the gait cycle during which the hamstring muscles were likely to be injured as well as to characterize the biomechanical condi-tions associated with the injurious situation. At the time of 16.5 %GC during the late swing phase, it was found that the hip joint was maximally flexed and started to extend and that the nEMG values of each hamstring muscles started to increase gradually, corresponding to the last 18.8 %GC before the time of foot contact. This result of nEMG values corresponded with that of previous studies [10, 27], which have indicated that the ham-string muscles are activated during the last ~20 % of late swing phase. Because at this time the hip joint is maximally flexed, the knee joint is extending, and all of the hamstring muscles are lengthening at this time, it would appear to represent an injury-Table 2 The greater 3 peak times and the magnitudes of the GRFs.
GRF 1st peak GRF 2nd peak GRF 3rd peak
Time Magnitude Hip angle Knee angle Time Magnitude Hip angle Knee angle Time Magnitude Hip angle Knee angle
( %GC) (N/kg) (deg) (deg) ( %GC) (N/kg) (deg) (deg) ( %GC) (N/kg) (deg) (deg)
37.9 25.2 ± 14.8 34.6 ± 6.5 30.7 ± 6.8 43.0 10.2 ± 6.0 34.5 ± 6.8 46.2 ± 11.1 54.4 8.5 ± 7.6 19.1 ± 9.3 49.5 ± 11.4
Table 3 The greater 3 peak times and the magnitudes of TFIs in the BFlh and ST muscles during the stance phase.
TFI 1st peak (stance) TFI 2nd peak (stance) TFI 3rd peak (stance)
Mus-cle
Time Magnitude Hip angle Knee angle Time Magnitude Hip angle Knee angle Time Magnitude Hip angle Knee angle
( %GC) (deg) (deg) ( %GC) (deg) (deg) ( %GC) (deg) (deg)
BFlh 35.6 51.8 ± 10.3 37.0 ± 2.0 29.1 ± 1.9 38.8 40.3 ± 37.6 34.1 ± 6.3 32.9 ± 8.7 44.7 31.7 ± 38.3 35.0 ± 6.9 50.3 ± 10.0 ST 35.6 50.0 ± 10.5 37.0 ± 2.0 29.1 ± 1.9 37.7 38.2 ± 28.5 34.8 ± 6.5 30.4 ± 6.7 54.2 34.3 ± 30.1 19.8 ± 9.3 50.1 ± 11.2 Table 1 Peak time ( %GC) and magnitude of the muscle-tendon ratio (/L0), nEMG ( %MVC) and TFI in each hamstring muscle.
Mus-cle
Length EMG TFI
Time Magnitude Hip angle Knee angle Time Magnitude Hip angle Knee angle Time Magnitude Hip angle Knee angle
( %GC) (deg) (deg) ( %GC) ( %MVC) (deg) (deg) ( %GC) (deg) (deg)
BFlh 27.0 1.33 ± 0.11 46.8 ± 6.5 42.3 ± 9.7 35.6 42.8 ± 32.1 37.0 ± 6.8 29.1 ± 1.9 31.6 52.4 ± 43.1 41.3 ± 6.6 31.9 ± 6.0 ST 23.7 1.43 ± 0.09 51.2 ± 6.8 55.6 ± 12.9 31.6 40.9 ± 33.6 41.3 ± 6.6 31.9 ± 1.7 31.6 56.4 ± 47.3 41.3 ± 6.6 31.9 ± 6.0 SM 23.0 1.38 ± 0.09 52.1 ± 6.9 58.6 ± 13.3 27.0 34.6 ± 24.5 46.8 ± 6.5 42.3 ± 2.8 27.0 37.5 ± 26.9 46.8 ± 6.5 42.3 ± 9.7
related muscle strain situation. However, most of those kine-matic parameters had not reached peak values (knee joint angle, 91.6 ± 9.0 °; BFlh length, 1.28 ± 0.07 and nEMG, 16.0 ± 11.8 %MVC; ST length, 1.38 ± 0.09 and nEMG, 16.2 ± 14.5 %MVC; SM length, 1.33 ± 0.09 and nEMG, 14.6 ± 14.9 %MVC). As it has been clarified that the gluteal muscles are active during hip extension/flexion in sprinting [8, 17], relatively lower activities of hamstring mus-cles at this time could be due to the activation of gluteal musmus-cles as a hip extensor.
During the period from the maximum hip flexion to the foot contact, which is called the late-swing phase, the values of MTL ratio and nEMG in each hamstring muscle were increasing and peaked in the same temporal order: SM, ST, and BFlh. Since the hip and knee joints were both extending during this period, lengthening of hamstring muscles was due to knee extension, and muscles were forced to contract eccentrically to decelerate the lower leg. A previous study found that the activation pat-terns among the hamstring muscles during eccentric/concentric knee flexion movement were not uniform and that the degree of activation was greater in the ST muscle than in the BFlh and SM muscles [19]. The results of the magnitudes of MTL, nEMG and TFI during late-swing phase showed the highest value in the ST muscle and were consistent with the finding of the previous physiological study. These findings would suggest that the ST muscle is exposed to higher risk of a strain injury during the late-swing phase of sprinting.
Most of the previous studies investigated the likely time period of hamstring muscle strain injury during sprinting and con-cluded that the BFlh was most susceptible to the injury during the late-swing phase of sprinting [3, 7, 24, 25]. As the results of the current study demonstrate, some characteristic finding could indicate the reason why the BFlh has the greatest inci-dence of injury. Namely, peak MTL in the BFlh occurred later in the gait cycle than that of the ST and SM muscle, and the magni-tudes of TFI in the BFlh peaked during late-swing phase. How-ever, the magnitudes of MTL ratio and nEMG values in the BFlh were still smaller than those in the ST, and the total magnitude of TFI in the BFlh was smaller than that of ST. The critical differ-ence of those parameters in 2 muscles was the peak time of nEMG, which was immediately after foot contact in the BFlh. It would thus appear that the high estimated TFI value during late-swing phase was due to the relatively stretched state of the BFlh itself.
In the present study, we have identified some typical changes of biomechanical parameters which indicated the potential risk of hamstring muscle injury during stance phase. During the moment (about 0.01 s in real time) from the foot contact (35.3 %GC) to the peak GRF (37.9 %GC) in early-stance phase, the nEMG value of the BFlh reached the maximum value (35.6 %GC), and the knee joint was extended maximally (36.3 %GC). The TFIs of BFlh and ST increased steeply at the same time (35.6 %GC), and those magnitudes were the largest within the stance phase. Orchard [23] argues that the early-stance phase seems to be the highest risk period of strain injury for hamstring, and the GRF is likely culprit for hamstring failure in sprinting. To support this argument, he cites the Ralph Mann’s original thesis [13, 14], which posits that it is a strong force (moment, torque) in the opposite direction(s) (causing hip flexion and knee extension) that strains a hamstring, which is most likely to be due to the GRF. Pursuing this thesis further, the present study has identi-fied that the maximal knee extension and the first and largest GRF occurred at almost the same time in the early-stance phase.
Moreover, during the mid-stance phase described by the second peak of GRF (43.0 %GC), an obvious change of the hip joint angle in flexing direction was observed, and the knee joint was extend-ing to push off the ground durextend-ing the late-stance phase described by the third peak of GRF. However, during the stance phase, i. e., the period the GRF is measured, the MTL ratios of all the ham-string muscles were decreasing, i. e., none of the hamham-string mus-cles were lengthened. Thus, it is important to understand that the culprit for hamstring injury would be the internal force gen-erated by muscle contraction to be transformed into the exter-nal force against the ground, although the elongation of the muscle-tendon could trigger the event. Previous studies have revealed that higher muscle activation would be conducive to a higher risk of muscle strain [11, 25]. In the present study, steep increases of nEGM values in the BFlh and ST muscle were meas-ured at almost the same time as peak GRFs and changes in direc-tion of hip and knee joint angles. Determining whether those times coincide would help ascertain the occurrence of ham-string strain injury. Proper timing during the gait cycle would prevent hamstring muscles from being excessively activated or lengthened.
A limitation of the present study is that the relevance of TFI has not been established. We applied the following formula to calcu-late TFI
TFI = [normalized MTL (/L0)] × [nEMG ( %MVC)]
based on the rationale that muscle strain injuries occur when excessive tensile force was exerted on the voluntarily contract-ing muscle [5, 9]. However, many other biomechanical parame-ters must be considered to estimate the tensile force within a muscle, such as moment arm on a joint in which the joint mus-cle inserts, contracting velocity, the degree of body segment rotation angle, innate flexibility of soft tissues, etc. The TFI value calculated in this study could have a certain implication as far as the first trial predicting the likely time period of hamstring mus-cle strain injury during sprinting is accurate, though further investigations examining the effects of every biomechanical parameter on the tensile force within a muscle are warranted. As a practical application for hamstring injury rehabilitation or prevention, we recommend, based on our findings in this study, that “single-leg deadlift” exercises be conducted in training pro-grams. If the TFI value defined in this study is an accurate hypothesis, the early stance phase would be the most likely period that the BFlh would be strained, and it is important for the muscle to be adapted to and overcome such injury-related situations. Our previous study found that BFlh and SM were selectively recruited during the hip extension exercise [20], and that continuous deadlift training could be practical for improv-ing hip extension torque [21]. Future studies should investigate whether training is effective in decreasing the injury and/or reinjury rate of hamstring muscle strain.
Conclusion
▼
Considering the biomechanical conditions associated with the injury-related situation, we conclude that the biceps femoris long head muscle is susceptible to a strain injury during the early stance phase of the sprinting gait cycle as far as the esti-mated TFI value is an accurate hypothesis.
Affiliations
1 Faculty of Sports Sciences, Waseda University, Tokorozawa, Japan
2 Department of Health and Sports, Niigata University of Health and Welfare,
Niigata, Japan
3 Faculty of Sports Science, Kyusyu Kyoritsu University, Fukuoka, Japan 4 Department of Sports Science, Waseda University, Tokyo, Japan 5 Faculty of Sport Sciences, Waseda University, Saitama, Japan References
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