Introduction
The hamstrings consist of the biceps femoris long heads and biceps femoris short heads (BFlh and BFsh, respectively), semitendinosus (ST) and semimembranosus (SM), working as hip extensors and/or knee flexors. It is known that there are morphological and functional differences among the individual hamstring muscles. For example, BFlh and SM possess a large physiological cross-sectional area, whereas ST has long muscle fibers (15 11 1,4 6 8 5,14,15,18 3
Muscle activity level during training exercises has been evaluated by surface electromyogram (EMG). The EMG activity of hamstrings was shown to be influenced by the hip joint position (21 12 13 16 13 7
The purpose of this study was to investigate the effect of hip joint position on the level of activity of the individual hamstring muscles during stiff-leg deadlift.
Methods
Experimental Approach to the Problem
To determine the effect of hip joint position on the individual hamstring muscles, stiff-leg deadlift was chosen. This is because stiff-leg deadlift has been widely prescribed by strength and conditioning professionals to strengthen the hamstring muscles, and the hip joint positions (internal/external rotation and adduction/abduction) can be easily manipulated by adjusting foot placement and angle during the exercise. Subjects performed stiff-leg deadlift in 6 hip joint positions in a random order after maximal voluntary contraction (MVC) of isometric knee flexion. During the deadlift, EMG signals were recorded from the proximal and distal regions of BFlh, ST, and SM for the analyses. Root mean square values of EMG data (RMS-EMG) were calculated for each of the concentric and eccentric phases of the deadlift and were normalized by RMS-EMG during MVC as % MVC.
Subjects
Fourteen male collegiate sprinters (age: 19.6 ± 1.0 years, body mass: 64.5 ± 4.1 kg, height: 175.4 ± 5.6 cm, personal best time of a 100-m race: 11.03 ± 0.25 seconds, mean ± SD ) participated in this study. The age range of these subjects was from 18 to 21 years. They had a minimum of 3 years of experience of sprint running and no injury in the hamstrings at the time of experiment. All subjects were fully informed of the purpose and procedure of this study and gave written informed consent before participation in the study. All protocols of this study were approved by the Doshisha University Research Ethics Review Committee regarding Human Subject Research [number: 18002].
Procedures
Bipolar electrodes (DL-141; S&ME, Tokyo, Japan, interelectrode distance: 12 mm) were placed over 40 and 60% of the thigh length (the distance between the greater trochanter [0%] and the popliteal crease [100%]) for BFlh; 30 and 50% for ST and 50 and 70% for SM to evaluate the activity in the proximal and distal regions of each muscle. The electrode placements were determined on the basis of innervation patterns of the individual hamstring muscles (19,20
Subjects lay prone on a bed of a dynamometer (BIODEX SYSTEM4; Biodex Medical Systems, Upton, NY) with the right knee joint at 45° (full extension = 0°). The pelvis and right ankle were fixed with belts. They held the bed with their arms. After a warm-up session including 3 submaximal contractions, they performed 2 MVCs of isometric knee flexions for 2 seconds.
After MVC measurement, the subjects performed stiff-leg deadlift in the following 6 hip joint positions (Figure 1 ): (a): internally rotated by 20° (IN20), (b) neutral (NT), (c) externally rotated by 20° (EX20) and (d) by 40° (EX40), (e) adducted (ADD), and (f) abducted (ABD) positions. In IN20, EX20, and EX40, the distance between the middle of the heels was set at 20% of their height. In ADD, NT, and ABD, the feet were placed parallel with a distance between the middle of the heels set at 5, 20, and 40% of their height, respectively. They put their feet on white tapes preset to adjust the hip joint positions. The deadlift was started from the upright position with a barbell positioned in front of the thighs. They slowly lowered the barbell to their tibial tuberosities with the knee joints extended and then lifted the barbell to the start position. As a familiarization session, subjects performed a few repetitions of deadlift in each of the hip joint positions with a load of 60% of their body mass with particular attention to the range of motion and body position during the deadlift. After this session, they performed 2 sets of 2 repetitions of deadlift with the same load as the familiarization session. Each of the concentric and eccentric phases of deadlift was completed in 2 seconds with a metronome set to 60 b·min−1 . The order of the hip joint positions was randomized across the subjects. Sufficient rest was allowed between the different hip joint positions to avoid an influence of muscular fatigue on EMG data. When they were unable to perform the deadlift at the prescribed pace or with proper technique, an additional set was requested to perform after 3-minute rest.
Figure 1.: Images demonstrating the positions of the hip joint. A) IN20: Internally rotated by 20°, NT: neutral, EX20: Externally rotated by 20°, and EX40: Externally rotated by 40°. B) ADD: Adducted, NT, and ABD: Abducted.
Raw EMG signals were preamplified and filtered at 5–500 Hz with the electrodes and recorded on a computer at a sampling frequency of 1000 Hz via an A/D converter (PowerLab 16SP; AD Instruments, Sydney, Australia). The EMG signals were high-pass filtered at a cutoff frequency of 5 Hz with computer software (LabChart ver.8; AD Instruments). The knee flexion torque signal was sampled with the A/D converter (PowerLab 16SP; AD Instruments) at 1000 Hz and transferred to the computer. The RMS-EMG during MVC was determined for 0.5 seconds around the peak torque. The higher RMS-EMG value of the 2 MVC trials was used for the normalization of EMG. The RMS-EMG during the stiff-leg deadlift was calculated from the EMG data of each of concentric (from the start to the end of upward movement) and eccentric phases (from the start to the end of the downward movement). To determine the concentric and eccentric phases, the motion of the subject during the deadlift was recorded with a digital video camera (EX-100; Casio, Tokyo, Japan) at 30 Hz. The video was synchronized with the EMG data by using an electrical signal from a synchronizer (PH-1250A-6SW; DKH, Tokyo, Japan). The RMS-EMG values during deadlift excluding the largest and the smallest values among the 4 data sets (2 sets × 2 repetitions) of each hip joint position were extracted, and the mean of the 2 values was normalized by RMS-EMG during MVC as %MVC. The intraclass correlation coefficients (ICC) of the individual hamstring muscles were analyzed by the 2 extracted RMS-EMG values in each hip joint position. Based on the previous study (10
Statistical Analyses
All statistical analyses were performed using a statistical software package (IBM SPSS Statistics, ver. 25.0, IBM). A three-way analysis of variance (ANOVA) with repeated measures was used to examine the effects of muscle (BFlh, ST, and SM), region (proximal and distal) and hip joint position on RMS-EMG of the concentric and eccentric phase, separately. The ANOVA was separately performed for the internal/external rotation (IN20, NT, EX20, and EX40) and adduction/abduction (ADD, NT, and ABD) of the hip joint. If a significant interaction was found, two- and one-way ANOVAs with Bonferroni multiple comparison tests were performed to test for any differences in RMS-EMG among the hip joint positions. The statistical power, effect size (partial η2 and Cohen's d ), and confidence interval (CI) were calculated for these tests. The statistical significance level was set at p ≤ 0.05.
Results
Three-way ANOVA for the internal/external rotation showed no significant interaction of muscle × region × hip joint position for RMS-EMG in the concentric phase (p = 0.165, partial η2 = 0.108, statistical power = 0.575). Because no significant interactions of muscle × region (p = 0.689, partial η2 = 0.028, statistical power = 0.104) or region × hip joint position (p = 0.996, partial η2 = 0.064, statistical power = 0.053) were found, RMS-EMG values in the proximal and distal regions were pooled for each muscle. A significant interaction of muscle × hip joint position was found for the pooled RMS-EMG in the concentric phase (p < 0.001, partial η2 = 0.563, statistical power = 0.999, Figure 2A ). The post hoc test revealed that RMS-EMG of BFlh was significantly higher in EX20 than that in NT (p = 0.008, 95% CI: 1.4–11.5, d = 0.446, difference = 6.3%). The RMS-EMG of BFlh in EX40 was significantly higher than that in NT (p = 0.001, 95% CI: 4.1–14.7, d = 0.627, difference = 9.4%) and IN20 (p = 0.021, 95% CI: 1.0–13.9, d = 0.497, difference = 7.4%). The RMS-EMG of ST was not significantly different among IN20, NT, EX20, and EX40 (p = 0.051, partial η2 = 0.179, statistical power = 0.634). The RMS-EMG of SM was significantly higher in IN20 than that in EX40 (p = 0.004, 95% CI: 1.2–7.3, d = 0.407, difference = 4.3%).
Figure 2.: A) Root mean square of electromyograms (RMS-EMG) of (a) the biceps femoris long head (BFlh), (b) semitendinosus (ST), and (c) semimembranosus (SM) in the concentric phase of the deadlift in the internally/externally rotated positions of the hip joint. B) RMS-EMG of (d) BFlh, (e) ST, and (f) SM in the concentric phase of the deadlift in the adducted/abducted positions of the hip joint. The asterisks between the bars indicate a statistically significant difference (p < 0.05). NS = not significant. Hip joint positions: ADD = adducted; NT = neutral; ABD = abducted; IN20 = internally rotated by 20°; EX20 = externally rotated by 20°; EX40 = externally rotated by 40°.
Three-way ANOVA for the adduction/abduction showed no significant interaction of muscle × region × hip joint position for RMS-EMG in the concentric phase (p = 0.770, partial η2 = 0.034, statistical power = 0.148). There were no significant interactions of muscle × region (p = 0.911, partial η2 = 0.007, statistical power = 0.063) or region × hip joint position (p = 0.749, partial η2 = 0.022, statistical power = 0.091). Hence, RMS-EMG values in the proximal and distal regions were pooled for each muscle. Although no significant interaction of muscle × hip joint position was found for the pooled RMS-EMG in the concentric phase (p = 0.792, partial η2 = 0.031, statistical power = 0.140), there was a significant main effect of hip joint position (p = 0.001, partial η2 = 0.434, statistical power = 0.972, Figure 2B ). The post hoc analysis showed that the RMS-EMG in ABD was significantly higher than that in NT for BFlh (p = 0.015, 95% CI: 0.9–8.4, d = 0.320, difference = 4.6%), ST (p = 0.047, 95% CI: 0.0–7.5, d = 0.444, difference = 3.8%), and SM (p = 0.005, 95% CI: 1.2–6.6, d = 0.367, difference = 3.9%).
Three-way ANOVA for the internal/external rotation showed no significant interaction of muscle × region × hip joint position for RMS-EMG in the eccentric phase (p = 0.806, partial η2 = 0.037, statistical power = 0.192). Because there were no significant interactions of muscle × region (p = 0.604, partial η2 = 0.038, statistical power = 0.125) or region × hip joint position (p = 0.808, partial η2 = 0.024, statistical power = 0.107), RMS-EMG values in the proximal and distal regions were pooled for each muscle. There was a significant interaction of muscle × hip joint position for the pooled RMS-EMG in the eccentric phase (p < 0.001, partial η2 = 0.444, statistical power = 0.999, Figure 3A ). The post hoc analysis demonstrated that RMS-EMG of BFlh was significantly higher in EX40 than that in IN20 (p = 0.038, 95% CI: 0.3–13.4, d = 0.684, difference = 6.8%). The RMS-EMG of ST was not significantly different among the hip joint positions (p = 0.660, partial η2 = 0.048, statistical power = 0.176). The RMS-EMG of SM was significantly higher in IN20 than in EX20 (p = 0.019, 95% CI: 0.5–6.3, d = 0.440, difference = 3.4%) and EX40 (p = 0.023, 95% CI: 0.5–7.8, d = 0.548, difference = 4.1%).
Figure 3.: A) Root mean square of electromyograms (RMS-EMG) of (a) the biceps femoris long head (BFlh), (b) semitendinosus (ST), and (c) semimembranosus (SM) in the eccentric phase of the deadlift in the internally/externally rotated positions of the hip joint. B) RMS-EMG of (d) BFlh, (e) ST, and (f) SM in the eccentric phase of the deadlift in the adducted/abducted positions of the hip joint. The asterisks between the bars indicate a statistically significant difference (p < 0.05). NS = not significant. Hip joint positions: ADD = adducted; NT = neutral; ABD = abducted; IN20 = internally rotated by 20°; EX20 = externally rotated by 20°; EX40 = externally rotated by 40°.
Three-way ANOVA for the adduction/abduction showed no significant interaction of muscle × region × hip joint position for RMS-EMG in the eccentric phase (p = 0.983, partial η2 = 0.007, statistical power = 0.068). There were no significant interactions of muscle × region (p = 0.804, partial η2 = 0.017, statistical power = 0.081) or region × hip joint position (p = 0.502, partial η2 = 0.052, statistical power = 0.157). Thus, RMS-EMG values in the proximal and distal regions were pooled for each muscle. There was no significant interaction of muscle × hip joint position (p = 0.650, partial η2 = 0.046, statistical power = 0.191) or significant main effect of hip joint position (p = 0.672, partial η2 = 0.030, statistical power = 0.108, Figure 3B ) for the pooled RMS-EMG in the eccentric phase.
Discussion
The results of this study showed that the level of muscle activity of BFlh in EX20 and EX40 was higher than that in NT during the concentric phase of stiff-leg deadlift, whereas that of SM was higher in IN20 than in EX40 during the concentric and eccentric phases. In addition, the activity levels of the individual hamstring muscles in ABD were higher than that in NT during the concentric phase. However, no significant differences were found in the activity level of ST among the hip joint positions during the concentric or eccentric phases except for the difference between NT and ABD during concentric phase. It has been reported that the internal/external rotation of the hip joint affects the muscle activity level of the hamstrings during single-leg deadlift (13 13
The muscle activity level of BFlh was higher in EX20 and EX40 than in NT during the concentric phase of stiff-leg deadlift. On the contrary, the activity level of SM was higher in IN20 than in EX40 during the concentric and eccentric phases. As the stiff-leg deadlift is a hip-dominant exercise, the hip extensors including the individual hamstring muscles need to generate hip extension torque. The magnitude of muscle torque is the product of the muscle force and its moment arm. It is possible that the moment arm of BFlh and SM at the hip joint in the sagittal plane increased when these muscles were located at the most posterior position in the externally and internally rotated position, respectively (Figure 4A ). If so, the long moment arm greatly contributes to generating muscle torque, and thus high activity level of the muscle with the long moment arm could be efficient for exerting the required torque. Taken together, the possible difference in the moment arm at the hip joint might be related to the difference in the activity level of BFlh and SM in the internally/externally rotated positions, respectively.
Figure 4.: A) The schematic illustration of the biceps femoris long head (BFlh) and the semimembranosus (SM) at the externally and internally rotated position in the sagittal plane during stiff-leg deadlift. The dotted lines indicate moment arms of BFlh and SM at the hip joint in the sagittal plane. B) The schematic illustration of the component of the force generated by the individual hamstrings at the neutral and the abducted position in the frontal plane (posterior view) during stiff-leg deadlift. The downward arrows indicate the component of muscle force contributing to the hip extension torque.
The activity levels of the individual hamstring muscles were higher in ABD than in NT during the concentric phase of the deadlift. To lift the barbell, the hip extensors including the hamstring muscles in ABD must generate the hip extension torque equivalent to that in NT. However, the contribution of the component of the force generated by the individual hamstrings to the production of hip extension torque was supposed to be smaller in ABD than in NT (Figure 4B ). Thus, higher activation of the hamstring muscles may be required in ABD as compared to NT.
There was no difference in the muscle activity level of ST among the hip joint positions during concentric or eccentric phases of the deadlift except for the difference between NT and ABD during concentric phase. A previous study reported that muscle activity level of ST during stiff-leg deadlift was lower than that of BFlh and SM (16
The activity level was not different between the proximal and distal regions within the individual hamstring muscles during stiff-leg deadlift. This result was inconsistent with the previous finding that the distal region of BFlh showed a higher activity level than the proximal region during stiff-leg deadlift (7 7 7
This study has several limitations. The surface EMG is known to be susceptible to cross-talk from adjacent muscles. To reduce the effect of cross-talk, we carefully identified the belly of each muscle using ultrasonography. Moreover, our subjects were male sprinters, who have larger hamstring muscles than other populations (9 Practical Applications
Stiff-leg deadlift is commonly used for preventing strain injury of the hamstring muscles and enhancing the strength of hip extensor muscles in rehabilitation and training fields. When performing the stiff-leg deadlift, athletes and their coaches may not pay attention to the hip joint position. Meanwhile, this study revealed the effects of hip joint position on the muscle activity levels of the individual hamstrings during the stiff-leg deadlift. The activity level of BFlh was higher in the externally rotated and abducted positions than in the neutral position. It is known that BFlh is the most frequently injured muscle among the hamstrings, probably due to the weakness of the muscle strength (17 2
Acknowledgments
The authors show their greatest appreciation to the participations in this study and all members of their laboratory. The results of this study do not constitute endorsement of any product by the authors or the National Strength and Conditioning Association.
The authors have no conflict of interests to disclose.
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