exercises

: The dorsal muscles of the lower torso and extremities have often been denoted the "posterior chain." These muscles are used to support the thoracic and lumbar spine as well as peripheral joints including the hip, knee, and ankle on the dorsal aspect of the body. This study investigated relative muscle activity of the hamstring group and selected surrounding musculature during the leg curl, good morning, glute-ham raise, and Romanian deadlift (RDL). Twelve healthy, weight trained men performed duplicate trials of single repetitions at 85% 1RM for each lift in random order, during which surface electromyography and joint angle data were obtained. Repeated measures analysis of variance (RMANOVA) across the four exercises was performed to compare activity from the erector spinae (ES), gluteus medius (GMed), semitendinosus (ST), biceps femoris (BF), and medial gastrocnemius (MGas). Significant differences (p<0.05) were noted in eccentric muscle activity between exercise for the MGas (p<0.027), ST (p<0.001), BF (p<0.001), and ES (p=0.032), and in concentric muscle activity for the ES (p<0.001), BF (p=0.010), ST (p=0.009), MGas (p<0.001), and the GMed (p=0.018). Bonferroni post hoc analysis revealed significant pairwise differences during eccentric actions for the BF, ST, and MGas. Post hoc analysis also revealed significant pairwise differences during concentric actions for the ES, BF, ST, MGas, and GMed. Each of these showed effect sizes that are large or greater. The main findings of this investigation are that the ST is substantially more active than the BF among all exercises, and hamstring activity was maximized in the RDL and glute-ham raise. Therefore, athletes and coaches who seek to maximize involvement of the hamstring musculature should consider focusing on the glute-ham raise and RDL. The purpose of this study was to identify the amount of involvement of specific muscles in the thigh, shank, and lower back during various weight training exercises. All four exercises being investigated (glute-ham raise, good morning, RDL, prone leg curl) are suggested as targeting the hamstring muscle group. The current study was designed to quantify activity of the BF and ST/SM as well as the surrounding stabilizing muscles including the gluteus medius (GMed), longissimus subdivision of the erector spinae (ES), and medial gastrocnemius (MGas). This investigation was the first to compare the glute-ham raise, good morning, RDL, and prone leg curl exercises, which are all suggested to primarily activate the hamstrings and surrounding musculature. We hypothesized that there would be no difference in activation within muscles for the prone leg curl and glute ham, due to the kinematic similarities between the two exercises. We similarly hypothesized no difference in activation within muscles for the good morning and RDL due to similar kinematics.


INTRODUCTION
The dorsal muscles of the lower torso and extremities have often been denoted as part of the "posterior chain." These muscles are used to support the thoracic and lumbar spine as well as peripheral joints including the hip, knee, and ankle on the dorsal aspect of the body (19). This terminology, apparently first coined by Françoise Mézières as far back as 1947, was used in the context of describing this musculature as "too short" and "too strong." While many would still describe the posterior chain as too short (i.e. lacking adequate flexibility), it is seldom discussed as too strong. While several exercises involve this musculature, many of those specifically target the hamstrings. Therefore, this study investigated relative muscle activity during four weight training exercises (stressing the hamstrings) which are widely used and typically available for both elite and recreational athletes.
The hamstrings muscle group is made up of three muscles in the posterior thigh, including the biceps femoris (BF), semitendinosus (ST), and semimembranosus (SM) (10) . The hamstrings are responsible for actions at the hip and knee, as they cross both joints at origin and insertion (5) . This muscle group is typically recognized for producing flexion of the knee, but with the trunk flexed and knees extended, the hamstrings are powerful hip extensors (10) . The actions and levels of activation of muscles have been posited as a method by which bodybuilders and athletes may prioritize certain exercises in an attempt to maximize regional hypertrophy (2).
Surface electromyography (sEMG) has been used to examine several aspects of muscle activity during various weight training exercises (4,6,8,14,15,18,20,21). Wright et al. (22) studied normalized, integrated EMG activity of the hamstrings (BF and ST) to compare the efficiency of three resistance training exercises, including the leg curl, stiff-leg deadlift (similar to the Romanian deadlift; RDL), and back squat. That study found no significant differences in activation between exercises, but reported much greater activity and peak amplitudes on the concentric portions of the exercises compared to the eccentric (22) . To date, no studies have investigated the glute-ham raise, which is another common exercise for the hamstrings. The purpose of this study was to identify the amount of involvement of specific muscles in the thigh, shank, and lower back during various weight training exercises. All four exercises being investigated (glute-ham raise, good morning, RDL, prone leg curl) are suggested as targeting the hamstring muscle group. The current study was designed to quantify activity of the BF and ST/SM as well as the surrounding stabilizing muscles including the gluteus medius (GMed), longissimus subdivision of the erector spinae (ES), and medial gastrocnemius (MGas).
This investigation was the first to compare the glute-ham raise, good morning, RDL, and prone leg curl exercises, which are all suggested to primarily activate the hamstrings and surrounding musculature. We hypothesized that there would be no difference in activation within muscles for the prone leg curl and glute ham, due to the kinematic similarities between the two exercises. We similarly hypothesized no difference in activation within muscles for the good morning and RDL due to similar kinematics.

Experimental Approach to the Problem
Four exercises that have been used to target the hamstrings include the glute-ham raise, good morning, RDL, and prone leg curl. While there are numerous muscles involved in these exercises, five muscles were chosen for assessment, including synergistic musculature. Using a sample of experienced resistance-trained men, we used a cross-sectional design examining four exercises: glute-ham raise (Figure 1), good morning, RDL, and prone leg curl. Since all subjects were experienced, resistance-trained athletes, 85% of their one-repetition maximum (85% 1RM) represents a common training load, and their one-repetition maximum (1RM) values are believed accurate due to the reported reliability of the 1RM protocol used herein (7,17).

A C C E P T E D
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Subjects
Twelve healthy, weight-trained (8.6±5.5 yrs) men (27.1±7.7 yrs, 88.7±15.9 kg) participated in the study for a total of four sessions. During the first session, subjects were informed of the procedures involved in the study and, after clarifying any possible questions, gave written informed consent and filled out medical history and physical activity questionnaires. All procedures were approved by the University Human Subjects Review Board. Potential subjects were excluded if their medical history suggested a musculoskeletal or other issue that would prevent them from safely performing maximal weight training exercises.

Procedures
Subjects reported to the lab on four separate occasions to complete the study. During the first session, anthropometric measures such as height, body mass, and estimated body fat percentage via 3-site skinfold (1) were measured. Also in this session, the subjects were tested on their 1RM (3) on the glute-ham raise and good morning exercises, in order, with approximately 10 minutes of rest between exercises. During the glute-ham raise each subject held the resistance load against their chest, making sure that the center of the weight was aligned with the xiphiod process. Range of motion was pre-determined as 90° of extension and flexion at the knee while barbell held with a pronated grip at the anterior aspect of the thighs and slight bend in the knee (near full extension). This lift was mechanically similar to the good morning, with the difference being the resting position of the barbell. The prone leg curl was conducted on a modified plateloaded leg curl machine. The subject was placed on the machine so that the axis of rotation on the lever arm was at the center of the knee joint. The required ROM was from the start position on the machine until the lever arm contacted the back of the leg, approximately 110°.
On the third and fourth visits, subjects performed duplicate trials of single repetitions at 85% 1RM for each lift in random order, during which sEMG and joint angle data were obtained.
A standardized warm-up protocol was performed prior to lifting during all sessions, similar to the warm up for the 1RM tests. One test repetition was performed prior to the two experimental repetitions to ensure sensor viability. Two to three minutes of rest were given between trials to ensure fatigue was minimized without reducing the effects of the warm-up. Each subject was given 2-5 days of rest between sessions.

sEMG
In order to determine muscle activity during each lift, surface sensors were placed on the right side of the body, over the ES, GMed, ST/SM, BF, and MGas, according to Hermens et al. (11).
These sensor placements are suggested to reduce cross-talk in the EMG signal. Due to the deep position of the SM (10) and some discrepancy in the ability to obtain separate sEMG data from the SM and ST (9,12,16,22) , we elected to use the ST placement of Hermens (11) to represent the combined activity of the ST/SM group. Two round Ag-AgCl bipolar surface sensors (Ambu Blue Sensor SP, 2 cm inter-sensor distance) were used for each muscle. Sensors were placed on a line between anatomical landmarks (11) so that the same fibers intersected both sensors, distal to the motor point. A ground sensor was placed on the patella for signal noise reduction, and all sensor placement was standardized between sessions by tracing the outline of the sensors with a permanent marker. The sensors used were passive, therefore pre-amplification was not possible.
A common mode of rejection of 90dB, a band pass filter (10-450Hz), and input impedance of 10MΩ were applied to incoming data. Prior to sensor placement, the subject's skin was shaved, abraded with fine sandpaper, and cleaned with alcohol. Signals were recorded and processed

Statistical Analyses
Data are expressed as mean ± (SD). Repeated measures analysis of variance (RMANOVA) across the four exercises was performed for each muscle group (1 x 4), and the a priori significance was set at p<0.05. Bonferroni correction was utilized for pairwise comparisons in the instance of significant main effects, and standardized effect sizes for repeated measures were calculated. Statistical procedures were conducted with SPSS20.

A C C E P T E D
Copyright Ó Lippincott Williams & Wilkins. All rights reserved.
The training status of the subjects is substantial based on the reported training history, and the 1RM values achieved (Table 1) GMed activity was significantly greater during the leg curl and glute-ham vs. the good morning ( figure 9). Each of the aforementioned pairwise comparisons had effect sizes that are considered large or greater (13) . In addition, there were several muscles that did not display significant pairwise differences between exercises, but did show large or greater effect sizes (13) . These include eccentric contractions from ES (between prone leg curl and good morning), BF (between prone leg curl and glute-ham), and MGas (between prone leg curl and glute ham); and concentric actions from the ES (between prone leg curl and good morning), and GMed (between RDL and good morning).

DISCUSSION
The main findings of this investigation demonstrate that there are significant differences in activation within muscles when comparing all exercises. While one might expect similar activation for a given muscle for activities of similar kinematics such as the prone leg curl and glute-ham raise, this is not the case with the data herein. These findings may also indicate that the kinematics are not as similar as they appear to be, especially when you consider possible greater activity from the BF during the eccentric RDL as compared to the eccentric prone leg curl. We also noted that qualitatively, each of the tested muscles were more active during the concentric versus eccentric action, consistent with Wright et al. (22).
It has been previously reported that the concentric leg curl and stiff-leg dead lift produced greater activity for the ST in comparison to a dead lift and back squat (22). Our investigation showed that the concentric action during the prone leg curl and RDL were actually the least active for the ST/SM compared to the good morning and glute-ham raise. The glute-ham raise also elicited the greatest activity during the concentric contraction of the BF, and was significantly more active compared to the concentric portion of the prone leg curl. These results suggest that when considering the concentric actions of the BF and ST/SM, the glute-ham raise is the most effective exercise for maximizing sEMG activity of these muscles. Oliver and Dougherty (16) reported no significant difference in EMG activity from the medial hamstrings, BF, gluteus maximus, and GMed when comparing the razor curl to the prone leg curl but reported that 90 degrees of both hip and knee flexion (as seen at the completion of the razor curl) is optimal for increasing hamstring contractibility. This statement is best supported by the finding that the razor curl demonstrated elevated activity compared to the prone leg curl when EMG activity was expressed as a percent MVIC (16). The suggestion that simultaneous hip and knee flexion is optimal for producing hamstring contractibility (16) is not supported by our findings since our data demonstrate significantly greater concentric activity from the BF and ST/SM during the glute-ham raise compared to that seen during the prone leg curl, despite no motion at the hip.
It has been suggested that the position of the more medial hamstring muscles allow them to resist shear tibial forces that are likely to contribute to ACL injuries; Therefore, it may be optimal to maximize involvement from the more medial hamstring musculature in order to reduce the likelihood of ACL injuries (16), but this hypothesis has yet to be thoroughly supported. It was also noted that ES involvement was greatest during the glute-ham raise. The elevated activity during the glute-ham raise could be related to the possibility that there is greater torque about the knee and thus greater demands from the hamstring and ES muscles when considering the mechanical actions during these exercises.
This investigation has expanded upon previous analyses of muscle activation during lower body resistance exercise (8,22) by including analysis of GMed and MGas. For both eccentric and concentric actions, GMed was significantly more active during RDL compared to the good morning. MGas was also significantly more active during RDL in comparison to good morning and prone leg curl during both concentric and eccentric actions. Escamilla et al. (9) evaluated the MGas during the deadlift but many researchers failed to analyze activity from the MGas during various lower body exercises (8,22) which is a major limitation since this biarticular muscle crosses at the ankle and knee joints (10). Additional research is needed to determine MGas activity relative to the RDL and other exercises that are meant to stress the MGas specifically.

PRACTICAL APPLICATIONS
Since each of the tested lifts are performed with the intention of stressing the hamstrings, it is important to note that activity was maximized for the BF during the RDL and glute-ham raise.
The concentric action from the ST/SM was highest in the glute-ham raise, while the eccentric action of the ST/SM was highest in the RDL. Therefore, the current findings suggest that athletes