The aim of this study was to study the influence of different BE execution forms on the spine, hip and knee RoMs, external joint moments and on the muscle activity. In total, 16 subjects performing six different versions of BE were measured and analysed.
Due to the different exercise specifications, higher RoMmub, RoMlmb and Ct,rp and smaller RoMplb were expected for BEs compared to BEh and could be analysed (Tables 2 and 3). It is remarkable that a similar lumbar spine flexion was found for BEh compared to BEs. However, whereas a thoracic spine flexion was analysed during BEs, a spine extension was observed during BEh from the starting to the reversal point (Fig. 2c and d). This difference resulted in an opposed movement for the dorsal spine muscles in BEh. This opposed movement in the spine during the eccentric part of BEh might affect the dorsal back muscles. ErecLum becomes stretched and experiences an eccentric force, whereas ErecThor contracts in a concentric manner.
The RoMs in the lumbar and thoracic spine of non-fatigued BE reported by Larivière and co-workers  were within the values of BEh and BEs of this study. This finding supports the assumption based on the FFT analysis of the EMG data that the BE in the present study were performed in a non-fatigued state. With fatigue, Larivière and co-workers  observed RoMs in the lumbar spine that were closer to those of this study’s BEh, whereas the values of the RoMs in the thoracic spine were closer to those of the BEs (Table 2). Although the participants tried to start in the exact same position for all execution forms, the significant differences in Ct,s, especially between BEh and BEs, indicated that this was not entirely the case (Tables 2 and 3). Although BEs involved an isolated spine flexion and BEh a stabilised spine, the RoM in Cl seemed to be comparable between BEh and BEs, which indicates that it was not possible to completely isolate one movement.
BEh resulted in a slightly higher Mb,max (Fig. 2c, Tables 2 and 3). A higher moment in the back is assumed with a greater exerted strain on the lumbar back muscles, which probably led to a larger training effect on those specific muscles. This difference could be explained by a bigger lever arm of the segmental mass of the upper body in BEh. For comparison, the back moments estimated for L5/S1 by Plamondon and co-workers  were normalised with a mean body weight (BW, 1.42 Nm/kg). The smaller values for L4/L5 (0.94-1.05 Nm/kg) found in this study can be argued as followed. First, a less horizontal leg position causes a smaller external torque  and second, a smaller lever arm due to L4/L5 being more cranial compared to L5/S1 is presented. Furthermore, Plamondon and co-workers  described that the maximal back moment occurred at or near a horizontal trunk position, which agrees with this study's Mb,max occurring at the reversal point along with the smallest Cl (Fig. 2c).
Comparing found back moments to other back strength exercises, such as deadlifts and goodmornings, the maximal moment in the back, Mb,max, is two to three times smaller during BE [maximal back moment = 2.75–2.81 Nm/kg with 25% extra barbell load for deadlifts and goodmornings; 27].
Because of different exercise specifications, RoMh was expected to be higher for BEh compared to BEs. This assumption was confirmed by the results (Fig. 2b, Tables 2 and 3). Larivière and co-workers  achieved a RoMh of 13°, which was in accordance with this study’s BEs values. The smaller RoMh for 1L compared to 2L (Fig. 2b) could be explained by the inability of the participants to fully extend their hip in the starting position of 1L executions due to a lack of stabilising capacity.
Similar to the kinetic results in the lower back, the maximal moment in the hip, Mh,max, was slightly higher for BEh compared to BEs (Fig. 2b, Tables 2 and 3), which presumably resulted in a higher training effect for the dorsal hip muscles during BEh. Additionally, a significantly higher Mh,max and, thus, probably a better training effect were achieved for 1L compared to 2L for both types of BE (Fig. 2b, Tables 2 and 3).
Comparing to other strength exercises, Mh,max during 2L BE was similar to squats without any extra load [27, 28] and was approximately half for 2L and two thirds for 1L of the load reached by deadlifts and goodmornings with 25% BW extra barbell load . The latter is quite astonishing, considering that the BE were performed without any extra load. For comparison with this study's results, the maximal hip moment for 45° BE calculated by Contreras and co-workers  was normalised with BW (5.43 Nm/kg). Because they used an external weight of approximately 50% BW, it is reasonable that their value is approximately five times larger than what the present study measured for Mh,max.
Unsurprisingly, a small RoMk was expected and confirmed in this study because the BE were performed with straight legs (Fig. 2a, Tables 2 and 3). Similar RoMk values were obtained for the straight leg exercise goodmornings . The RoMk is significantly higher for BEh, and the starting positions differ between BEh and BEs (Fig. 2a, Tables 2 and 3). To increase the RoMh for BEh, participants might have tilted their pelvis forward, which could have promoted knee extension and thus increased the RoMk. However, compared to other strength exercises, the RoMk is still extremely small. In the starting position of 1L, the knee might have been more extended than in the starting position of 2L due to stabilising reasons and higher moments (Fig. 2a, Tables 2 and 3).
For the knee joint not being directly involved in the movement, high Mk,max were obtained, especially for BEh and 1L executions. It is important to note that Mk,max is an extension moment and is thus supporting the isometric contraction of the hamstrings and providing a training effect for these muscles. In addition to the positive effect on the hamstrings, the extension moment might exert forces on the passive structures in the knee, which must be considered in patients with current or past knee injuries. As far as we know, no one has ever looked at the kinematics and kinetics in the knee during BE although this exercise might shift the quadriceps/hamstring ratio towards hamstring as part of an Anterior cruciate ligament (ACL) prevention program. The knee extension moments obtained for goodmornings were slightly higher, probably due to the extra barbell load of 25% BW .
Because BE are a dynamic exercise, the EMG results must be treated and interpreted with caution due to skin artefacts. This caution is especially important for the comparison between BEh and BEs, which are different movements.
In general, all muscles were more active during the concentric phase than during the eccentric phase (Tables 4 and 5). For the concentric phase, the activity levels for gluteal muscles and hamstrings agree with the literature . The activity level for lumbar and thoracic dorsal back muscles are considerably lower (lumbar: 39-56% MVC, thoracic: 43% MVC; ). For gluteal muscles and hamstrings, no significant differences between BEh and BEs were found, which agrees with the results of Mayer and co-workers , who did not find any influence of lumbar posture on these muscles. Moreover, they also observed that an accentuated lumbar lordosis increased the activity of the lumbar extensors, whereas the present study found, for ErecLum, no significant differences between BEh and BEs. However, due to the different MVC measurement settings, electrode placements, starting positions and exercise executions, a direct comparison between different studies is difficult. The only study looking at the muscle activities in the eccentric phase of the BE exercise was a study by De Ridder and co-workers , who considered BE performed in a horizontal position and with an extra load of 60% 1RM. Due to the extra load, a comparison of the values between their study and the present study is not reasonable.
As expected, the ventral trunk muscles were almost inactive (2-5% MVC), which agrees with the results of Callaghan and co-workers . The high standard deviations (see Table 4) suggest that some participants needed to activate those muscles more than others to stabilise the trunk.
Comparison of types BEh and BEs
Apart from the obvious differences in RoMmub, RoMlmb and RoMh, there were other relevant differences between BEh and BEs. BEh showed higher external moments in hip, knee and lower back with higher or similar RoMs (Fig. 2a-c, Tables 2 and 3). This finding suggests a higher training effect for BEh for dorsal back and hip muscles and for hamstrings. However, the above-mentioned opposed movement in the spine for BEh must be considered.
Comparison of versions 1L and 2L
The 1L version showed higher external moments in the hip and knee with smaller or similar RoMs (Fig. 2a-c, Tables 2 and 3). Because strength training requires high moments with high RoMs, we are unsure whether 1L provides a greater training effect for the dorsal hip muscles and hamstrings than 2L. These results suggest that the version should be chosen according to the specific requirements, i.e., whether high RoM or high external moments are desired.