Low back pain (LBP) is the most common injury among rowers (Thornton et al., 2017; Trease et al., 2020). It accounts for 32–52% of all reported rowing injuries (Bahr et al., 2004; Wilson et al., 2010). According to a study, 18% of rowers who reported LBP missed training in excess of 1 month (O’kane et al., 2003), and in other studies, 53–79% reported LBP recurrence (Teitz et al., 2003; Newlands et al., 2015). Despite the high incidence of LBP in rowers, there is no clear consensus on prevention strategies, and the development of effective interventions remains challenging.

Dysfunction of the trunk muscles that control the spine is a factor in the development of LBP (Panjabi, 1992; Richardson et al., 1999). Bergmark (1989) proposed grouping muscles by functional characteristics into global (e.g., the erector spinae, rectus abdominis, internal oblique, external oblique, quadratus lumborum, psoas major, and vastus lateralis muscles) and local (transverse abdominis and multifidus muscles) muscles. Global muscles contribute to the regulation of spinal orientation, the balancing of extrinsic burdens, and the production of a substantial amount of torque for spinal movement. People with LBP are preferentially biased toward posture-specific activation of global muscles, which are larger and more superficial, than local muscles, which fine-tune intersegmental movement (van Dieën et al., 2003a; Ferguson et al., 2004; Marras et al., 2004; Claus et al., 2018). Although excessive activity of these muscles contributes to spinal stability, it also increases the load on the spine and is considered a risk factor for LBP (van Dieën et al., 2003b; Hodges et al., 2013).

Despite the importance of examining muscle activity during rowing when considering prevention strategies and interventions for LBP in rowers, the differences between rowers with and without LBP regarding trunk muscle activity during rowing at maximal effort (such as a 2,000-m race) are unclear. A previous study reported that the lumbar erector spinae activity was significantly higher during incremental rowing tests in rowers with a recent history of LBP than in those without LBP (Martinez-Valdes et al., 2019). Therefore, even when rowing under maximal effort, trunk muscle activity may differ between rowers with and without LBP. The purpose of this study was to investigate the differences in trunk muscle activities during rowing at maximal effort between rowers with and without LBP, assuming a 2,000-m race.

There were no differences in age, height, weight, or BMI between the LBP and control groups (P = 0.346, P = 0.456, P = 0.323, and P = 0.356, respectively; Table 1). The ODI, RDQ, and VAS scores were significantly higher in the LBP group than in the control group (P < 0.001, P = 0.004, and P < 0.001, respectively; Table 1). There were no significant differences in the 500-m measurement time or stroke rate between the two groups (P = 0.112 and P = 0.909, respectively; Table 2). All participants completed the 500-m rowing measurement without LBP.

Significant interactions were found in the TES and LES muscle activities (P < 0.001 and P = 0.047, respectively; Figure 3). In the post hoc test, the TES activity in the LBP group was significantly higher than that in the control group at the 10% to 20% and 20% to 30% stroke cycles (P = 0.013 and P = 0.007, respectively). The LES activity in the LBP group was significantly higher than that in the control group at the 0% to 10% stroke cycles (P < 0.001). There was a main group effect in the LD activity, with significantly higher activity in the LBP group than in the control group (P = 0.023). There were no significant interactions or main effects in the activities of the RA and EO muscles (Figure 4). There were significant main effects of stroke cycle on the TES, LES, LD, EO, and RA muscles (P < 0.001, P < 0.001, P < 0.001, P < 0.001, and P < 0.001, respectively). For kinematics data, there was a significant difference of the range of motion at L3 (P = 0.027; Table 3).

In this study, we investigated the differences in the amplitude of trunk muscle activity during rowing at maximal effort in a 2,000-m race between rowers with and without LBP. This study revealed that the amplitudes of the LES and TES muscle activities were significantly higher in the LBP group than in the control group, especially in the 0–30% of one stroke cycle. A main group effect was observed in LD muscle activity, with higher muscle activity observed in the LBP group than in the control group. However, no significant interaction or main effects were found in the abdominal muscle groups, such as the EO and RA.

This is the first study to show that LES, TES, and LD muscle activities during rowing under maximal effort, simulating a 2,000-m race, were significantly higher in the LBP group than in the control group. Martinez-Valdes et al. (2019) reported that the amplitude of LES activity was higher in rowers with a history of LBP than in asymptomatic rowers during an incremental rowing test. A previous study reported that the cross-sectional area of the back muscles of rowers with LBP was significantly larger than that of rowers without LBP (McGregor et al., 2002), suggesting that rowers with LBP may exhibit excessive activity of the back muscles than of the lower or upper limbs to generate force during strokes. The results of this study support those of the above-mentioned previous studies.

The TES, LES, and LD muscles that showed significant differences in this study are classified as global muscles (Bergmark, 1989). Previous studies have reported that individuals with LBP overactivate their global muscle groups. Increased global muscle activity increases spine stress (van Dieën et al., 2003a; Ferguson et al., 2004; Marras et al., 2004), and this may contribute to LBP (van Dieën et al., 2003b; Hodges et al., 2013). In addition, repeated excessive muscle activity is believed to cause muscle pain (Johansson and Sojka, 1991; Visser and van Dieën, 2006). The finding of this study that rowers with LBP exhibit excessive activity in the back muscles suggests that pain may occur in the muscles. Tsao et al. (2011) reported that experimentally induced LBP increased the excitability of superficial trunk muscles, including the OE and LES, to cortical stimulation by transcranial magnetic stimulation, indicating that LBP may rapidly induce plasticity of cortical motor pathways. Experimental LBP may elicit long-term plasticity of the cortical motor pathways related to global muscle overactivity, even after pain extinction (Rohel et al., 2022). Therefore, the differences in muscle activity observed in this study may be related to central nervous system plasticity. Furthermore, in this study, significant differences were found in LES and TES muscle activity from 0–30% of the stroke cycle. The first half of the stroke cycle is called the “drive phase” and is the phase that generates the driving force for rowing. In a previous study, peak forces estimated at L4/L5 during rowing using ergometer under maximal effort were 2694 N of compressive force (4.6 times the rower’s body weight) and 660 N of shear force (Fl et al., 2000). The “drive phase” requires the back muscle group activties to generate propulsive force, and muscle activity during this phase may be important in examining its relationship with LBP. To prevent and improve LBP, attention should be paid to the activity of global back muscles during rowing, especially during the “drive phase.”

The symptoms of the rowers in the LBP group in this study were relatively mild. The rowers in this group were capable of rowing at maximum performance, and the impact of LBP on their daily lives as assessed by ODI and RDQ was mild. Therefore, the results of this study can be generalized to rowers with mild LBP and may be related to rowers with worsening LBP.

In this study, there were no statistically significant differences in 500-m trial time or stroke rate between the LBP and control groups. Therefore, the differences in muscle activity of the LES, TES, and LD muscles observed between the LBP and control groups in this study were not likely to be caused by differences in rowing performance. The force applied to the oar handle depends on the musculoskeletal forces or joint torques generated by knee and hip extension and upper limb pull movements, in addition to that generated by the back muscles (Baudouin and Hawkins, 2002). The pelvic muscles, in particular, are so important to posture and technique during rowing. The control group exhibited comparable performance with significantly lower muscle activity in the back muscle group compared to that observed in the LBP group, which may have generated more upper and/or lower extremity muscle activity.

Concurrent with the electromyography analysis in this study, we have analyzed the range of motion in sagittal plane angles of the spinal segments, pelvis, hip, and knee joints. We found that the range of motion of L3 was significantly larger in the control group than the LBP group. However, it is difficult to relate the present kinematics results to the EMG results. It may be possible to explain the relationship between EMG and kinematics in the present results by measuring not only trunk muscles but also other muscle activities such as pelvic muscles.

This study has some limitations. First, it is unclear whether the abnormalities in EMG activity indicate a cause or an effect of LBP because this study was a cross-sectional study. Prospective studies are required to clarify this causal relationship. Second, EMG recordings were obtained using a surface EMG system. Although the electrode placements in this study were consistent with those in a previous study (Pollock et al., 2009), it is likely that the surface EMG detected the activity of other muscles. Third, muscle activity measurements were limited to trunk muscle measurements. Further analysis of the muscle activity of other muscles, including the pelvic muscles, would provide further evidence related to the study results. Fourth, MVIC was used to normalize the muscle activity of the rowers with LBP. Although MVIC is not accurately measured in individuals with LBP (Larivière et al., 2003), the most appropriate normalization method for individuals with LBP has not yet been established (Besomi et al., 2020). Finally, although this study used rowing ergometers to analyze muscle activity during rowing, it has been reported that sagittal lumbar motion differs between rowing ergometers and rowing on water (Wilson et al., 2013). Since changes in trunk kinematics also affect muscle activity, it should be noted that the results of this study are limited to the characterization of muscle activity on the rowing ergometer as different muscle activities may be observed when rowing on water.

The present study showed that rowers with LBP compared with those without LBP exhibited significantly higher TES, LES, and LD muscle activities during the drive phase of ergometer rowing. These results indicate that rowers with LBP exhibit excessive back muscle activity during rowing under maximal effort.