Back pain (point) prevalence in adolescent athletes is reported at a rate of 8–20%, with a relevant increase beginning around age 14 and featuring sport-specific differences (Schmidt et al., 2014; Müller et al., 2017). Consequently, back pain can be considered a relevant risk factor in the careers of young elite athletes.

In the context of back pain, great emphasis has been placed on the importance of trunk stability, especially in situations requiring compensation of repetitive, intense loading induced during high-performance activities, e.g., jumping or landing (Cholewicki et al., 2000; Borghuis et al., 2008; Simons and Bradshaw, 2016). In etiology, repetitive micro-trauma and insufficiency of the muscle-tendon complex based on an inadequate neuromuscular and postural control, and a reduced maximum strength capacity in addition to trunk muscle fatigue during dynamic loading are discussed as an explanatory model (George and Delitto, 2002; Sassmannshausen and Smith, 2002; Standaert, 2002, 2008; Trainor and Wiesel, 2002; Bono, 2004; Trainor and Trainor, 2004; Lawrence et al., 2006). In particular, trunk muscle forces providing stability are considered meaningful in counteracting high-impact loading during high-intensity activities (Kibler et al., 2006; Borghuis et al., 2008; Larivière et al., 2015; Prieske et al., 2016). When compensating high loading, a reduced trunk strength capacity as well as delayed muscle onset, increased co-contractions, and increased SEMG variability has been shown in back pain patients (Cholewicki et al., 2000; Radebold et al., 2000, 2001; Marras et al., 2005). In a recent systematic review, Abboud et al. could moreover show evidence for back pain related decreased erector spinae and increased external obliquus muscle reflex amplitudes (Abboud et al., 2016). In addition, trunk strength capacity is considered essential for the compensation of external forces and loads in young and adult athletes (Kibler et al., 2006; Zazulak et al., 2006, 2007; Wirth et al., 2016).

As reported in previous studies, different types of sports reveal specific demands on core stability (Helge and Kanstrup, 2002; McGregor et al., 2004; Iwai et al., 2008; Baur et al., 2010; Mueller et al., 2012). Nevertheless, Kibler et al. (2006) described the role of core stability for all types of sports, whether running, throwing or jumping tasks. High-impact forces acting on the trunk are reported in judo, rowing, weight lifting, (rhythmic) gymnastics, and jumping (Liemohn et al., 2005; Peate et al., 2007; Hibbs et al., 2008; Ripamonti et al., 2008). Repetitive loading with large components of translation, rotation and reclination movements are believed to result in stress of the structures involved (Hutchinson, 1999; Sassmannshausen and Smith, 2002; Adirim and Cheng, 2003; Jones et al., 2005). It could be shown that athletic tasks like running, hopping, jumping and landing increase the impact forces that need to be compensated (Dufek and Bates, 1990; Keller et al., 1996; Simons and Bradshaw, 2016). Simons and Bradshaw (2016) reported an additional loading of the trunk with up to eight times the body weight during repetitive hopping or drop landing. The importance of trunk strength capacity was recently reported as beneficial not only for compensating loading and stabilizing the trunk, but also for enhancing athletic performance (Kibler et al., 2006; Zazulak et al., 2006, 2007). Furthermore, Zazulak et al. (2007) reported an association between trunk muscle activity and lower limb kinematics during landing tasks. Decreased neuromuscular activity is attributed to higher knee valgus, increasing injury risk at the knee. Hence, an inadequate (neuromuscular) compensation strategy is discussed as a common cause of overloading and injury. As a possible consequence, there is a need to identify relevant trunk muscles that must necessarily be addressed in injury and overload prevention. Nevertheless, the role of back pain as one factor influencing neuromuscular activity of the trunk-encompassing muscles has not been clarified.

Therefore, this study aims to evaluate trunk muscle activity during drop jump (DJ) in adolescent athletes with back pain compared to their healthy counterparts. An altered neuromuscular activity strategy driven by higher ventral and reduced dorsal muscle SEMG amplitudes in athletes suffering from back pain while performing and compensating high-impact loading during drop jumps is expected compared to healthy athletes.

Altered neuromuscular activity in back pain patients compared to healthy subjects is known (Radebold et al., 2001; Maaswinkel et al., 2016). This study aimed to evaluate neuromuscular activity of the trunk muscles during high-impact loading represented by drop jump performance in adolescent athletes with back pain compared to healthy counterparts. The main findings of the investigation are an altered neuromuscular activation pattern in adolescent athletes with back pain, with increased SEMG amplitudes for the M. obl. ext. abd (EO) and M. obl. int. abd (IO) especially during reactive ground contact (Post_i). Regarding the total jumping cycle the abdominal and transverse muscle groups showed increased SEMG amplitudes with similar absolute drop jump performance measurements.

There is existing evidence that athletic performance and function is an outcome of the appropriate (neuromuscular and kinetic) coordination of body segments (Kibler et al., 2006). Back pain is often discussed as an influencing factor, but, despite the existence of back pain, the adolescent BP athletes do not show a reduction in (drop) jump performance capacity. Jump performance alone might therefore not serve as a valid indicator for deficits in adolescent athletes with back pain. In this context, the level of back pain and chronification might play a relevant role. It could also be speculated that high pain levels compared to our cohort might show reduced performance as described in other papers (Balagué et al., 2012; Bauer et al., 2015).

However, in terms of trunk muscle activity, BP is associated with an altered neuromuscular activity level as reported previously (Radebold et al., 2001; Nelson-Wong and Callaghan, 2010; Abboud et al., 2016; Maaswinkel et al., 2016). A decreased muscle activity for the dorsal muscle group (e.g., erector spinae), as reported by Ramprasad et al., and Shenoy et al., for patients with back pain, could not be supported by our data (Ramprasad et al., 2010; Shenoy et al., 2013). But, increased muscle activity in the abdominal and transverse muscle groups correspond to higher co-contractions, already presented in other trunk loading experiments (Radebold et al., 2001; Nelson-Wong and Callaghan, 2010). Furthermore, Liebetrau et al. could present in their musculoskeletal model that delayed abdominal muscle reflex lead to a reduced trunk stability. Information's about detailed association for pain and function (higher pain—higher functional deficits) are rare since experimental studies mainly remain to the dichotomous comparisons of pain to identify functional differences (Liebetrau et al., 2013; Schinkel-Ivy et al., 2013; Abboud et al., 2016). To compensate delayed reflex activity, higher activation amplitudes of the abdominal muscles are valid (Liebetrau et al., 2013). Transferring this to drop jumps, it could be speculated that the shown increased muscle activity (Mm obl. abd. intern./extern.) serves as a compensation strategy to enhance core stability and protect the trunk from further negative loading (Kibler et al., 2006; Hibbs et al., 2008; Prieske et al., 2015; Wirth et al., 2016). It is also known that the stability provided by the trunk muscles is meaningful in counteracting single and repetitive loading during high-intensity performance (Kibler et al., 2006).

The combination of a similar (jump) performance and yet increased neuromuscular trunk activity level in adolescent BP athletes compared to NBP athletes still appears controversial. It could be speculated that BP athletes present a less efficient jump performance execution. Otherwise, the additional muscle activation might show the functional adaptive response in athletes supporting an appropriate protection from overloading (Maaswinkel et al., 2016). Therefore, drop jumps could serve as a suitable test situation to analyze alterations in the neuromuscular trunk activity of athletes with and without back pain. Additionally, SEMG activity might reflect the compensation capacity for repetitive high loading. It remains to be seen whether this might not only serve to identify athletes suffering from back pain, but also athletes at risk for developing it. In addition, this analysis might allow a valid evaluation of athletes in order for them to return to sports after a period of back pain. Nevertheless, these points of discussion need further verification.

Consequently, optimizing neuromuscular core stability is considered beneficial for protection against repetitive and excessive overloading of the trunk (Kibler et al., 2006; Borghuis et al., 2008; Saragiotto et al., 2016; Wirth et al., 2016). In line with the results presented here, core stability exercises (with additional repetitive loading) addressing the complex interaction of transverse, straight, ventral and dorsal muscles in adolescent athletes with back pain should be preferred (Pedersen et al., 2004; Saragiotto et al., 2016).

Certain limitations have to be considered when interpreting the results. Acute but no average pain was assessed and association of pain with outcome variables is not presented since correlation analysis would not be valid for the existing skewed pain distribution (no subjects with pain intensity between 0 and 2 VAS). SEMG normalization is often used to acquire comparable values between different individuals and groups, but contains some crucial points. The pain might influence the MIVC measurement used, since the intended 100% activation might not be reached. This could lead to reduced strength values and a systematic overestimation of the normalized SEMG amplitudes during the jumps for the patients (Marras et al., 2005). Our data showed for the MIVC measurement in extension and flexion (18/23%) lower strength values for BP. But, the higher (BP) activity during jumping was only for the flexor muscles, but not for the others present. Therefore, considering this limitation, the SEMG method and normalization procedure used seem valid. Furthermore, the highly standardized dynamometer-based MIVC extension and flexion test might not challenge all ventral/dorsal or straight/transverse muscles identically (Iida et al., 2011). This should be noted to put the normalized SEMG values (e.g., LD > 100%) into perspective. The SEMG setup used has been shown as valid in previous studies, but the possibility of a bit of cross-talk between muscles cannot be totally denied (Radebold et al., 2001). Concerning the investigated athletes performing mainly rowing, the applicability to other sports needs to be proven in future investigations. Finally, only the current, but not the average back pain intensity across days or weeks was assessed and might have influenced the results.

In conclusion, adolescent athletes with moderate back pain intensity are capable of presenting (jump) performance comparable to that of their healthy counterparts. Higher activity of the transverse, but not the straight, trunk muscles indicates a specific compensation strategy to support trunk stability in athletes with back pain during drop jumps. For prevention and therapy, specific sensorimotor exercises addressing the transverse trunk muscles with e.g., 3-dimensional loading situations might be beneficial.

The author contributions are distributed as followed: conception or design of the work (SM, JM, FM), data acquisition (SM, JS, JM, MC), data analysis (SM, JM) and interpretation (SM, JM, FM); drafting (SM) or revising (JS, JM, MC, FM) the work; final approval of the version to be published (SM, JS, JM, MC, FM); agreement to be accountable for all aspects of the work (SM, JS, JM, MC, FM).

This study was supported by a research grant from the National Institute of Sport Science of Germany (Bundesinstitut für Sportwissenschaft BISp: IIA 1-080126/09-13). We acknowledge the support of the Deutsche Forschungsgemeinschaft and Open Access Publishing Fund of University of Potsdam.