Trunk strength plays an important role from different aspects--related to health and physical performance. Most researchers who compared healthy subjects' trunk strength in different planes of movement found the greatest strength in sagittal plane extension (Smith et al., 1985), followed by sagittal plane flexion, frontal plane bending (Guzik et al., 1996) and transversal plane rotation with the smallest force output (Beimborn and Morrissey, 1988).
Although maximal voluntary isometric trunk strength is not the best predictor of low back pain (LBP) (McGill, 2007; Lindsay and Horton, 2006), there are studies, which suggest that good global trunk muscle performance may protect back-related problems (Rissanen et al., 2002). When LBP patients and healthy controls were compared, different conclusions have been reported. Kumar et al. (1995) showed that healthy subjects and patients both were stronger in trunk lateral flexion compared with axial rotation, but patients demonstrated just 45%-55% peak isometric torques of asymptomatic controls. Nouwen et al. (1987) found different muscle activity of abdominal and back muscles during dynamic muscle actions of sagittal flexion only. On the contrary, Ng et al. (2002) found decreased isometric muscle strength in all planes of trunk movement in LBP subjects. Back in 1980 McNeill et al. found a deficit of isometric trunk extensor muscles in LBP subjects, while later studies (Gomez, 1994; Leino et al., 1987) did not confirm the importance of trunk isometric strength as a predictor of low back troubles. Studies on athletes showed similar values during isometric maximal voluntary contractions (MVCs) between asymptomatic subjects and LBP sportsmen. Klein et al. (1991) reported that MVC is a poor indicator of LBP in rowers, Maus et al. (2010) did not find differences in MVC between soccer players and Renkawitz et al. (2006) found similar trunk extension strength in amateur tennis players.
Low fitness level, with not much use of trunk extensors, leads to histomorfologic and structural changes, mainly the atrophy of type 2 muscle fibers. These changes are the causes of decreased strength in these muscles (Parkkola et al., 1993). Dannels et al. (2002) found that healthy subjects had higher activation of multifidus and iliocostalis lumborum during strength training than sub-acute LBP patients.
Because of different patterns in lumbo-pelvic motion in sagittal plane in LBP subjects (Esola et al., 1996) and significantly higher compression forces on the lumbar spine during axial rotations (McGill, 2007), movements in sagittal and transverse planes are among the most investigated. Data by McGill et al. (2003) suggested that in contrast to a single plane strength analysis, perturbed flexion-extension ratio is related to back problems. McGill (2007) concludes that higher values of trunk extension-flexion strength ratio are more frequently seen in people with LBP. On the other hand, Lee et al. (1999) found greater trunk flexors' strength than trunk extensors' strength in people with back troubles.
While often studied independently, trunk and hip muscles act functionally together. Some studies compared static flexion and extension strength in relation to hip joint position. Keller and Roy (2002) found out higher values of extension-flexion ratio with increased hip flexion. Cartas et al. (1993) and Wessel et al. (1994) found a reduced peak isometric trunk flexion torque with increased trunk flexion. Gallagher (1997) showed decreased peak torque of trunk extensors in kneeling compared to standing body position, in contrast with another study (Graves, 1990; Tan et al., 1993), where they found peak torque values of trunk extensors in full hip flexion. Szpala et al. (2011) compared trunk extensor's torques and spinal muscles activity during sitting and lying body positions. They found significantly higher values of electromyographic activity in m. erector spinae during lying and peak torque values during sitting position. Tan et al. (1993) suggested that increased erector spinae efficiency in more flexed postures during trunk extension tasks is a consequence of its increased mechanical advantage.
Measurement of trunk strength represent an important insight into either the individual's performance or back health. Although different approaches exist (static, isoinertial, isokinetic), isometric computerized dynamometry offers good reliability (Azghani et al., 2009), relatively cheap testing and a good pelvic fixation. Despite brand and protocol differences between various dynamometers, Demoulin et al. (2012) demonstrated significant inter-system correlations of absolute maximal voluntary contractions values. The aim of this study was to assess trunk flexion and extension strength in different positions of hip joint in sagittal plane. Our second objective was to compare correlations in trunk strength between all three planes of trunk movement in the seated position.
Sixty healthy adults volunteered for the study. The structure of the participants of the study, including age, gender, body mass, and body height is presented in Table 1. Participants with acute or chronic LBP, or systemic neurological disease were excluded. Subjects were informed about the study protocol before the beginning of the experiment and confirmed their voluntary participation by signing the informed consent. The study was approved by the National Medical Ethics Committee.
A multi-purpose dynamometer was custom developed (S2P Ltd., Ljubljana, Slovenia) to measure isometric trunk strength in all three planes of trunk exertion (Figure 1). Maximal force was recorded via force sensor (Z6FC3 -200 kg, HBM, Darmstadt, Germany), which was traction loaded, depending on movement direction. The signal was 400x amplified, analog-to-digital converted and acquired at a sampling rate of 1000 Hz (NI-USB-6009, NI, Austin, USA). The signals were stored on a personal computer for later analysis. Subjects were instructed to perform three maximal voluntary isometric contractions of trunk flexion and extension in standing, kneeling and sitting position. The order of the tasks was random. Additional testing contains both side bandings and rotations in sitting position. Trunk was maintained in an upright position...