Beyond the Legs: Reconsidering Sprint Performance through the Lens of Spinal Engine Theory

Abstract

Traditional sprint models have advanced understanding of lower-limb force production, but they do not fully explain the active contribution of the trunk and lumbo-pelvic system to sprint performance. This thesis aimed to examine the role of lateral trunk flexion in sprinting by first reviewing the relevant biomechanical literature and then testing whether isometric lateral trunk flexion strength was associated with 40-m sprint times and discrete performance variables. The narrative review identified that conventional models, including the SMM and 2MM, largely represent the upper body or above the lower leg as a single passive mass, whereas emerging imaging, kinetic, morphological, and coordination evidence suggests that the trunk contributes actively to sprinting through muscle activation, pelvic rotation, COM control, and integrated arm-trunk-leg coordination. This evidence provided the rationale for the experimental study, as the literature describes what the trunk does during sprinting but has rarely tested whether lateral trunk flexion strength is directly related to sprint performance. Eighteen trained sprint and field-sport athletes (age 18.9 ± 2.9 years; body mass 84.2 ± 8.9 kg) completed isometric lateral trunk flexion testing using fixed dynamometry and maximal 40-m sprints. Split times over 0–10 m, 10–20 m, 20–40 m, and 0–40 m, peak acceleration, peak velocity, and distance to peak velocity were recorded. The lateral flexion peak force protocol demonstrated good-to-excellent intrasession reliability across sides (ICC = 0.84–0.90; CV = 7.81–9.15%). Pearson product-moment correlations showed that greater lateral flexion peak force was associated with faster 0–10 m (r = −0.63, p = .007), 10–20 m (r = −0.56, p = .018), and 0–40 m sprint times (r = −0.70, p = .002). The association with 20–40 m sprint time was moderate but non-significant (r = −0.48, p = .054), and no significant relationship was observed with distance to peak velocity (r = .41, p = .101). These findings suggest that lateral trunk flexion strength may be most relevant to acceleration and early sprint performance rather than later, higher-velocity sprinting. However, the results should be interpreted with caution because the study was cross-sectional, involved a small and predominantly male sample, and did not include concurrent kinematic or GRF data to explain the mechanism underpinning the observed relationships. Practically, lateral trunk flexion strength may be worth assessing and developing in athletes whose performance depends on acceleration, but further research with larger samples using predictive experimental designs is required to elucidate the contribution of lateral trunk flexion strength to linear sprinting performance.