X-Factor: Trunk and Hip Dissociation

Hip-shoulder separation (X-factor, pelvis-trunk separation) pertains to axial rotation observed in the loading/cocking stage of the serve & ground strokes. It is a critical component for high velocity serves & groundstrokes & defined as the angular difference, in the transverse plane, b/w: 1) a line connecting the hip joint centers & 2) a line connecting the shoulder joint centers. Pro tennis players show on avg. a 25-30° separation angle.

It is critical because it accommodates the stretch-shorten cycle (SSC) of the trunk musculature during serve & groundstroke production. The SSC allows the muscles of the trunk (i.e., Lat dorsi & internal/external obliques) to eccentrically contract (lengthen) to harness potential energy before concentric contraction (shorten) during the acceleration phase of serve. This mechanism increases power that the muscles produce at ball contact.

Timing of eccentric to concentric contraction is a key element of the SSC as stored elastic energy in the muscle dissipates as a fx of time. In other words, delay from loading to acceleration phase will reduce elastic energy & decrease stretch reflex.

Max separation is observed in early cocking phase as the pelvis (hips) rotate before the torso. This further potentiates elastic energy storage in the core. These mechanics are also observed during the downswing in golf & swing/pitch in baseball. Engaging pelvis rotation “lead w/ hips” while maintaining torso posture maximizes rotation power during hitting.

However, these mechanics may lead to Injury to the lumbar region due to high torsional loads. Although important to have optimal mechanics for performance, it is imperative to have adequate core stability/strength to prevent injury.

Furthermore, several studies have examined that impaired core proprioception, measured by active proprioceptive repositioning of the trunk, predicted knee injury risk! (Zazulak 2007)

In Summary: Optimal hip-shoulder separation relies on:
1) 25-30° separation
2) Rapid transition from back to forward swing
3) Pelvis prior to thorax rotation.

Pelvic Rotation Exercise:

These exercises promote motor control of pelvic on trunk rotation during serve & groundstrokes. A resistance band can be used to provide more input to the body!

Citations:
1) Zazulak, B. T., Hewett, T. E., Reeves, N. P., Goldberg, B., & Cholewicki, J. (2007). The effects of core proprioception on knee injury: a prospective biomechanical-epidemiological study. The American journal of sports medicine, 35(3), 368-373.
2) Dines, J. S., Bedi, A., Williams, P. N., Dodson, C. C., Ellenbecker, T. S., Altchek, D. W., ... & Dines, D. M. (2015). Tennis injuries: epidemiology, pathophysiology, and treatment. JAAOS-Journal of the American Academy of Orthopaedic Surgeons, 23(3), 181-189.
3) Davies, G., Riemann, B. L., & Manske, R. (2015). Current concepts of plyometric exercise. International journal of sports physical therapy, 10(6), 760.
4) Maquirriain J, Ghisi JP. Uncommon abdominal muscle injury in a tennis player: internal oblique strain. Br J Sports Med 2006;40(5):462–463.
5) Elliott B, Reid M, Crespo M. Technique Development in Tennis Stroke Produc- tion. Valencia, Spain: International Tennis Federation, 2009.