Short-term effects of thoracic spine manipulation on the biomechanical organisation of gait initiation: a pilot randomized study

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Keywords:

• Anticipatory postural adjustments
• gait initiation
• spinal manipulation HVLA
• T9 vertebrae
• posturo-kinetic capacity

1. Introduction

Gait initiation, which is the transient phase between quiet standing posture and ongoing walking, is a functional task that is classically used to investigate postural control (Brenière et al. 1987). It is composed of a postural phase preceding the swing foot-off, which corresponds to the ‘anticipatory postural adjustments’ (APA), followed by an execution phase ending at the time of swing foot contact with the ground (cf. Figure 1(a)). During these APA, the center of pressure is shifted backwardly, which provides the initial propulsive forces necessary to reach the intended progression velocity. The larger this shift, the greater the progression velocity (Brenière et al. 1987; Lepers and Brenière 1995). According to the concept of ‘Posturo-Kinetic Capacity’ (Bouisset and Do 2008), any factor that improves the mobility of the postural chain, in particular the mobility of the spine, facilitates the development of APA and the motor performance (in terms of velocity, force, precision and so on).

Figure 1. (A). Stick representation of gait initiation. P: standing posture; APA: anticipatory postural adjustments; EXE: step-execution; (B) HVLA Manipulation.

‘Spinal Manipulative Therapy High-Velocity, Low- Amplitude’ (SMT-HVLA), which is a healing manual technique applied to the spine, has been used for centuries by healthcare professionals to relieve painful patients or to improve spine mobility (Wiese and Callender 2005). However, no studies to date investigated the effects of vertebral manipulation on APA and gait initiation performance. The present study was designed to test the hypothesis that SMT- HVLA increases spine mobility, which in turn, facilitates the development of APA and motor performance.

2. Methods

This randomized study included twenty-two young healthy adults. Eleven participants (6 women, 5 men, 28 ± 4 years, 64 ± 8 kg, 169 ± 8 cm) were assigned to the HVLA group and eleven participants (5 women, 6 men, 29 ± 4 years, 63 ± 8 kg, 170 ± 8 cm) were assigned to the control group (sham). Participants of each group performed series of ten gait initiation trials on a force-plate (AMTI, Watertown, MA, USA) at a spontaneous velocity before and after a vertebral manipulation or a sham protocol. The manipulation protocol consisted of a manipulation of the SMT- HVLA applied to the ninth thoracic vertebrae (T9, cf. Figure 1(b)). The following biomechanical parameters were quantified in each trial, APA duration, APA amplitude (peaks of center of pressure (COP) and COM velocity at toe off in the sagittal plane), step length, step-execution phase duration, anteroposterior (AP) COM velocity at the time of foot contact (step performance), COM speed was achieved with a simple integration of COM acceleration. The COM acceleration was obtained thanks to Newton’s second law. The evaluation of the spine range of motion in flexion-extension was evaluated by an inclinometer (Bubble® Inclinometer, Manufacturing Enterprises, USA). A mixed model analysis of variance (ANOVA) was used, followed by Tukey’s post hoc test when appropriate. The threshold of significance was set at alpha = 0.05.

3. Results and discussion

The results showed that the maximal thoracic flexion increased by 20% in the HVLA group after the manipulation, which was not the case in the sham group. In the HVLA group, results further showed that each of the following gait initiation variables reached a significantly lower mean value in the post- manipulation condition as compared to the pre- manipulation condition (cf. Figure 2): APA duration, peak of backward anticipatory center of pressure displacement, center of gravity velocity at foot-off, peak of center of gravity velocity and step length. In contrast, for the sham group, results showed that none of the gait initiation variables significantly differed between the pre- and post-manipulation conditions.

Figure 2. Comparison of selected biomechanical parameters between the pre- and post-manipulation conditions in the sham and HVLA groups. xP_APA: peak of anticipatory backward centre of pressure displacement, x’G_TO: centre of gravity velocity at toe- off, x’G_MAX: peak of centre of gravity velocity (motor performance). **, ***: statistical difference with p < 0.01, p < 0.001, respectively.

These results suggest that the HVLA manipulation applied to T9 has an immediate beneficial effect on the mobility of the spine but, contrary to our expectations, it has a detrimental immediate effect on the development of APA and related motor performance. In addition to its mechanical effect on spine mobility (Millan et al. 2012), SMT-HVLA is known to induce transient changes in sensorimotor pathways and structures involved in coordination between posture and movement (Haavik and Murphy 2012, Lelic et al. 2016). The negative motor outcome reported in this study may thus have been meditated by these neural changes.

4. Conclusions

It is concluded that HVLA manipulation applied to T9 has an immediate beneficial effect on spine mobility but a detrimental effect on APA development and step performance during gait initiation. We suggest that a neural effect induced by SMT-HVLA, possibly mediated by a transient alteration in the early sensory-motor integration, might have masked the potential mechanical benefits associated with increased spine mobility. HVLA manipulation should be considered with caution by participants who seek an immediate increase of motor performance during locomotor tasks. This is the case for athletes who often use this technique with the goal to increase their speed performance.