The impact of compressive force magnitude on the in vitro neutral zone range and passive stiffness during a flexion–extension range of motion test

Figures & data

Figure 1. A schematic of the material testing system set-up.

Figure 2. Images of a specimen in the material testing system: specimen in flexion (left) and extension (right). The moment angle data were sampled from an independent servomotor connected in series with a torque cell.

Figure 3. Defining neutral zone range using a method of fitting a fourth-order polynomial curve to the ascending and descending curve (solid line) to the raw data (thin line), and first derivative of the fitted curve (dotted line) was calculated (Thompson et al., 2003). The neutral zone range was defined by the angle where it reached  ± 0.05 Nm/degree (flexion and extension limit defined).

Table 1. Mean (SE) neutral zone range, passive stiffness and flexion/extension limits (n = 33)

Dependent measure	Compressive force magnitude (N)
	10	300	900	1800
Neutral zone range (degrees)	7.21^A (0.41)	4.64^B (0.25)	3.28^C (0.15)	2.80^D (0.15)
Passive stiffness (Nm/degree)	0.03^A (0.003)	0.11^B (0.01)	0.23^C (0.01)	0.41^D (0.02)
Flexion limit (degrees)	5.48^A (0.91)	6.49^B (0.81)	6.36^B (0.82)	6.67^B (0.91)
Extension limit (degrees)	−1.74^A (0.84)	1.82^B (0.75)	3.04^C (0.78)	3.96^D (0.88)

Notes: The neutral zone range decreased, whereas the stiffness increased as the magnitude of compressive force increased.

Flexion limits (positive limits denote flexion) did not change at higher compressive force, while the extension limits significantly shifted more towards flexion as the compressive force increased.

Superscript letters in the table within each row indicate the results of pairwise comparison, where the significant difference (p < 0.05) of compressive force is indicated by different superscript letters.

Thompson, R. E., Barker, T. M., & Pearcy, M. J. (2003). Defining the neutral zone of sheep intervertebral joints during dynamic motions: An in vitro study. Clinical Biomechanics, 18, 89–98.10.1016/S0268-0033(02)00180-8

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