*4.1.2.1 Plumb line method*

The two-dimensional evaluation of posture, using a plumb line, is very common, due to its low-cost and simplicity [23]. Kendall et al. postulated guidelines to evaluate posture in accordance with the alignment of the ideal plumb line for the measurement of the sagittal and frontal planes [24]. Kendall et al. state that the ideal alignment of sagittal plane posture is when the plumb line intersects the ear lobe, through the shoulder joint; then through the greater trochanter of the hip, just in front of the knee joint; and finally slightly in front of the lateral malleolus of the ankle before it reaches the floor. Williams and McClay reported that the plumb line method had a good intra-rater reliability for measuring postural variables with an average ICC of 0.80 in both 10 and 90% of body weight bearing scenarios in standing [10]. The standard error of the mean (SEM) reported was between 2 and 5 mm for the lower limb indices and from 5 to 10 mm for patients with a trunk list or lateral shift. List is defined as 'the lateral displacement, in millimetres, of a surface marking of the spinous processes of T12 from that of S1' (McKenzie and May [25], p. 214). Furthermore, Hickey et al. evaluated the reliability of using the plumb line to measure resting head posture in a large sample size of 122 healthy volunteers (80 women and 42 men, ages 18–60 years) [26]. In this study, all participants were screened for cranial, cervical and/or upper thoracic dysfunction. The results of this study demonstrated the plumb line method to have high intra-rater reliability with ICCs ranging from 0.83 to 0.84 for the measurement of resting head posture. Although the plumb line method has been reported to have good intra-rater reliability and is a useful and easy to use instrument for measuring posture, its limitations include the difficulty of minimising movement error or postural sway [9, 27]. Additionally, this plumb line method only measures one plane.
