QUANTIFICATION OF TIME TO STABILISATION USING THE SEQUENTIAL ESTIMATION TECHNIQUE

Abstract

Dynamic postural stability is a key physical attribute for athletes and can be defined as the ability to maintain postural balance whilst moving from a dynamic to a static state (Wikstrom et al., 2005). This dynamic stability requires the complex integration of sensory afferent systems (visual, somatosensory and vestibular) with efferent responses in both upper and lower body neuromuscular systems. The effect of neuromuscular pathologies on dynamic postural stability has been the focus of a considerable body of research (e.g. Ross et al. 2009), particularly in the areas of chronic ankle instability and ACL injury. Underpinning this research are several functional tests that have been proposed to objectively quantify dynamic postural stability. One of the most commonly cited measures from these tests is known as ‘time to stabilisation’ (TTS) which is defined as the time required to reach stability after landing. As reaching stability is a somewhat non-specific event, several different techniques have been proposed for quantifying the TTS. The majority of techniques use a generalised approach that identifies when the resultant ground reaction forces (GRFs) reach some baseline threshold. One such TTS technique was proposed by Colby et al. (1999) and has since been used by numerous others (e.g. Shaw et al. 2008). This technique uses a process called sequential estimation, whereby a cumulative average of GRF data is calculated by adding one data point at a time. Stabilisation is deemed to occur when the cumulative average reaches and stays within 0.25 SDs of the overall mean. This sequential estimation procedure has been applied to mediolateral (M-L), anterior-posterior (A-P) and vertical GRF data from a range of jump types. While data from this technique has not always supported expected group or condition differences in stability, the technique is still well recognised and used in TTS calculation. To the author’s knowledge, no research has pointed to mechanistic flaws in the use of sequential estimation in calculating TTS values. However, on reviewing the technique, it appears that the use of both the cumulative average and SD values predispose the technique to inaccurate TTS assessments. Use of a cumulative average suggests that increased force oscillations can theoretically reduce the TTS, and similarly, the use of SD values based on the full landing sequence implies that the threshold range is larger for less stable landings and can thus also theoretically lead to shorter TTS values. In seeking to understand the links between dynamic stability and neuromuscular pathologies, it is clearly essential that the measures used are robust and do not provide spurious results. Therefore, the aim of this paper was to assess the validity concerns of the sequential estimation method of TTS calculation by using it to compare jumps with clear differences in dynamic stability.