Measurement error associated with gait cycle selection in treadmill running at various speeds

A common approach in biomechanical analysis of running technique is to average data from several gait cycles to compute a representative mean. However, the impact of the quantity and selection of gait cycles on biomechanical measures is not well understood. We examined the effects of gait cycle selection on kinematic data by: (i) comparing representative means calculated from varying numbers of gait cycles to global means from the entire capture period; and (ii) comparing representative means from varying numbers of gait cycles sampled from different parts of the capture period. We used a public dataset (n = 28) of lower limb kinematics captured during a 30-second period of treadmill running at three speeds (2.5m {middle dot} s-1, 3.5m {middle dot} s-1 and 4.5m {middle dot} s-1). Ground truth values were determined by averaging data across all collected strides and compared to representative means calculated from random samples (1,000 samples) of n (range = 5--30) consecutive gait cycles. We also compared representative means calculated from n (range = 5--15) consecutive gait cycles randomly sampled (1,000 samples) from within the same data capture period. The mean, variance and range of the absolute error of the representative mean compared to the ground truth mean progressively reduced across all speeds as the number of gait cycles used increased. Similar magnitudes of error were observed between the 2.5m {middle dot} s-1 and 3.5m {middle dot} s-1 speeds at comparable gait cycle numbers -- where the maximum errors were < 1.5 degrees even with a small number of gait cycles (i.e. 5-10). At the 4.5m {middle dot} s-1 speed, maximum errors typically exceeded 2-4 degrees when a lower number of gait cycles were used. Subsequently, a higher number of gait cycles (i.e. 25-30) was required to achieve low errors (i.e. 1-2 degrees) at the 4.5m {middle dot} s-1 speed. The mean, variance and range of absolute error of representative means calculated from different parts of the capture period was consistent irrespective of the number of gait cycles used. The error between representative means was low (i.e. <1.5 degrees) and consistent across the different number of gait cycles at the 2.5m {middle dot} s-1 and 3.5m {middle dot} s-1 speeds, and consistent but larger (i.e. up to 2-4 degrees) at the 4.5m {middle dot} s-1 speed. Our findings suggest that selecting as many gait cycles as possible from a treadmill running bout will minimise potential error. Analysing a small sample (i.e. 5-10 cycles) will typically result in minimal error (i.e. < 2 degrees), particularly at lower speeds (i.e. 2.5m {middle dot} s-1 and 3.5m {middle dot} s-1). Researchers and clinicians should consider the balance between practicalities of collecting and analysing a smaller number of gait cycles against the potential error when determining their methodological approach. Irrespective of the number of gait cycles used, we recommend that the potential error introduced by the choice of gait cycle number be considered when interpreting the magnitude of effects in treadmill-based running studies.