The use of yank-time signal as an alternative to identify kinematic events and define phases in human countermovement jumping

Abstract

Detailed examinations of both the movement and muscle activation patterns used by animals and humans to complete complex tasks are difficult to obtain in many environments. Therefore, the ability to infer movement and muscle activation patterns after capture of a single set of easily obtained data is highly sought after. One possible solution to this problem is to capture force-time data through the use of appropriate transducers, then interrogate the signal's derivative, the yank-time signal, which amplifies, and thus highlights, temporal force-time changes. Because the countermovement vertical jump (CMJ) is a complex movement that has been well studied in humans, it provides an excellent preliminary model to test the validity of this solution. The aim of the present study was therefore to explore the use of yank-time signal, derived from vertical ground reaction force-time data, to identify and describe important kinematic (captured using three-dimensional motion analysis) and kinetic events in the CMJ, and to relate these to possible muscle activation (electromyography) events that underpin them. It was found that the yank-time signal could be used to accurately identify several key events during the CMJ that are likely to be missed or misidentified when only force-time data are inspected, including the first instances of joint flexion and centre of mass movement. Four different jump profiles (i.e. kinematic patterns) were inferred from the yank-time data, which were linked to different patterns of muscle activation. Therefore, yank-time signal interrogation provides a viable method of estimating kinematic patterns and muscle activation strategies in complex human movements.

Keywords: bimodal; force platform; vertical jump; yank-time.

Figure 1.

Force-time signals and subsequent derivatives. Panel (a) shows an original force-time signal when a load is placed on a force platform. Panels (b–d) show the yank-time, tug-time and snatch-time signals, which are first, second and third time derivatives of the force-time signal (panel a). The complexity (the number of extrema [maxima and minima] and points of crossing the x-axis, i.e. where x = 0) of the signal-time relation increases with derivative order. Image is taken from Sahrom, S. [10] ‘Beyond jump height: Understanding the kinematics and kinetics of the countermovement jump from vertical ground reaction force data through the use of higher-order time derivatives’.

Figure 2.

Vertical ground reaction force (GRF_z)-time trace, with important kinematic events and jump phases identified (flight phase not shown), and the associated displacement, velocity, acceleration and yank-time traces from one subject. Five events are shown: Event 1, start of CMJ/first instance of movement; Event 2, meaningful CoM movement (downwards); Event 3, start of braking (deceleration); Event 4, peak yank braking; Event 5, point of lowest CoM height. The upwards phase, shown as phase 6, is divided into two sub-phases: 6a, the first 50% of the upwards phase; and 6b, second 50% of the upwards phase. Further information regarding the events and sub-phases is presented in table 2.

Figure 3.

Typical temporal changes in knee and ankle joint angles and the vertical ground reaction force (GRF_z). Ankle dorsiflexion and knee flexion, i.e. joint flexion (orange and green lines), due to the start of CMJ are reflected as an inflection point (black dotted line) and recognized as Event 1 on the yank-time signal (yellow line). Note that GRFz (brown line) does not noticeably decrease until approximately 0.128 s after this point.

Figure 4.

Jump profiles based on GRF_z-time signal shape, knee angles and EMG activity of the muscles within the cohort. Unimodal (left; one force peak), bimodal-primary (middle; two force peaks, first peak is higher) and bimodal-secondary (right; two force peaks, second peak is higher). The height of CoM (first row, blue line) starts from zero. The second row shows the knee and ankle angle while the third and fourth rows show the EMG activities of the respective muscles.

Figure 5.

Difference between the two different types of unimodal jump profiles. In a unimodal-secondary jump profile, the point of lowest height of CoM (Event 5) occurred before the point of maximum GRF_z. In a unimodal-primary jump profile, the point of lowest height of CoM (Event 5) occurred close to or at the point of maximum GRF_z.

Figure 6.

Comparison of GRFz-time relations for the three jump profiles normalized to jump time. Unimodal-secondary, bimodal-primary and bimodal-secondary are compared using normalized time. Based on the occurrence of Event 5, the three respective vertical lines in unimodal-secondary and the first local maximum for the bimodal jumpers, we speculate that the activation patterns of bimodal jumpers are similar to the unimodal-secondary jumpers.