Jump testing tools

In addition to jump testing protocols being scrutinised for inaccuracies, specific jump and power testing tools have also come under criticism. Numerous instruments measuring jump height and contact time, utilising different technologies and calculations, have provided varying results (Markovic et al., 2004). Force plates have been considered to be the “gold standard” for the measurement of jump tests, having been reported to provide excellent measurement accuracy for estimation of power via forward dynamics, calculated by the force applied to the jumping surface (Walsh, Ford, Bangen, Myer, & Hewett, 2006). Jump instruments such as optical measuring systems, that solely assess flight time and do not determine jump height from the impulse-momentum relationship, as used by Domire and Challis (2007) are at risk of

inaccuracy, as subjects can alter jump technique, such as a flexed foot on landing to increase flight time. By contrast, when using force plate assessment maximal vertical velocity of the centre of mass at take-off provides a more accurate measure of jump height, by using a forward dynamics approach. Cormack, Newton, McGuigan, and Cormie (2008) highlighted the

importance of analysing the flight time to contraction time ratio as a measure sensitive to fatigue, with the time from initiation of the counter movement until the subject leaves the jumping surface being key. It is, however, important for practitioners to note that despite the

research by Cormack, Newton, McGuigan, and Cormie (2008) reporting flight time to

contraction time as the measure of jump performance, the methodology used actually assesses only the initial movement when utilising velocity of the centre of mass jump testing protocol. Movement may have also have occurred in the upper body prior to contraction of the lower body during velocity of centre of mass assessment using the protocol by Cormack, Newton, McGuigan, and Cormie (2008), and one cannot therefore assume that the initial movement reported is contraction time. Also of note for practitioners is that, despite the research by Cormack, Newton, McGuigan, and Cormie (2008) not stating the sampling frequency used for the force plate assessment; typical Ballistic Measurement Systems use a frequency of 40 ms. As a result, it could be argued that this default sampling frequency option may prove inaccurate, when compared against research which involved a different sampling frequency. Practitioners are therefore advised to note both the thresholds set for the onset of movement and the calculation used for flight time, as this may have an effect upon the resultant measurement variables collected.

Force plates have been shown to possess excellent reliability as performance measures for both jumping and landing tasks and strength assessment (Cormack, Newton, McGuigan, & Doyle, 2008; Cormie, McBride, & McCaulley, 2007; Walsh et al., 2006), while use of saliva to assess hormonal response post rugby match is commonplace (Beaven, Cook, et al., 2008; Crewther et al., 2013; Elloumi, Maso, Michaux, et al., 2003; Elloumi, Maso, Robert, et al., 2003; Maso, Lac, Filaire, Michaux, & Robert, 2004; West et al., 2014). Force plates use the impulse-momentum relationship to determine velocity, which calculates power through the forward dynamics approach (Cormie et al., 2007). Previous research has utilised the flight time calculation via the height of rise of centre of mass (French et al., 2004; McBride, Triplett-McBride, Davie, & Newton, 1999, 2002). The vertical take-off velocity of the centre of mass is calculated, whereby

acceleration (a) is determined by dividing vertical ground reaction forces (F) by the mass of the system (SM) at each time point:

𝑎=𝐹/𝑆𝑀

With jump height from velocity of centre of mass calculated via the following equation: 𝐽𝐻=(𝑣^2)/(2𝑥9.81)

v = displacement/time

This method of forward dynamics calculation has been regularly used to assess bodyweight jumps in previous research (French et al., 2004). However, the main disadvantages associated with force plates are that they are expensive to use and often impractical in field-testing scenarios (Casamichana et al., 2013). Additionally, methods of assessing power output via kinetic methods such as the force plate have been questioned (Cormie et al., 2007), with force plate methodology typically used to compare performance of bodyweight jumps, rather than to measure the influence of loads upon power output. During Olympic lifting movements, for example, the barbell moves at a differing rate to that of the participants centre of mass (COM), as the barbell starts below the lifter’s COM and finishes above their COM meaning the power applied will differ. On summary the advantages of force plates outweigh the disadvantages mainly due to the vast amount of data that force plates can collect including such parameters as PRFD and impulse (McLellan & Lovell, 2012; McLellan et al., 2011a, 2011b; McLellan, Lovell, & Gass, 2011c; McLellan et al., 2011d).

Recently, contact mats and optical measuring systems have become commonplace for field based assessments (Bosquet, Berryman, & Dupuy, 2009). CMJ and SJ performed on a contact mat were deemed reliable at estimating anaerobic power in a study by Moir, Button, Glaister, and Stone (2004). Contact mats were also considered practical for use in laboratory and field settings, for squat jump and CMJ (Markovic et al., 2004). This was mainly due to the nature of the testing protocol in the field and the ability for data to be produced relating not just to jump height, but also to various other parameters to assess power (Casartelli, Muller, & Maffiuletti, 2010). Bosco, Luhtanen, and Komi (1983) were the first to derive jump height from flight time via a contact mat, with infrared optical device now replacing contact mats to measure flight time. However, due to error in time keeping by subjects taking off and landing in different locations, this method of calculating jump height has been questioned (Garcia-Lopez et al., 2005). The main criticism, that contact mats cause inaccuracies, has been based on the fact that the subject’s feet are not directly in contact with the specific sport surface in question, with the result that athlete surface interaction varies (Markovic et al., 2004). Although this inability to test on specific surface is also seen in force plates, the advantages of force plates outlined above mean that this is not considered a major concern.

If methodological protocol is standardised throughout jump testing, the effect of differing surfaces and other dependent variables such as technique used will be minimised. Accuracy of data collected on contact mats was researched and evidence for its use questioned by Klavora (2000), who found that subjects recorded higher jump scores with a contact mat than a jump and reach style test as used in NFL combine testing (Kuzmits & Adams, 2008). In recent research (McMahon, Jones, & Comfort, 2015) a commonly used contact mat (JustJump) was compared against a force plate, with suggested use of a corrective equation when using a

JustJump system, despite ICC demonstrating excellent within-session reliability of CMJ height (ICC = 0.96, p < 0.001). Jump and reach tests have also come under scrutiny as the techniques used by participants, as a subject-performing jump and reach tests can use the wall as an aid to extend jump height. Harman (1990) reported that jumping and touching a wall during a vertical jump is more restrictive than jumping straight up and down, hence their study found jump height to be 5.3 cm greater when using a total body centre of mass displacement, compared to the jump and reach test. Additionally, it is assumed that participants on a jump and reach test will mark the wall or displace a marking vane at the peak of their jump. As reported by Klavora (2000) this is often not the case, therefore influencing results collected. Early research, assessing the validity and reliability of methods for testing vertical jump performance are being questioned due to methodological considerations (Hatze, 1998). In the research by Hatze (1998) the methodological limitations noted include; invalid assumptions regarding performance calculations and the jumping technique utilised. This is despite the use of force plate methods supported by research (Walsh et al., 2006). Jump mats, for example, use flight time as the calculation of performance, yet this calculation is an approximation, as flight time is no direct measurement of force. By contrast, force plates give direct measurement of force based upon the forward dynamics discussed earlier. If the calculations used to assess performance incorporate displacement time data involving direct measure of velocity, it is important for practitioners to acknowledge that this measure is an approximation of force, therefore its accuracy should be questioned. This view was supported by Hatze (1998) who noted that the validity and reliability of the jumping ergometer method for evaluating certain aspects of athletic performance are highly questionable, due to the methods of calculation

identified above. More recent research has, however, shown jump testing that uses modern methods as reliable (Markovic et al., 2004). Technological development of contact platforms and mobile devices (My Jump) has been reported to produce reliable intra and inter-session data during drop jump, CMJ and SJ movements (Gallardo-Fuentes et al., 2016).