Isometric strength of knee extensors has been widely assessed since decades due to its close relationship with functional performance [1
] and as effective tool for monitoring injury rehabilitation success [4
]. In recent years, the use of the isometric knee extension exercise during explosive or ballistic contractions has attracted increasing attention as an effective method to test and improve force production during rapid muscle activation, with positive transfers to sport-specific performance such as jumping or sprinting [6
]. Despite its practical implications, however, the stable measurement of rapid force production remains of ongoing concern [9
]. Hence, further understanding of best-practice and testing procedures that can aid the development of effective assessment continues to be of importance.
Current resistance training practices involve the use of technology to obtain real-time data about neural and muscular determinants of performance [11
]. Muscular determinants during isometric exercises are conventionally assessed by the maximal voluntary concentric force (MVC), which refers to the highest force the individual is able to produce during the test [7
]. The MVC is demonstrated as a reliable and easy-to-obtain variable during the isometric knee extension test with portable force sensors [14
]. However, while most of sport-related movements (i.e., jumping, sprinting, kicking) involves rapid contraction times (50–250 ms), the MVC is typically reached at later phases ≥300 ms, which may limits its ability to explain performance for rapid actions [13
In turn, the ability to produce force rapidly (<250 ms) mostly relies on neural determinants such as motor unit recruitment, discharge rates and force twitches [8
]. Neural factors are commonly assessed during isometric tests by the rate of force development (RFD) exerted within the early phase of rising muscle force, and the contractile impulse that can be produced within a given contraction time [16
]. Despite the rising interest in the use of isometric testing to determine improvements in ballistic sport-specific movements [6
], the design of practical field-based methods to obtain reliable RFD and impulse measures at different phases of the contraction is an ongoing challenge [18
]. This is further confounded by the fact that the measurement of both RFD and impulse is highly dependent to the testing procedures [9
], the signal filtering [19
] and the warm-up [22
]. Hence, a further examination of the quality and consistency of the RFD and impulse measures is of critical importance.
To provide these insights would have practical implications for coaches and therapists, since nowadays one can easily collect automatically RFD and impulse data while providing real-time visual feedback using low cost, portable force sensors attached to a bench or table [9
]. To the best of our knowledge, only one previous study has examined the reliability of rapid force production variables during the knee extension [21
]. Albeit showing promising results, the fact that they conduced time-consuming testing procedures (separate tests for MVC and RFD which doubled the time required) makes difficult to transfer their results into the sport daily practice thus requires a field-based replication. Furthermore, despite side-to-side asymmetry in quadriceps RFD has gained interest as a screening tool for injury management [23
], little is known about the differences between dominant and non-dominant limbs when testing the rapid force production during the knee extension test in athletes. Besides, there is no quantitative data describing the influence of a familiarization session to maximise the reliability of RFD during the knee extension strength test [9
Therefore, the current study aimed to determine the intra-session and test-retest reliability of a rapid, isometric knee extension test to evaluate the MVC, RFD and impulse on both limbs in young athletes unfamiliar with the test, following a field-based approach. Based on the existing literature, we hypothesize that late contractions >250 ms would reach sufficient reliability since the first session, whilst earlier phases would require a previous familiarization.
The main findings of this study indicate that: (i) RFD and impulse during the isometric knee extension tests can be assessed with high reliability during “late” phases of the contraction (0–250 ms) since the first session, following a field-based approach, in young athletes unfamiliar with the test; (ii) earlier phases of the contraction (0–150 ms) can be measured with moderate reliability after one familiarization session; (iii) in contrast, measures at 0–50 ms requires larger sampling rates and/or longer familiarization to reach sufficient reliability; (iv) the results confirm previous findings that knee extension MVC can be accurately assessed using portable force sensors [14
The development of practical methods to obtain reliable RFD and impulse parameters from sport-related actions is an ongoing challenge in sport practice [9
], since they may inform about the neural efficiency of motor skill performance [8
]. According to our findings, RFD and impulse at 0–150 ms and 0–250 ms can be measured with sufficient reliability (CV < 10 %) after one familiarization session during the isometric knee extension tests, with better results if filtering the data. These good results were automatically obtained using a portable low-cost force sensor at 80 Hz which make possible to easily reproduce this assessment in both athletic and clinic (e.g., hospitals, rehabilitation centres or nursing homes) environments. In contrast, the lower reliability even with fitted data (CV~14%) found in earlier phases of contractions (0–50 ms) suggests the need of higher sampling rate equipment and/or experienced participants. This is in line with Buckthorpe et al. [21
] who found good within-participants reliability from 100 ms onward but high variation up to CV = 19% at earlier phases (RFD0–50
) during a rapid, isometric knee extension test after familiarization. Since we used a much lower sampling rate (80 Hz vs. 2000 Hz), the current improvements could be attributable to methodological differences such as the inclusion of a rapid-response warm-up [22
]. Further, in light of our findings, to collect MVC and RFD together from the same trial via evidence-based guidelines [9
] seems advantageable as may increase performance and reduce fatigue as a result of performing less trials while saving time. Of interest, adopting an external focus of attention (e.g., “try to touch this target”) seems to be beneficial when testing rapid contractions of the knee extensors [32
]. Future studies should examine whether the use of external attentional focus may have an acute impact on the reliability of rapid force production measures during the isometric knee extension tests, specially at early phases of the contraction (0–50 ms).
A main practical resource herein provided for a better understanding of the isometric knee extension test reliability is the reporting of intra-session and test-retest differences in absolute terms (i.e., SEM). This information assists in the interpretation of results from a practical viewpoint. A large SEM relative to the between-participant variance contributes to poor reliability. In other words, if we would like to compare the differences after a training program, changes greater that the SEM would be likely to be a result of the intervention rather than a measurement error [29
]. Accordingly, taking the Session 2 fitted readings and the dominant leg reference (Table 2
), the current field-based method would permit us to identify, at least, changes in MVC over 35.9 N or changes in RFD0–250
from 126 N·s−1
and 7.4 N·s respectively. Hence, despite we cannot deny the existence of a high intra-individual variability during an isometric leg extension test [21
], the use of evidence-based protocols, rapid-response warm-up and visual feedback make it possible to obtain a reliable RFD/Impulse measurement with field-based, portable and low-cost equipment. This is particularly relevant for coaches and sport clubs dealing with athletes with similar strength status than our sample (MVC from ~500 to ~850 N; 7- to 10-fold their body mass).
The implications of these findings are that using a portable force sensor under proper measurement guidelines allows to collect reliable RFD, Impulse and MVC data (altogether from the same trial) to evaluate the rapid isometric contraction of the knee extensors in young athletes. Such knowledge, along with the interpretation of the measurement errors, will aid both in the assessment of performance at a given time-point (e.g., pre-season, tapering, diagnostic analysis) and the identification of true changes due to training-induce adaptations (e.g., training program, injury rehabilitation). In addition, we provide new quantitative data describing the influence of familiarization during RFD assessment [9
], with some variables reaching sufficient reliability since the first session and the others requiring just a single previous session (Table 2
). Of final note is that important inter-limb asymmetries were identified, with non-dominant limbs describing lower records and larger variations during rapid force assessment. Although these differences could be anticipated [30
], it seems to be the first time presenting data about asymmetries and RFD performance during a ballistic knee extension test in athletes. According to our findings, this test would allow to identify severe asymmetries >15% but requiring a previous familiarization. All in all, given that improvements in RFD can be expected within the first weeks of training [6
], and the benefits of a previous familiarization, it is advisable to evaluate the rapid isometric contraction of the knee extensors since initial stages of the season and conducted frequent monitoring to verify short-term progresses.