Knee isokinetic test scores and functional hop tests findings: Are they related?

1. Introduction

It is important for health practitioners to be able to identify patients’ ability to cope with the physical demands placed upon them [1]. According to Barber et al. [2] in order to evaluate functional limitations of the knee joint, an objective measurement simulating sporting activity is required.

Functional performance tests, such as the hop tests, are closed chain activities in nature and therefore assimilate the joint loading forces and kinematics that functionally occur [3]. Hop testing has been proposed as a practical, outcome measure that reflects the integrated effect of neuromuscular control, muscular strength and confidence in the limb requiring minimal equipment and time [4, 5, 6]. Functional activities require not only muscle tension production but also appropriate motor synergies and postural adjustments. Hop tests are the most common used type of closed kinetic chain [CKC] functional tests in the clinical setting due to the use of the contralateral leg [e.g., uninjured] as control for comparisons between limbs [6] and as a reference point for assessing the progress of the rehabilitation program and the return to a competitive functional level [2, 3, 7, 8, 9, 10]. They incorporate a variety of motion principles [i.e., change direction, speed, acceleration, deceleration] imitating dynamic knee stability requirements during sports or simple activities [10, 11, 12]. Functional performance tests are cost effective, they exhibit a high correlation with functional performance [5, 8] and they have become useful in the clinical decision making process [6]. The most frequently used hop tests for the knee joint include the single hop for distance (SHD), triple hop for distance (THD), and single timed hop [STH] test. High reliability for hop tests has been reported in healthy subjects [13, 14].

An alternative strength evaluation test in the clinical setting is the lower limb isokinetic evaluation [15]. Isokinetic dynamometry has become a standard tool for evaluation of muscular strength due to the benefits of range of motion, control, quantifying resistance, and being safe for the patient [2, 11, 16]. It has long been regarded as the principal method for assessing muscle function and imbalances in clinical, research, and sports environment [17]. Some researchers have established the mechanical reliability of the Biodex [18] and have shown that reciprocal, concentric peak moment measures obtained at 60 and 180 ∘ /sec are very consistent from day to day when a standardized protocol is used [19, 20]. Peak moment is the most often studied strength parameter [21]. Most studies yield results on maximum peak moment [peak moment represents the highest moment output generated by an activating muscle] in relation to the functional tests.

However, human movement cannot be represented only by a peak moment measurement. Another para-meter not so extensively studied is total work [22, 23]. Work has been suggested to be a better indicator of dynamic muscle activity than peak moment since it is a measure of force production throughout the whole range of motion as opposed to one point in the range [20]. Manca et al. [24] in their study tried to quantify the extent of variability in strength variations. They found that the extent of asymmetry between the stronger and the weaker limb was differently portrayed by maximal work (significant inter-limb deficit of 20–24%) and by peak moment (non-significant inter-limb deficit of 12–14%). In 2019 Manca et al. [25] examined people with multiple sclerosis and found that when they have a reduced maximal work they may benefit from strength training approaches aimed at increasing gait speed. They concluded that maximal work was a better indicator of the function of a muscle group than peak moment, as moment must be maintained throughout the ROM and over time. In another study Morrissey [26] suggested that total work may be a better indicator for muscle function than peak moment since peak moment may not reflect the ability of the knee joint to emit force throughout the range of motion after injury. Iossifidou and Baltzopoulos concluded that work depended on the range of motion and added information about muscle and joint function throughout an identified range of motion when compared with peak moment [27]. Both work and peak moment have been found to be reliable measures of muscle performance [28].

Another parameter that needs to be considered is power which indicates how quickly a muscle can produce force. Knee power may better reflect the capacity of muscle to manage knee loads than peak moment. For example, the role of knee extensor strength on the incidence of radiographic knee OA is controversial. In the Multicenter Osteoarthritis Study, knee extensor strength did not predict incident radiographic evidence of knee OA 30 months later [29]. There are studies in the healthy-aging literature that have shown muscle power to be a stronger predictor of functional performance than strength [30, 31, 32]. With aging, the knee extensors show ample losses of power [33]. The authors assumed that the mean and overall values were more accurate in estimating the actual muscular performance as opposed to the peak moment values alone which virtually shows only a maximum performance during a repetition and by itself may not adequately reflect the trend of growth in a range of motion. Peak moment (PM), total work (TW) and average power (AP), are relevant measures of muscle performance [25] and they will be useful as an evaluation tool of the knee joint.

At the same time, the isokinetic assessment is more expensive than a simple hop test. Aditionally, some researchers consider it a poor reflection of functional ability [3, 18] since a single joint assessment of individual muscle groups may not adequately address patients’ functional ability, particularly in sports. Open kinetic chain (OKC) training alone cannot adequately prepare the patient to return to a dynamic functional level. Since both methods are valuable for their own merit it would be of clinical value to know the correlation between performance in the functional tests and flexors’ and extensors’ muscle group isokinetic strength test. Although hop tests are reported to test components of strength, power, and balance, to our knowledge the extent to which deficits on a single-leg hop test correlate with any of these factors has not been thoroughly investigated [6]. Understanding these relationships would improve the clinical usefulness in detecting imbalances and deficits in preseason screening and after injury [6].

Some studies have shown a lack of strong correlation between isokinetic and functional measures [34, 35] with some researchers showing fair and moderate to good correlations [22, 23, 34], while others show little or no correlations [3, 36, 37]. These differences may emerge from the variety of the pathological conditions evaluated, since these correlations have been shown in patients with conditions like anterior cruciate ligament reconstruction [23, 38] with patellofemoral pain syndrome [39], knee osteoarthritis [40], total knee replacement [41, 42] etc. Differences in the results may also stem from differences in methodologies, in the selected individual populations evaluated, the test methods, equipment, etc. If the literature is divided between small and large correlations it would be important to have a firm ground for a clinical decision making in order to facilitate the use of the hop tests as a standardized performance based outcome measure. Specifi-cally, further information regarding not only the peak moment but also average power and total work is necessary to accurately plan rehabilitation programmes and to more confidently make clinical decisions about individual patients.

Isokinetic testing supplemented by the hop tests can create a better approach to the patient’s assesment. Moreover, in the lack of an isokinetic device, a strong correlation between the two would suggest that hop tests can provide the clinician with an important hindsight of the condition of the patient.

The aim of the urrent study was therefore to explore the correlations among isokinetic knee flexors and extensors strength-related variables normalized to bodyweight (BW): mean peak moment to body weight – MPM/BW; total work to body weight – TW/BW; average power to body weight - AP/BW and three functional hop tests.

2. Methods

3. Results

The anthropometric data and the descriptive statistics (mean, standard deviation, range) for the 25 participants are presented in Table 1. Table 2 presents the mean, standard deviation and range for all functional data and Table 3 presents descriptive statistics (mean, standard deviation, range) of all isokinetic variables for the dominant limbs. Correlations between normalized to BW functional tests and isokinetic parameters are presented in Table 4. Observing each functional test individually, the results show that with regard to the SHD/BW test, the only correlation found was a positive fair correlation ( r = 0.419, p = 0.037) with extensors’ MPT/BW at 180 ∘ /s (Table 4).

Regarding the THD/BW test, positive moderate to good correlations were found in extensors’ MPM/BW ( r = 0.585, p = 0.002) and AP/BW ( r = 0.659, p = 0.000) at 180 ∘ /s and in flexors’ AP/BW ( r = 0.528, p = 0.007) at 180 ∘ /s. Positive fair correlations were found in extensors’ TW/BW ( r = 0.408, p = 0.043) at 180 ∘ /s and positive fair correlations in extensors’ MPM/BW ( r = 0.405, p = 0.044), in TW/BW ( r = 0.411, p = 0.041), and also in AP/BW ( r = 0.403, p = 0.046) at 300 ∘ /s. Aditionally, positive fair correlations with flexors’ MPM/BW ( r = 0.472, p = 0.017) at 180 ∘ /s and TW/BW ( r = 0.425, p = 0.034) at 300 ∘ /s, were found (Table 4).

In the STH/BW test, negative fair correlations were found with extensors’ TW/BW ( r = - 0.406, p = 0.044) and also AP/BW ( r = - 0.408, p = 0.043) at 60 ∘ /s. Aditionally, there was negative fair correlations with MPM/BW ( r = - 0.432, p = 0.031) and AP/BW ( r = - 0.436, p = 0.029) at 60 ∘ /s as well as AP/BW ( r = - 0.423, p = 0.035) at 300 ∘ /s (Table 4).

This study investigated whether the use of 3 para-meters of isokinetic evaluation i.e., AP, MPM and TW normalized to body weight, would yield a correlation between functional and isokinetic strength testing. The authors hypothesized that since participants have to control their body weight when performing the functional tests, the three isokinetic variables if they are expressed in terms of BW they will show an even greater correlation with function.

5. Conclusion

Due to their high reliability, hope tests constitute a group of useful tool for clinicians in order to build and also assess rehabilitation programs. On the other hand, according to our results, hop tests could not substitute isokinetic parameters such as Mean Peak Moment, Total Work and Average Power because of the moderate correlations between the functional hop values (CKC evaluation) and the isokinetic parameters (OKC evaluation). Thus it seems that isokinetics should be used in conjunction with the functional tests for a more objective evaluation of knee muscular performance. A combination of OKC and CKC evaluation can provide better information for functional assessment and performance of the knee muscles, and for the proper organization of a muscle strength and function restoration program.