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The effects of COVID-19 lockdown on jumping performance and aerobic capacity in elite handball players

The aim of this research was to analyse the capacity of a home-based training programme to preserve aerobic capacity and jumping performance in top-level handball players during the COVID-19 lockdown. Eleven top-level male handball players from the same team participated in the study. A submaximal shuttle run test and a counter-movement jump test were used to measure the players’ aerobic fitness and lower limb explosive strength, respectively. A 9-week home-based training programme was followed during lockdown. Pre-test measurements were assessed before the pandemic on 29 January 2020 and ended on 18 May 2020. Moderate significant mean heart rate increases were found in the late stages of the submaximal shuttle run test after the lockdown (stage 5, 8.6%, P = 0.015; ES = 0.873; stage 6, 7.7%, P = 0.020; ES = 0.886; stage 7, 6.4%, P = 0.019; ES = 0.827). Moderate significant blood lactate increases were observed immediately after the submaximal shuttle run test following the lockdown (30.1%, P = 0.016; ES = 0.670). In contrast, no changes were found in jump performance. A structured home-based training programme during the COVID-19 lockdown preserved lower limb explosive strength but was an insufficient stimulus to maintain aerobic capacity in top-level handball players.


INTRODUCTION
During the test, heart rate (HR) was registered using a Garmin HR strap. The HR monitor was linked to the WIMU PRO system (Realtrack Systems, S.L., Almeria, Spain) and data were analysed thereafter using mean HR values for each submaximal shuttle run test stage.
One minute after the end of the test, players were pricked in the earlobe to analyse blood lactate levels [20,21,22]. The analysis was performed with a Lactate Scout + lactate analyser and Lactate Scout test strips (Nova Biomedical, Waltham, MA, USA).

Jump
The CMJ test was used to assess vertical jump performance as an indicator of lower limb explosive strength [23]. Players performed a fast flexion movement of the knee joint followed by a maximumeffort vertical jump, maintaining the hands-on-hips position until the final phase of the jump. A contact platform (Chronojump Boscosystem, Barcelona, Spain) was used to assess CMJ height. The hardware was connected to a computer which displayed the vertical jump height (cm) using free software (2.0.2., Chronojump Boscosystem Software, Barcelona, Spain). This type of technology has proven its reliability and validity in other types of research with vertical jump tests [24]. Players performed two bilateral CMJs and two unilateral CMJs with each leg. The best result of each test (height, cm) was recorded and used for further analysis. sessions. All sessions conducted at home followed the medical recommendations derived from the COVID-19 pandemic [9,25]. All sessions were preceded by a general warm up consisting of ~10 min of low intensity cycling (stationary bike), mobility and lumbo-pelvic stability exercises. In the first four weeks, strength training was enduranceoriented and over the last four weeks strength training was hypertrophy-oriented [26]. Individual hypertrophy-oriented training programmes were organized in super-sets in which a combination of low specificity level exercises (i.e., bilateral squat-based exercises) preceded slightly more specific exercises (more dynamic correspondence with limb explosive strength and aerobic capacity) in top handball players during the COVID-19 lockdown. To the best of our knowledge, only one previous study [14] has investigated the effects of a given training programme in the aerobic capacity of elite handball players. Such studies might provide valuable insights about the real impact of home training programmes to prevent detrimental effects on the general fitness of elite handball players.

Home training programme during COVID-19 lockdown
Accordingly, the aim of this research was to analyse the effectiveness of a home training programme to preserve aerobic capacity and lower limb explosive strength in top-level handball players during the COVID-19 lockdown.

Design
A retrospective design was used to compare the change in submaximal shuttle run test and jump test performance. A 9-week home-based training programme was followed during lockdown. Pre-test measurements were assessed before the pandemic on 29 January 2020 and ended on 18 May 2020. The tests were conducted on the same day, first performing the submaximal shuttle run test in two groups of five and six players, respectively, and then the counter-movement jump (CMJ) test. The submaximal shuttle run and the CMJ tests were used to measure the players' aerobic fitness and lower limb explosive strength (both bilateral and unilateral), respectively.

Subjects
The study was conducted on 11 top-level male professional handball players from the same team throughout the same season. These were all international players with their respective national teams during the season in which they participated in this study. The play- 84.0 ± 0 kg). The data were obtained from the periodic monitoring of the players during training sessions. All players signed a contractual clause accepting their participation in research projects; therefore approval by an ethics committee was not required [18]. However, all players were informed about the purpose of the study, the known risks and possible associated hazards. The research was in accordance with the Declaration of Helsinki, and professional players gave informed consent prior to participation through their contracts.

Submaximal shuttle run test
To assess the aerobic capacity of the players, the multistage 20-metre shuttle run test [19] was performed up to stage number 8. The test consisted of running continuously between two lines placed 20 m apart at running speeds increased by appropriate intervals at a pre-recorded beep. Mean velocity started at 8.5 km · h -1 for the first minute (stage 1), increasing by 0.5 km · h -1 every minute up to handball-specific movements, i.e., vertical jump exercises) [26]. Regarding endurance training, players performed an average of 19 sessions. In the first four weeks, players performed individual strengthbased high-intensity interval training (HIIT) circuits, and from the fifth week onwards they were prescribed general aerobic fitness training sessions based on continuous and progressive exercises. Both subjective ratings of perceived exertion (RPE) [27,28] and the OMNI Perceived Exertion Scale (OMNI-Res Scale) for Resistance Exercise [29] were used to prescribe intensity during training sessions. See   [30]. All data were reported as mean ± standard deviation and the level of significance was set at P < 0.05. All statistical analyses were conducted using SPSS version 23.0 (SPSS Statistics, IBM Corp., Armonk, NY, USA).

Submaximal shuttle run test
Moderate, non-significant mean HR increases were observed in the early stages of the submaximal shuttle run test (aerobic capacity) (from stage 1, 108 ± 15 and 117 ± 11 bpm; stage 2, 127 ± 11 and 141 ± 21 bpm; stage 3, 133 ± 12 and 147 ± 20 bpm; stage 4, 140 ± 12 and 153 ± 19 bpm of HR mean values from before and after the home training programme, respectively) and moderate, significant changes were observed in later stages (from stage 5, (see Figure 2). This might be indicative of a loss of aerobic capacity [31,32]. It has been well established that detraining, due to training suppression or inadequate training, induces maximum HR increases (between 5% and 10%) [11]. Although this was not exactly the case during the lockdown scenario, the home training programmes probably failed to provide a sufficient stimulus to maintain aerobic capacity in elite handball players. This was also previously described by Fikenzer et al. [14], who found that endurance capacity, measured by the maximum mean velocity achieved in a multistage 20-metre shuttle run test, was diminished in most elite handball players from a given team due to the unspecific and inadequate stimuli provided by a home-based training programme during the COVID-19 lockdown. Dauty et al. [32] obtained similar results with the yo-yo test in young football players. The dependence on volume of endurance training responses [33] would explain the incapacity of home training programmes to maintain aerobic capacity in highly trained top handball players. In fact, the home training volumes were approximately 40% lower than those achieved during the regular season immediately before the lockdown. Moreover, our players only received running-specific stimuli during the last two weeks of the lockdown (see Figure 1), reinforcing the notion that the lack of training specificity also contributed, to some extent, to the loss of aerobic capacity [14,32].
Finally, only small, non-significant increases were found in the last stage (stage 8, 163 ± 10 and 168 ± 13 bpm of HR mean values from before and after the home training programme, respectively).
The results from this test were derived from 9 players due to HR band registration problems with 2 of the players from the sample.

Jump test
No changes were found in jump performance (41.  Figure 4).

DISCUSSION
The aim of this research was to analyse the ability of a home training programme to preserve aerobic capacity and jumping performance in top-level handball players during the COVID-19 lockdown. The home training programmes followed by the players maintained lower limb explosive strength, measured as CMJ performance (jump height), but appeared to be insufficient to maintain aerobic capacity.

Aerobic capacity
Moderate, significant HR increases were observed in the last stages of the submaximal shuttle run test after the COVID-19 lockdown muscle [11] and present a high correlation with endurance capacity in trained populations [35]. Together with HR values, these results confirmed that home-based endurance training was insufficient to maintain aerobic fitness in top-level handball players. However, it must be considered that since only moderate (ES) changes in aerobic fitness indicators (HR and lactate) were found after the lockdown, it seems reasonable to expect a rapid recovery of pre-lockdown values when players returned to on-court sport-contextualized training regimes.

Lower limb explosive strength
Regarding CMJ performance as an indicator of lower limb explosive strength [23], no changes were found between the two test periods (see Figure 4), showing that the training stimuli provided by the strength home-based training programme ( Figure 1) were adequate to preserve jump capacity. Despite certain signs of detraining in neuromuscularrelated qualities and peak power output, similar results have been previously reported in the literature about home training programmes' capacity to preserve jump performance (height) in professional football players [36,37] and futsal players [38]. Specifically, Rampinini et al. [37] analysed fifty professional football players and found that 2-3 bodyweight or small weight strength training sessions per week at home during the COVID-19 lockdown preserved CMJ height despite a moderate (ES) loss in peak power output. Those authors [37] also obtained similar results following the transition period, where similar bodyweight training strategies were implemented. In this regard, it has also been observed in national level handball players that a 7-week interruption of the external weight-based strength training, where players only performed sport-specific training and bodyweight exercises, was enough to maintain jump performance (height) [39]. Therefore, and although a certain degree of loss in jump-related neuromuscular qualities might be expected, home-based lower limb strength training programmes, despite the differences in training contents and strategies (including equipment), seem to be capable of maintaining jump performance measured as CMJ height.

Limitations
An important limitation of this study was the impossibility of assessing the whole team after the lockdown because many players were in their respective home countries. Despite this limitation, 11 toplevel handball players were analysed, all of whom were international players with their respective national teams. Finally, since the findings of this study come from 11 high-level handball players from a single team, caution is advised when generalising from these results, as different home training strategies in different team sport athletes might induce different adaptations.

Practical applications
A structured home-based training programme based on body weight and low weight exercises provides a sufficient stimulus to maintain jump performance (jumping height), an indicator of lower limb explosive strength, in top-level handball players. In contrast, the homebased training programme described did not succeed in preserving aerobic fitness in the cohort under study. Earlier implementation of aerobic fitness training strategies might have helped in the preservation of players' endurance capacity. However, since the loss in aerobic fitness indicators was moderate (ES), a rapid recovery of prelockdown values may be expected when players return to on-court sport-contextualized training regimes. Overall, the results of this study support existing general recommendations on the training approach during COVID-19 lockdown periods [40].

CONCLUSIONS
In conclusion, a structured home-based training programme during the COVID-19 lockdown preserved lower limb explosive strength but was an insufficient stimulus to maintain endurance capacity in toplevel handball players.