Effects of individualised training programmes based on the force-velocity imbalance on physical performance in rugby players

1. Introduction

An elite rugby player must be competent in the different phases of sprinting speed, including the initial acceleration and maximum speed [1]. Rugby players are required to perform maximal and explosive movements for contact situations and sprint actions [2]. In this regard, due to the nature of the modern game, it requires players to be strong to successfully tackle, agile to quickly evade opposition, have high cardiovascular endurance to sustain performance more than 80 minutes of activity, and have rapid force production capabilities for accelerating and tackling [3].

According to load magnitude employed in resistance training, Mora-Custodio et al. [4] reported that low-loads (40–60% 1RM) training programmes produce similar or more beneficial effects on neuromuscular performance than moderate loads (60–80% 1RM). Likewise, Pareja-Blanco et al. [5] showed that squat training with low/moderate loads combined with light loads resisted sprints may be an effective stimulus for improving leg strength, jumping ability, change of direction (COD), and sprint performance. In rugby players, the combination of two different complex training loading, both enhanced positive adaptations across the entire force-velocity spectrum, while simultaneously improving maximum strength and sprint [6].

Samozino et al. [7] proposed a model, known as Fv profile, based on the measurement of three simple variables, which allows practitioners to accurately assess strength, speed and power output in lower-limbs during a squat jump (SJ) in field conditions. In addition to being simple and low cost, test validation has been reported recently [8, 9]. The comparison of an actual Fv profile with a theoretical optimal Fv profile (i.e. balance between force and velocity) could help identify which component (force or velocity) should be prioritised during training to improve performance [7, 10]. For a given maximum power (Pmax), an imbalance between force and velocity (Fvimb) could lead to a 30% loss of jump performance [7]. The loss of performance, depending on the calculated imbalance, would indicate how far (expressed in %) an athlete is from an optimal Fv profile. In practical terms, individualised training programmes aiming to improve vertical push-off performance, should focus on increasing Pmax and/or reducing the Fvimb [11]. Morin and Samozino [11] suggested that athletes with a significant Fvimb, should follow an individualised RT program that prioritises the development of the unbalanced mechanical variable to optimise jumping performance. According to a recent study [1], both coaches and sports professionals involved in the development of rugby players should consider a sprinting force dominant Fv profile to improve acceleration ability, particularly on the initial acceleration phase. Another study [12] mentioned that to improve sprint speed in rugby players, training strategies should aim to optimise the athlete’s power to weight ratio, and that RT for lower limbs should focus on movement velocity.

In the light of the above, only two studies [13, 14] have examined the effects of an individualised resistance and plyometric programme based on Fvimb. These studies [13, 14] concluded that individualised training programmes based on the Fvimb improve jumping ability in soccer and rugby players. However, they did not examine the effects of these protocols on lower limbs strength (i.e. 1RM) and sprint performance. In contrast, regarding sprint Fv profile, it has been reported that an individualised sprint-training based on the Fvimb was not more effective in improving sprint performance than a generalised sprint-training programme [15].

It seems that no research has analysed the combination of resistance and plyometric programme based on Fvimb on strength, jump and sprint performance. It was hypothesised that if the Fvimb is reduced, it would increase vertical jump height along with a positive impact on lower limbs strength and sprint. Therefore, the aim of this study was to experimentally test the effects on lower limbs strength, jump and sprint performance after following an individualised RT programme based on Fvimb.

3. Results

4. Discussion

The aim of this study was to experimentally examine the effects on lower limbs strength, jump and sprint performance after following an individualised RT programme based on Fvimb. In the present study, results showed that individualised RT programme based on Fvimb induce improvements in sprint performance, whereas the NI athletes showed improvements only in SM. Moreover, individualised RT programmes based on Fvimb induced better adjustments of the Fv profile to the theoretical “optimal” Fv profile, although none of the groups improved 1RM-SQ strength, jump height or Pmax.

In rugby, sprint speed, BM and SM discriminate between levels of play [23]. Additionally, WB athletes improved not only sprint times in all intervals but MSS and SM. Moreover, the other “individualised” groups (Vimb and Fimb) also improved sprint performance. Young et al. [24] mentioned that sprints from a static start in a relatively short distance, such as 10 m, is a reflection of the acceleration ability, and therefore it could be expected an improvement in their initial sprint momentum. Baker and Newton [25] suggested that rugby players should focus on increasing strength and power along with muscle hypertrophy, improving on this way sprint momentum, which would be beneficial to cope with the contacts and tackles demands during games [26].

Likewise, and despite the substantial changes found in Fvimb across all “individualised” groups, no improvements were observed in jumping ability or Pmax after the intervention programme in highly trained rugby players. These findings do not support previous findings by Jimenez-Reyes et al. [13], who reported that reducing Fvimb without even increasing Pmax lead to clearly beneficial jump performance changes. Our findings do not support this suggestion, since none of the intervention groups increased jump performance despite obtaining a more “optimal” Fv profile. Regardless of this lack of “positive” results, Fvimb could be considered as a potentially useful variable for prescribing optimal RT loads to improve sprint and lower limbs strength. It is plausible to suggest that the lack of effectiveness in jumping ability in all groups could be due, in part, to the presence of neuromuscular fatigue that is accumulating throughout the season, as reported recently with high level athletes [27]. Moreover, another possible explanation could be that rugby players participating in the present study possibly needed more weeks to show positive effects on jumping ability and Pmax.

Regarding RT, it has been shown that higher levels of strength are associated with improvements in Fv characteristics, general and specific sports skills [28]. The results reported in the current study confirm that when imbalances are not individually addressed, as for NI athletes, the training programme was not effective in reducing Fvimb or improving jumping ability. In line with the findings of Jiménez-Reyes et al. [13] athletes participating in training programmes based on their Fvimb will get better results (at least they did not decrease jump performance) than those who participated in traditional training. It is likely, therefore, that an individualised RT based on Fvimb will improve strength and sprint performance in lower limbs, although a question arises as improvements in jumping ability did not occur. In this regard, Jiménez-Reyes et al. [13] only analysed changes in jumping ability. In addition, although no significant intra-groups improvements were observed for 1RM-SQ, the Fimb group was the only one that attained nearly significant gains ( P = 0.07) in 1RM-SQ. In this regard, it seems that carrying out an individualised RT programme based on Fvimb induced increases in 1RM-SQ strength. However, it should be recognised that improvements in maximal strength through RT is not unexpected. Previous studies have clearly shown the effectiveness of strength training aiming at specifically increasing maximal force [29, 30, 31, 32].

The results in the mechanical variables (Table 5), showed positive changes in the area of the curve that was prioritised in their training and, in turn, reduced the opposite one. These findings are in agreement with Jiménez-Reyes et al. [13], who reported that when training was focused on increasing V0, a parallel decrease in F0 was observed, and vice-versa. Therefore, caution should be taken when only is prioritised an extreme of the Fv spectrum aiming to attain the “optimal” profile, because it can induce decreases in the opposite extreme, being this a plausible explanation of the lack of positive adaptations in squat strength and jump performance. Therefore, the interpretation of Fv profile should be considered taking into account the changes in F0 and V0, since a decrease in both variables (F0 and V0) could result in lower Fvimb but could induce a lower athletic performance. Results in Vimb players, are consistent with previous findings in youth soccer players, in which after a RT program with low loads and low volume combined with jumps and sprints, induced important improvements in performance [20]. Regarding the use of heavy loads in RT, a study reported improvements in muscle performance in the upper portion of the force-velocity curve [33]. In Fimb athletes, results showed almost significant ( P = 0.07) improvements in 1RM-SQ, which is in line with the increases in F0.

The aim of a RT programme with different loads is to target all areas of the Fv curve as an attempt to increase adaptations across the whole Fv spectrum [33]. Although no changes were found in jump height and 1RM-SQ, WB players improved sprint performance across distances from T10 to T30. Therefore, these findings suggest that strategies combining low and high loads could be useful for the development of sprinting speed in rugby players with a well-balanced Fv profile. An interesting finding of the current study was the fact that WB had significant changes in some testing variables, whereas the NI athletes did not, despite an identical loading regime was carried out. Consequently, it is plausible that more significant improvements in athletic performance may occur following an individualised RT programme when taking into consideration individual Fvimb. However, this hypothesis needs to be further investigated. Present findings are limited to the specific population analysed, thus caution must be made, as when training is applied to high-level team sports athletes, the effects of periodised RT programmes on strength and power seem to follow the law of diminishing returns, while training exposure increases, adaptation rates are reduced [34].

We invite further research to address some limitations of this study and analyse; a) the effect of individualised loading prescription using individual Fvimb, without modifications on the approach used by Jimenez-Reyes [13, 14]; b) the effect of training programmes with more training sessions and weeks, since only 14 training sessions during 7 weeks were performed in this study. Additionally, a larger sample size would be needed since recruitment led to a Vimb group of only 6 participants while Fimb, WB and NI groups included 11, 9 and 8 participants, respectively.

In practical terms, rugby player’s assessment should include Fvimb to individualise training loads according to the needs of each athlete, aiming to identify in which area of the Fv spectrum an athlete needs to prioritize their RT. However, it seems that the entire Fv spectrum should be targeted (although one zone is prioritised) to avoid decreases in the untrained zone. Therefore, strength and conditioning coaches should also assess 1RM strength (to determine exactly the relative and absolute loads to be used) and sprint speed. The combination of these assessments together with well-designed training programmes can lead to improvements in lower limbs absolute strength and sprint performance, which are key qualities in rugby union players.

5. Conclusion

In conclusion, this study presents data of the effects on key physical characteristics of rugby union players when addressing force-velocity imbalances. The individualisation of a training programme based on Fvimb could be an effective method to improve sprint and strength performance. Reducing the actual force-velocity imbalances, may have a positive impact in other maximal actions such as acceleration ability, speed and strength. However, no positive changes were observed in jump performance for any of the intervention groups.