Seventy–eight climbers were assessed (22 women and 56 men, 25.7 ± 6.7 years old, 64.9 ± 8.6 kg, 173.0 ± 9.0 cm) for the finger strength profile (including flexor strength, extensor strength and flexor-to-extensor ratio). Participants' climbing levels ranged from intermediate to elite on the International Rock Climbing Research Association (IRCRA) scale (25), with an average of 20.3 ± 4.3 in their self-reported best red-point grade in the past six months. They had all practised climbing (indoors and/or outdoors) at least twice a week for the past two years, and had had no upper limb injuries in the previous six months. In addition, although carrying out regular practice, no climber had followed a specific training protocol lasting several weeks in the six months prior to this study. All participants volunteered and signed an informed consent form. The study was conducted with the formal approval of the CERSTAPS Ethics Committee.

The 78 climbers were tested in a pre- and post-format described below. Of the initial sample, 52 climbers (15 women and 37 men, 25.7 ± 6.9 years old, 65.4 ± 8.5 kg, 172.9 ± 9.7 cm; 19.0 ± 4.3 in their best red-point grade) participated in the experiment by following a specific training protocol. The climbers were randomly assigned into four different training protocols. Based on previous research done on finger-specific training in climbing (26, 27), the training program lasted 4 weeks (weeks 1–4) with 2 sessions per week and started the week after the pre-test session (week 0). A post-test session, identical to the pre-test, was performed the week after the end of the training sessions (week 5) (22). All climbers were instructed to continue their climbing activities normally and regularly outside of the study throughout week 0 to week 5.

The pre- and post-tests consisted in measuring the finger flexion and extension strengths using a hangboard (SmartBoard, Peypin d'Aigues, France) instrumented with force sensors (strain gauges) measuring the vertical force applied on the holds (0.8 N accuracy, 50 Hz acquisition, 0–4,000 N range of measurement). The associated app provided real-time feedback on the force exerted, allowing precise modulation of the force intensity during training. Before each test session, participants first underwent a 20-min standardized warm-up and familiarization with the instrumented hangboard, consisting of muscular awakening (scapular retractions, shoulder and wrist circles, finger grips, etc.) traverses and specific exercises (pull down, push up with fingers) on the hangboard with increasing intensity. Then, they performed the tests, which consisted of assessing maximum finger flexor strength (MFS) and maximum finger extensor strength (MES). Four trials were performed in each condition (two warm-up trials and two maximum trials). Participants were asked not to train or climb the day before the experiment and to be ready to perform as much as possible. The same experimenter was present during all test sessions (before and after training), checked the correct execution of the tasks for each test and verbally motivated the participants to ensure maximum performance.

The participants were first randomly divided and followed three different types of training (TFlex, TExt, TPaired described below) and a control group (CTRL). Taking into account dropouts, other participants were recruited so that the climbing level, gender and age matched between the groups. TFlex focused on training finger flexion strength only, TExt focused on finger extension strength only, while TPaired aimed to train both flexion and extension strengths simultaneously. The same grip types (in flexion and extension) were used for all training sessions as for the test sessions. The CTRL group (n = 20) did not follow any specific training and only continued their normal climbing activity. The three training sessions were best matched in terms of the duration of effort, the perception of the load during the pre-test. In this sense, 10 s of effort in flexion appeared as an equivalent perception of effort of 5 s for extensors. We therefore added a set of repetitions for TExt compared with TFlex in order to achieve, at best, a similar duration. For TPaired, the duration of effort was longer than for the others, as we took into account the time needed to switch from flexion to extension.

Data are reported as mean ± SD. Descriptive statistics were used to verify whether the basic assumption of normality was correct for all the variables studied. As we were testing a mixed gender group, we first tested for the presence of any differences between men and women using ANCOVA (with climbing level as a co-variate) or non-parametric ANCOVA when variables did not follow a normal distribution. Then, to categorize the participants, the results of the MFS/MES ratio were divided into eight classes allowing them to be listed from “very low” to “extreme” ratio. The number of classes was determined using Sturges' rule, appropriate for n < 200 (29). Considering a normal distribution, the value of Z-score for a probability of <0.05 was computed and the confidence interval of the MFS/MES ratio was computed. Pearson's correlations were used to observe the relationship between the climbing level and the different parameters (MFS, MES, and the MFS/MES ratio). The effects of training on MFS, MES and the MFS/MES ratio were assessed by comparing the training groups (CTRL, TFlex and TExt and TPaired) over time (pre- and post-tests) using a 2-factor repeated-measures ANCOVA (Time × Group, with climbing level as a co-variate), with Tukey post-hoc analysis and power (1-β) when ANCOVAs were significant. In addition, effect sizes (partial eta squared, η²) were computed and were defined as small for η²>0.01, medium for η²>0.09 and large for η²>0.14 (30).

The finger strength profile variables for all participants are presented in Table 1. Analysis of the data performed after dividing the groups based on gender indicated that MFS and MES were higher in men than in women when expressed in N but when normalized to body weight, no differences were observed between men and women in MFS (p = 0.82) and MES [F(1,75) = 0.004; p = 0.95; η²=0.00]. The MFS/MES ratio (p = 0.16) was also similar between men and women.

A significant correlation between the climbing grade and MFS and the MFS/MES ratio was observed but there was no correlation between the climbing grade and MES. The same results were observed in men and women for MFS, MES and the MFS/MES ratio.

The eight MFS/MES intervals are shown in Figure 2. Since no significant difference was observed between men and women, the histogram was based on pooled data and made it possible to classify intervals from “very low ratio” to “extreme ratio”. The ratios of the lowest class were less than 4.15 while the extreme ratios were above 9.75, meaning that the finger flexors were 9.75 times stronger than the finger extensors in this class. The mean value was 6.27 and the confidence interval was within the range of 5.94 and 6.61. The Z-score for a p < 0.05 probability corresponded to a value of 8.75.

The anthropometric data and climbing ability of participants involved in the different training groups are summarized in Table 2. No statistical differences were observed between groups for all variables.

MFS, MES and the MFS/MES ratio results before and after training according to the different groups are presented in Table 3. There were significant Time × Group interaction effects for MFS [F(3,47) = 7.4; p < 0.001; η²=0.32; 1-β=0.91], MES [F(3,47) = 6.0; p = 0.001; η²=0.28; 1-β=0.86] and a tendency in the MFS/MES ratio [F(3,47) = 1.37; p = 0.099; η²=0.12; 1-β=0.48]. Post-hoc tests revealed that MFS was greater after training than before in the TFlex and TPaired groups, and MES was greater after training than before in the TExt group, and the MFS/MES ratio seemed to decrease after training in the TExt group.

Our results allowed us to determine a finger strength profile for the climbers, as well as ratio classes that allow us to measure the degree of imbalance between finger flexor and extensor strengths. Our results showed a positive correlation between the finger flexor capacity (MFS) with the climbing level which is in line with previous studies (8). As our sample was mixed-gender, the analysis enabled us to measure any gender-related effect. As no differences were observed between men and women when strength was normalised by body weight, the rest of the analysis was based on pooled data. The gender effect in our study differs from the literature, as Mermier et al. (31) found a higher strength in men than in women, despite body mass normalisation. In their study, the gender difference was explained by a lower climbing level in female participants compared with male participants. However, in our study, the climbing level of women (18.4 ± 4.1) was also lower (p = 0.013) than that of men (21.0 ± 4.1). Faced with this problem, we used ANCOVA with climbing level as a co-variate to correct for its effect on the variables analysed. This statistical approach may thus explain the different conclusion compared with Mermier et al. (31) who only performed a t-test without considering the effect of the climbing level. Future studies should thus take into account the climbing level as a co-variate to isolate the main effect of the factors tested and provide robustness in any conclusions.

Contrary to the results for MFS, MES results were not correlated with the climbing level which is in line with the literature (5, 8, 14). This confirms previous findings by Vigouroux et al. (14), who showed that practising climbing develops primarily the flexors, so it is justified to ask whether the balance of the finger flexor-to-extensor ratio should be shifted, especially given the complexity of the hand, which requires the intricate balancing of a whole chain of joints. This equilibrium implies a major action of the finger extensors, as previously shown in other types of grip (32, 33) which, without appropriate capacity, can limit finger force-generating capacity (34).

With regard to the MFS and MES results, the averaged MFS/MES ratio showed a strong imbalance in both men and women which is correlated with the climbing grade level. In our study the ratio revealed that the finger flexors were on average 6.27 times stronger than the finger extensors. This result is similar to the ratio previously observed in the literature for climbers [6.10 in Vigouroux et al. (14)]. A relationship between the MFS/MES ratio and climbing level was also shown, so the more experienced the climber, the more unbalanced the ratio, and the higher the need to rebalance the extensors' capacity. The histogram (Figure 2) provides ratio values that allow the imbalance to be considered and classified. For example, a climber with a ratio in the class of 6 (6.02–6.95) could be considered a “standard” climber (where the confidence interval is included). The “extreme” climbers (with a ratio higher than 8.75 defined by the Z-score) represented 7.7% of our participants, and are included in the two highest classes of MFS/MES ratio. With such imbalanced results, it is legitimate for climbers and trainers alike to decide whether a rebalancing should be undertaken since the extensors are highly solicited during climbing grips and such an imbalance could either limit performance or lead to overuse and injuries.

The main contribution of this first part is the easy-to-implement method which allows discriminating climbers from a muscular imbalance perspective. Although this method was based on external fingertip force measurements the results were in line with previous studies relying on more complex measurements and evaluating internal muscle capacities, confirming the validity of the present protocol. The main interest is that this method, unlike the one based on modelling by Vigouroux et al. (14), can be implemented in gyms for trainers and climbers. Given the complexity of the biomechanics (23 joint degrees of freedom) and muscles of the hand (more than 40 muscles), determining the capabilities of each muscle does indeed require a modelling approach using electromyography and kinematics, combining efforts on all the 23 joints of the hand under different force application conditions. This time-consuming method would not have been applicable to be consistent with our first objective and to use in daily training.

Few studies have focused on the flexor-to-extensor ratio in the upper limb of climbers, particularly in the shoulders and elbows (30, 31), and some differences have been found compared with non-climbers, but the impact of these consequences on the risk of injury needs to be confirmed as the climbers tested were all uninjured. Based on the method currently proposed, further studies are now needed to establish relationships between the occurrence of finger injuries and the value of the MES/MFS ratio, in order to investigate the pertinence of this ratio in the occurrence of injuries.

First of all, similar values in the control group between both pre- and post-tests showed that differences observed in other training groups are not attributed to a familiarization effect with the tests nor to other concomitant activities. Regarding the training effects, the hypothesis that flexor-based training increases MFS was confirmed by our results, which indicated an increase in MFS (+8.4% in the TFlex group and +11.9% in the TPaired group, on average). However, the hypothesis that extensor-based training increases MFS and MES was only partially confirmed: paired training increased MFS but not significantly MES (+18.1%), whereas extensor-only training increased MES (+41.4% in the TExt group on average) but not MFS. Thus, the MFS/MES ratio had a tendency to decrease with the extensor-only training (−27.8%) but seemed to remain stable in the other groups (between −1.9% and +1.6%). The increase in MFS after flexor-based training is in agreement with the literature (22, 26, 27, 35). The training with 80% MFS tested in the current study led to an 8.4% increase in strength. These improvements have been discussed in detail by Devise et al. (22) for this type of training. Briefly, the physiological phenomena activated are probably a combination of neural adaptation processes and metabolic stress that may be effective in increasing muscle strength.

No increase in MFS was observed in the extensor-only training. This differs from the literature focused on other joints, which showed an increase in hand grip (23) and elbow flexion (24) with antagonist training. This difference might be explained by several factors. First, the muscles analysed were not the same, especially as the fingers are at the end of the upper limb chain, so the adaptations may be different. In large muscles, hypertrophy can partly explain a strength gain, but the volume available in the forearms for the finger muscles is more limited and suggests more difficulties for development, which may explain the lack of increase (36). Secondly, the duration of our training protocols was shorter than in previous studies (4 vs. 6 weeks or more) and we can suppose that an increase may appear with a longer training program. Finally, climbers already have a higher initial flexor strength compared with non-climbers, which makes it more difficult to gain strength (22), whereas the population tested in the previous studies (23, 24) were untrained subjects. Thus, these effects would depend on the type of population studied, and it would appear that agonists in a trained population (i.e., with higher initial strength) would be less sensitive to strength gain.

The increase in MFS in the paired training is consistent with the literature concerning the APS training (28). This type of training was chosen because it might be beneficial for both strength development and injury prevention. As this type of training is an alternation of exercises involving the coupling of agonists and antagonists, it has the advantage of enhancing acute performance on agonists in a relatively short period of time [significant effects after 4-weeks of training (24)] and to be less-time consuming than traditional resistance training. Reported effects on antagonist strength are rarer but improvements may be expected as a previous study (37) has shown an increase in both flexor and extensor forearm strength in recreationally trained individuals. However, no significant increase in MES was observed in the paired training of our study, despite an average increase of 18.1%, which could be attributed to relatively high inter-individual variability. As MES is not correlated with climbing level, it cannot be the type of population (with a potentially higher initial MES) that affects our result. However, our results are similar to those of Fink et al. (38) who found no increase in one repetition-maximum for triceps, although the significance of their findings was questioned due to relatively large confidence intervals. It may also be that our training was not sufficiently optimum to be significant, but could probably be improved by simply changing the volume and/or rest periods during the training sessions.

There seemed to be a tendency for the MFS/MES ratio to decrease in the extensor-only training (−27.8%), due to a significant increase in MES without an increase in MFS. Again, the variability was relatively high. The mean ratio after the extensor-only training (3.16) was 47% lower than in the control group, close to or even lower than that found in non-climbers in the literature [3.66 in Vigouroux et al. (14)]. It can be assumed that extensor-based training may activate the same physiological phenomena as flexor-based training and as mentioned above. The effects of extensor-based training should be confirmed by an intervention longer than 4 weeks or with a higher training volume.

In the other training groups (the TFlex and TPaired groups), the MFS/MES ratio did not decrease so the flexor-to-extensor imbalance remained high. Although MFS increased, the ratio did not increase either, which means that MES must increase slightly, not enough to be significant but enough to keep the ratio similar. A certain amount of work was therefore done by the co-contraction of the finger extensors, which are the antagonist muscles, and are involved in the maintenance and stability of the joints (14, 33). However, the additional work on the extensors in the paired training was not sufficient as it did not increase the MES: it seems better to separate the training of the flexors from that of the extensors in order to obtain the best benefits.

From a practical point of view, the main conclusion is that improving MES is not obvious. Even if the extensors are highly engaged during climbing grip, TFlex or TPaired training is not suitable for improving their level. Only the TExt training over four weeks has been validated to rapidly enhance MES. Further studies with a longer training period should be conducted to explore whether this has a significant effect on the finger flexor-to-extensor ratio. In addition, the training volume of the TExt is only 15 min per session, so that it can be quickly and easily incorporated into a “classic” climbing training routine, making it potentially acceptable to climbers. It should be noted that the load applied in our study (>70% MES) is of high intensity to produce MES benefits. This training intensity is probably higher than that used in the popular exercise relying on elastic bands to train finger extensors. The amount of force exerted with elastic bands is not known and not constant throughout the extension phase. Unfortunately, to our knowledge, no studies have reported information on the effects of elastic band training on MES, but it can be expected that this exercise does not produce sufficient resistance and intensity to improve strength benefits (39).

This study presents some inherent limitations that should be considered. First of all, our results should be confirmed with higher-elite climbers as we only tested climbers from intermediate to elite climbers. In addition, our study lacked a population of non-climbers to exactly understand the adaptations associated with climbing. As a further analysis, it would be interesting to investigate the level of activation of the finger flexors and extensors before and after training using electromyography. This would highlight the neuromuscular adaptations that may have occurred and clarify the mechanisms that explain the strength gains whereas, in the current study, only assumptions of the phenomena can be made. In addition, the relationship between the MFS/MES ratio and the injury rate is only speculative given the current state-of-the-art. Further studies should thus focus on measuring the finger strength profile of previously injured climbers to provide more information. Furthermore, conducting a longitudinal study of climbers who have undergone rebalancing training and those who have not, and then observing the incidence of injury in both groups using the proposed assessment method would be a step forward in understanding injury prevention. Future research is therefore needed on this topic.