Exploring forearm muscle coordination and training applications of various grip positions during maximal isometric finger dead-hangs in rock climbers

Participants

Twenty-five participants (22 males and three females) (Table 1) were recruited for this observational cross-sectional study. Inclusion criteria were: (a) being 18 years of age or older; (b) having a minimum climbing experience of one year; (c) performing a minimum amount of regular climbing practice of one session/week in the previous 6 months; (d) self-report of a climbing grade equal to or higher than 6b on the French Rating Scale of Difficulty (Draper et al., 2015); (e) having a minimum experience of 4 weeks with DHs; and (f) being free of finger and upper body injuries. Rock climbing performance (i.e., climbing grade) was converted to the International Rock Climbing Research Association (IRCRA) reporting scale to compute descriptive statistics (Draper et al., 2015). This study was approved by the Ethics Committee for Clinical Research of the Catalan Sports Council (approval number: 25/2019/CEICGC) and was conducted in accordance with the Declaration of Helsinki. Written informed consent was obtained from all participants.

Procedures

Data collection and variables

Surface electromyography

The sEMG signals were recorded at 1000 Hz using a DataLog type no. P3X8 USB (Biometrics Ltd., Newport, UK) and SX230 sEMG sensors from the same manufacturer, which consisted of bipolar Ag-AgCl surface electrodes (10 mm diameter, 20 mm center-to-center distance) and a differential amplifier (gain 1000, input impedance 100 M Ω, an input noise <5 µV, common mode rejection ratio higher than 96 dB). Participants’ skin was shaved, abraded, and cleaned with alcohol. Then, surface electrodes were secured with double-sided tape on the flexor digitorum profundus (FDP), flexor digitorum superficialis (FDS), flexor carpi radialis (FCR), and extensor digitorum communis (EDC) (Fig. 2) of the participant’s self-reported dominant forearm. The electrodes on the FDS were placed approximately at 3/4 of the forearm length, slightly ulnarly on the line from the biceps tendon to the middle of the wrist. For the FDP, the electrodes were placed on the prominent bulge of the muscle at approximately 5–8 cm from the olecranon, slightly ulnarly and on the line between the olecranon and the lunate. For the FCR, electrodes were placed at around five cm from the medical epicondyle of humerus on the line between the medial humeral epicondyle and the proximal end of the second metacarpal. The EDC electrodes were placed around the 1/4 point on a line drawn from the lateral epicondyle to the styloid process of the ulna. The reference electrode was placed over the styloid process of the ulna. Electrodes were positioned on each muscle along a line between the origin and insertion of the muscle in the supinated hand, parallel to the direction of muscle fibers, determined using an anatomical atlas (Perotto et al., 2011), previous publications (Matthews & Miles, 1988; Vigouroux & Quaine, 2006; Dykes, Johnson & San Juan, 2019), and manual palpation. To check electrode placement and minimize crosstalk between electrodes, isolated contractions of each muscle were performed and electrode placement was adjusted when necessary. Flexion of the DIP of the fourth finger with immobilized PIP and neutral wrist position was performed to check the FDP electrode placement. Flexion of the PIP of the third finger with immobilized DIP and neutral wrist position was used to check the FDS electrode placement. Flexion of the wrist while maintaining extended fingers was used to check the FCR electrode location. Finally, extension of the fingers while maintaining a flexed wrist position was used to check the location of the EDC electrodes. Electrode placement was considered satisfactory when a clear and isolated activity of each of the muscles was obtained. In addition, when possible, inter-electrode distance between muscles was six cm or more, to minimize any possible crosstalk effect (Mogk & Keir, 2003).

sEMG data were analyzed using Spike 2.0 software (Cambridge Electronic Design Ltd., Cambridge, England). Raw sEMG data were filtered with a Butterworth Band-pass filter at 20–460 Hz. Root mean square (RMS) values were computed using a time window of 2 s of contraction, 1 s after the trial start. The average RMS for each muscle and grip position was computed between the two maximum DH trials (RMS_FDP, RMS_FDS, RMS_FCR, and RMS_EDC). Global muscle activity (RMS_Global) for each grip position was computed as the sum of the RMS of all muscles. In addition, neuromuscular efficiency (NME) was computed dividing the held force by the neuromuscular activity of each muscle (maximal load * 9.8/RMS) (Magalhães et al., 2016): NME_Global, NME_FDP, NME_FDS, NME_FCR, NME_EDC.

Statistical analysis

Data normality was checked via exploration of histograms and by the Shapiro–Wilks test. Variable transformation was used when necessary. One-way ANOVAs with repeated measures or Friedman’s test were used to test differences among grip positions. If a significant grip main effect was found in the ANOVAs or Friedman’s test, pairwise comparisons were carried out between grip positions. Bonferroni correction was used for the ANOVAs and pairwise comparisons. Statistical analysis was performed using SPSS.21. The level of significance was set at p = 0.05.

The effect size of the different tests was calculated according to Cohen (Cohen, 1988): d for t-test (0.2 small, 0.5 medium, and 0.8 large effect), ${\eta }_p^2$ for ANOVAs (0.01 small, 0.06 medium, and 0.14 large effect), and Kendall’s W for Friedman’s test (0.2 small, 0.5 medium, and 0.8 large effect).

Global muscle activity

When global muscle activity was compared, we observed that the CRIMP and SLOPE grips presented similar muscle activity levels, whereas the SLOPER presented greater global activity compared to the other two grip positions. To our knowledge, only one study has assessed forearm muscle activity aiming to compare two or more grip positions (Watts et al., 2008). Watts et al. (2008) compared six different grip positions, including the CRIMP and the SLOPE. Despite finding superior activity for the CRIMP, interpretation and comparison of their results is limited because they only measured sEMG from a single muscle on the anterior forearm and they did not equate intensity across grip positions. In addition, the study of Watts et al. (2008) did not assess forearm muscle activity in the SLOPER grip.

Our differences observed in global muscle activity could be caused by various factors. On one hand, it is well established that muscle activity assessed by sEMG amplitude is related to the magnitude of the muscle contraction and force or torque production (De Luca, 1997). Because we equated the relative intensity across conditions, the bigger usable surface of the SLOPER resulted in a superior mechanical advantage and allowed for higher loads, possibly favoring the production of higher sEMG in this grip position. The fact that SLOPER presented a higher NME indicates that this grip required lower muscle activity for each Kg of load used. Therefore, SLOPER can attain greater muscle activity, although the load necessary to reach this activity level is higher than for the other grip positions. On the other hand, differences in the gripping position itself could have caused changes in the contributory role of each of the joints involved, especially the interphalangeal and wrist joints (Charissou et al., 2017; Caumes et al., 2019a; Beringer et al., 2020). Therefore, changes in the activity of the muscle groups involved are presented below.

Individual muscles: SLOPER vs CRIMP and SLOPE

The SLOPER grip also presented greater activity of the FDS and the FCR compared to the other two grips, while showing lower activity of the EDC. The FDP activity remained similar across the three grip positions. However, SLOPER also presented greater FDP, FDS, and EDC NME, indicating a relevant role of the used load on these muscles’ activity.

Many climbing-related studies have focused on the role of the finger flexor muscles, as they are thought to be the primary agonist muscles during climbing and DHs (Vigouroux & Quaine, 2006; Macleod et al., 2007; Schweizer & Hudek, 2011; Philippe et al., 2012). FDP absolute activity was similar across grip positions, although SLOPER presented the highest FDP NME. Thus, comparable recruitment of the FDP could be achieved with any of the studied grip positions, but SLOPER may need higher absolute loads to achieve it. On the other hand, FDS showed increased activity in the SLOPER, indicating that this grip type can potentiate the use of an active flexion of the proximal interphalangeal joint (PIP) through the action of the FDS. Hence, high intensity training with the SLOPER could provide a greater stimulus for the FDS. However, this superior muscle activity could only be achieved at the expense of using a larger load than the one used with the other grip positons, as the SLOPER presented a greater FDS NME compared to the CRIMP. Although no previous research has compared the SLOPER grip with other grip positions in terms of differential muscle activity, we think that the differences found in the present study could be attributed to the necessity of maintaining friction with the curved sloper hold. To avoid slipping from SLOPER, the climber must maintain the center of pressure as close as possible to the flattest part of the hold; that is, parallel to the floor and close to the wall (Fuss et al., 2013). Because the sloper hold used in the present study had a depth of 60 mm, it allowed contact of the middle phalanx with the hold positioning it almost parallel to the ground. We believe that these conditions could have facilitated an increase in the vertical force produced at the fingertip through active flexion of the PIP generated by the FDS action.

Similarly, the increased FCR activity observed in the SLOPER could also be attributed to the need to maintain sufficient friction with the curved hold. This need to optimize friction may require greater wrist stability, which is usually attained through co-contraction of the wrist flexors and extensors (Caumes et al., 2019b; Caumes et al., 2019a). In addition, the greater activity of the FDS while pressing on the hold with the fingers could generate a certain level of mechanical extension moment at the wrist, which might be counteracted by an increase in the activity of some of the wrist flexors, such as the FCR (Charissou et al., 2017). The fact that no differences in the NME were found between grips for the FCR may support this reasoning. Thus, differences in the muscle activity of the FCR could not be solely explained by a greater load during the SLOPER, but they also may be explained by the hand and fingers configuration and action during this grip type. The present FCR results underline the importance of using SLOPER grip positions to better stimulate the wrist flexors.

Regarding the EDC, its activity was lower in the SLOPER grip compared to the other two grip positions. It has been proposed that a co-contraction of the finger and wrist extensors may be used as a control strategy to increase wrist joint stability when performing gripping tasks (Snijders et al., 1987; De Serres & Milner, 1991). Thanks to this co-contraction, deviations from the desired position and an unintended wrist flexion moment generated by the finger flexors’ action may be counteracted (Snijders et al., 1987; De Serres & Milner, 1991). However, different hand configurations during maximal gripping tasks may drastically affect the finger and wrist flexor-extensor co-contraction (Charissou et al., 2017). Specifically, Charissou et al. (2017) found that extensor activity was lower in a hand configuration consisting of finger-pressing on a surface similarly to our SLOPER grip, in comparison to power gripping. It was suggested that this difference in extensor activity was caused by a diminished need to counteract the unintended flexion moment generated by the finger flexors at the wrist. This hypothesis may support our results, where a greater activity of the FDS and FCR during the SLOPER would possibly facilitate maintenance of the center of pressure at the flattest part of the curved sloper hold while pressing on it. Thus, an opposition of the wrist flexion moment by the EDC might not be desirable, as observed by the lower EDC muscle activity and the greater efficiency we found in the SLOPER. Furthermore, previous climbing research using the SLOPE grip has proposed that the EDC may act alongside the FDS to stabilize the PIP joint, preventing FDP tension from driving this joint into hyperextension, a position also known as the swan neck position (Schweizer & Hudek, 2011). Although finger position during the SLOPE and the SLOPER might be similar, because the middle phalanx of the fingers was in contact with the hold during the SLOPER, it is unlikely that the PIP could hyperextend. Thus, lower stabilization of the PIP joint by the EDC might be required during the SLOPER.

Individual muscles: CRIMP vs SLOPE

The CRIMP and SLOPE grips presented similar levels of activity in the FDP, FCR, and EDC, but the CRIMP showed greater FDS activity and lower NME. These results indicate that the CRIMP grip can better stimulate the FDS than the SLOPE, even when similar Maximal loads are used. Although no studies have examined the activity of the FDS between these two grip positions under intensity-equated conditions, others have explored the contribution of the FDS and FDP by analyzing force and tendon tensions (Vigouroux et al., 2006; Schweizer & Hudek, 2011). Our results are supported by a study performed with cadaver fingers where a more predominant role of the FDS was observed when using the CRIMP grip compared to the SLOPE grip on holds of a depth equal to the distal phalanx or greater. The results reported by Schweizer & Hudek (2011) support the greater FDS activity in the CRIMP grip observed in our study, which was performed on a 21 mm deep hold, approximately the length of the distal phalanx.

Schweizer & Hudek (2011) have hypothesized that when performing the SLOPE grip using a hold of about the same size of the distal phalanx (similar to our 21 mm hold), the FDS may play a stabilization rather than an agonistic role, preventing the PIP from assuming a hyperextended position (swan neck position), thus less FDS activity would be expected when using the SLOPE grip. Furthermore, Schweizer & Hudek (2011) observed greater efficiency (greater tendon-fingertip force transmission) of the FDS during the CRIMP compared to the SLOPE grip. If this hypothesis is correct, it is possible that climbers involuntarily prioritize use of the FDS when hanging using a CRIMP grip. Our results support these hypotheses when using the SLOPE grip on a 21 mm deep flat hold.

Practical applications

Overall, our results may provide valuable information for coaches and practitioners as regards the design of climbing training programs. We suggest that the SLOPER has a greater recruitment potential of the agonist muscles (i.e., FDS and FCR) than the CRIMP and the SLOPE. Therefore, inclusion of the SLOPER when training grip strength through DHs might be advised, especially for those seeking to stimulate the FCR. In addition, this grip position could diminish the stress of the soft tissue of the fingers (pulleys and skin) compared to the other two positions. However, greater loads might be needed for this improved recruitment compared to other grip positions, which may increase the stress in the shoulders and elbows and reduce the comfortability of the DHs, as it requires the addition of a lot of external load. Furthermore, to our knowledge no study has yet modeled the real impact of the SLOPER on the annular pulleys like it has been done with the other two grip positions. On the other hand, CRIMP grip also seemed to have greater training potential than the SLOPE grip, especially for the FDS. Moreover, because the CRIMP presented the lowest NME, greater FDS activity might be expected with equal or even lower loads compared to the other two grip positions.

Study limitations and future directions

However, our study is not free of limitations. First of all, forearm is a difficult body region to measure sEMG because its muscles are rather small and are very close to each other. We were extremely careful placing electrodes following indications from previous publications and checking the electrode placement by obtaining isolate muscle activity. However, it is possible that our sEMG signals, and hence our results, were affected by an inevitable level of crosstalk between muscles. Furthermore, we did not take anthropometric or kinematic measurements of the fingers and wrist joints and we were therefore unable to compute torques and further discuss on the extent of the effect of load on muscle activity or fingers soft tissue stress across conditions. Although our results could have several applications in climbing and training, they should be interpreted with caution because generalization of our results to different hold characteristics, such as shape or depth, or real climbing may not be possible. In addition, despite the fact that most of the participants performed with the same maximal loads in both sessions and we left a minimum of 3 days between sessions while asking the participants to refrain from training for 24 h before sessions, it is possible that some degree of fatigue or delayed onset muscle soreness affected the participants (especially the less trained ones). To completely avoid this confounding factor, we would recommend tracking the participants’ activity to be sure that they really restricted their training activity for longer time periods. Lastly, our study mostly included intermediate and advanced level climbers with a wide range of anthropometric characteristics. Future studies should include anthropometrically homogenous groups of elite and higher elite level climbers to elucidate the possible effect of anthropometric characteristics and climbing expertise on forearm muscle activity.