Relationships between isokinetic knee and shoulder peak strength with maximal punch force in boxing athletes

1. Introduction

Boxing was first included in the Olympic program in 1904. To win a match, boxers must deliver powerful punches to knockout opponents. Alternatively, one can use the repeated execution of punches with high force to increase the possibility of a win based on points. Punch force is a key factor influencing competition results and an important indicator of boxing performance [1]. Smith et al. found that the maximal punching force (PFmax) was greater in elite boxers than in mediocre and novice boxers [2]. These results suggest that boxers benefit from the ability to punch with high force. A successful boxer must have the ability to generate well-developed muscle strength in order to improve PFmax. The lead (LA) and rear arm (RA) straight punches, the most frequent boxing punches, are influenced by both upper-and lower-body muscular strength [3]. Most studies have examined the correlation between strength performance-related testing and punching force Loturco et al. revealed positive relationships between countermovement jump (CMJ) height and PFmax [4]. Furthermore, propulsion power in bench press and bench throw was found to be significantly related to PFmax [4]. Dunn et al. indicated no correlation between countermovement bench throw force and punching force [5]. However, boxing is a non-symmetrical sport. Muscle strength imbalances are both joint-specific and punch movement-specific [6]. One of the critical limitations of previous bilateral testing is that symmetrical patterns may lack specificity for asymmetrical sports such as boxing. In other words, both limbs simultaneously apply force to determine jump or bench press performance, while there are distinct differences in muscle activation patterns when performing LA and RA punches [7].

Limited research has directly and accurately measured muscle strength in relation to punch force. It has been reported that isometric midthigh pull force is associated with the PFmax [5]. Maximum isometric force in squat exercises also has a positive relationship with LA and RA PFmax [4]. These studies have contributed to the understanding of a possible factor of punch forces and to optimizing strength training. However, the patterns of motor unit recruitment during isometric contractions differ from those seen during dynamic contractions [8]. More importantly, in whole-body strength testing, it is difficult to distinguish the specific contributions of the knee and shoulder. Dynamic evaluations are likely preferable to isometric tests in the physical evaluation of boxers who participate in high-velocity sports. In this sense, several studies have used isokinetic dynamometry to determine the link between sports performance and the peak torques (PT) of the knee and shoulder. In water polo athletes, the shoulder internal rotation PT at 60 ∘ /s has been identified as an important correlate of throwing performance [9]. Harrison et al. revealed a moderate correlation between knee extension strength and vertical jump height in rugby players [10]. In soccer, the associations between concentric knee flexion and extension PT in the non-dominant leg, PT at 180 ∘ /s and deceleration capacity showed the strongest associations [11].

High-level punching performance necessitates excellent muscle strength in both upper and lower limbs [12]. To accelerate toward a target, the punching arm rapidly extends with angular velocity generated by shoulder rotation strength [13]. Also, increasing ground reaction forces generated by knee extension thus improve the punch force [14]. Understanding the links between muscle strength characteristics and punch performance could lead to more effective strength training programs aimed at improving force. However, the relationship between PT of the knee and shoulder and PFmax has not been investigated. Therefore, the aim of the present study was to analyze correlations between PFmax and PT of the knee and shoulder at different velocities. The hypothesis of this study was that positive relationships exist between PT of the knee and shoulder and PFmax. Additionally, the ability to produce strength through high-velocity movements should be an important factor affecting punch force [4]. We also hypothesized, that at 180 ∘ /s, PFmax has stronger associations with PT than at 60 ∘ /s.

2. Materials and methods

4. Discussion

This is the first study that has analyzed relationships between shoulder and knee peak torque and maximal punching force. The first hypothesis that positive correlations exist between PFmax and PT of the knee and shoulder was found to be true. Our results partially supported the second hypothesis. SIR PT at fast velocity (180 ∘ /s) had stronger correlations with PFmax, whereas KE PT had higher correlations with PFmax measured at slow velocity (60 ∘ /s). These findings might be utilized to develop an effective strength and conditioning program for punching.

We observed that D-KE provided the highest correlation with PFmax LA and RA. ND-KE also shown strong correlations with PFmax LA and RA. Electromyographic research has suggested that, during the initiation and execution stages of punches, the right leg is seen to move the body toward the target and generate a peak vertical force [21]. According to Dyson et al., activation of the rectus femoris extension (KE) and biceps femoris flexion (KF) is critical for maximal force punching, particularly the influence of rectus femoris activity on a powerful rear straight punch. Similarly, the dominant leg provided 71% of the total ground reaction force (GRF) for the straight LA punch [22]. Loturco et al. also found significant correlation in boxers between maximum isometric force in squats and PFmax LA ( r = 0.69) and RA ( r = 0.73) [4]. In conclusion, the analyses of the present study showed that both LA and RA PFmax were significantly correlated with K-E strength, especially DK-E to PFmax, which further demonstrated the significance of the lower limbs in the generation of PFmax that have been previously highlighted [23].

In the present study, we found moderate to strong associations between KF strength and LA and RA PFmax. A possible reason for this is that biceps femoris strength relates to hip extension action during the punch phase. In track and field, the biceps femoris achieved hip extension during the ground contact phase, and athletes with a large Y-axis direction (anterior-posterior) of GRF had higher biceps femoris electromyogram activation [24]. Similarly, boxers require acceleration to approach a target along the Y-axis. Dowson et al. also revealed that KF strength had a large correlation to sprint performance ( r = - 0.666) [25].

The moderate to strong correlations for both LA and RA PFmax were also observed in ND-SIR and D-SIR. When performing straight punches, the arms of boxers must accelerate toward their target by enhancing concentric shoulder muscle contractions [26]. Furthermore, the correlation was smaller for PFmax with SER strength than SIR strength, likely related to the kinematic characteristics of punching. Arm movement during punching can be divided into: the extending phase and the returning phase. SER contributes more to increasing returning phase velocities viewed from the direction of applied forces. There are noticeable differences in velocities between the two phases [22]; however, we did not measure the returning phase performance. This highlights the importance of SIR strength for improving PFmax. The currently available evidence on the relationship between upper-limb strength and punching force is conflicting. A previous study reported a strong correlation between mean propulsive power in bench press and LA ( r = 0.76) and RA ( r = 0.79) PFmax [4]. In contrast, Dunn et al. concluded there was no relationship between peak strength in isometric bench press and PFmax [5]. The discrepancies in research outcomes may be due to methodological variations. Different neuromuscular recruitment patterns are observed during concentric and isometric contractions [8]. In summary, it is necessary to promote greater shoulder strength generation, as elite boxers outscore those from other divisions in peak shoulder strength in both limbs [19].

Another important result from the relationship between variables was the SIR. It had stronger associations with PFmax at higher velocities (180 ∘ /s). The effects on punching were highly dependent on the amount of force exerted at the time of contact. Thus, force production capability plays a key role in punching when the upper limb is moving at a high velocity. Indeed, the punch forces obtained in LA ( r = 0.76) and RA ( r = 0.79) PFmax measurements were closely related to mean propulsive power in bench press [4], while the lack of a relationship between maximum isometric strength in bench press and punch force supports the necessity of developing upper limb strength using high-speed exercise in boxers [5]. This could also explain the higher correlations between PFmax and SIR at 180 ∘ /s. A recent study in baseball athletes showed a stronger association between pitch velocity and SIR at 180 ∘ /s ( r = 0.481) than at 90 ∘ /s ( r = 0.439). These findings implied that stronger shoulder strength, particularly at higher angular velocities, contributed significantly to the capacity to increase punch force. Consequently, strength and conditioning programs are often aimed at increasing angular velocities produced at the shoulder in varied punch-specific posture [27].

Interestingly, a stronger association between the PFmax and KE was found to be at 60 ∘ /s, contrary to that in the shoulder. This might be explained by the kinematic and kinetic characteristics of the punch movement. When boxers begin punching, their horizontal motion is usually minimal [23], and lower limb segmental extension begins at “zero velocity,” relying entirely on the capacity to generate a large amount of ground reaction forces (GRF) to overcome inertia. The magnitude of GRFs correlates directly with the maximal strength of the lower limb [28]. In this context, a capacity for knee extension to generate more force is likely to be required by boxers. Dunn et al. also indicated that the rate of force generation may be less essential than the generation of maximal values of muscle strength in the lower limb [5]. In taekwondo athletes, researchers found a stronger association between five-jump test performance and knee strength at 60 ∘ /s than at 180 ∘ /s [29]. Fast motions have relatively little muscle strengthening impact, and heavy exercise requires slowing down the movement. In fact, Turner et al. also suggested lower-body maximal strength exercises as a crucial strategy for enhancing punching force [23]. Therefore, increased knee maximal strength production may contribute to improving punch force. A longitudinal study design should be performed in the future to confirm the effects of upper and lower extremity strength through intervention on punching force.

5. Conclusions

In summary, the present study highlighted the close relationship of knee PT, shoulder PT and punch performance. Boxers should focus on improving their knee and shoulder joint strength. Furthermore, these findings revealed that boxers with greater KE torques at slow angular velocities and SIR torques at high angular velocities, could produce higher punching forces. For further research, the change in punching performance in boxers after strength training of knee and shoulder muscles using different velocities should be investigated.

Author contributions

CONCEPTION: Dexin Wang.

PERFORMANCE OF WORK: Zixiang Zhou.

INTERPRETATION OR ANALYSIS OF DATA: Wenjuan Yi, Weijia Cui and Rui Wu.

FOR IMPORTANT INTELLECTUAL CONTENT: Chao Chen and Xin Chen.

SUPERVISION: Dexin Wang.

Ethical considerations

Participants received a detailed explanation of the purpose and methods of the study prior to signing a written informed consent form. This study was conducted in accordance with the Declaration of Helsinki on Work Involving Human Subjects. The study was approved by ethics committee of Shanghai University of Sport (102772021RT102).

Funding

This study was supported by the Shanghai Key Lab of Human Performance (Shanghai University of Sport) (No. 11DZ2261100), and Science and Technology Commission of Shanghai Municipality (No. 22010503800).