# Antagonist Muscle Co-Activation during Kettlebell Single Arm Swing Exercise
## 4 Discussion  
To our knowledge, this study is the first to quantify agonist-antagonist co-activation during single arm kettlebell swing exercise of the shoulder (AD/PD), core (ESL/RA, ESI/EO), and hip muscles (GM/RF). As hypothesized, the different agonist/antagonist muscle pairs showed significantly different co-activations in the ascending compared to the descending phase. In the shoulder joint, the anterior deltoid showed greater activity (\(\sim\)20% MVC) than the posterior deltoid (<10% MVC), especially during the ascending phase of the movement. This was expected, as the muscle is an agonist muscle during the flexion of the shoulder joint in the ascending phase. The activation of the anterior deltoid was specifically high during the first part of the upward movement to accelerate the kettlebell. When the kettlebell was close to its highest position, EMG activity decreased and increased again at the end of the downward movement to decelerate the movement of the kettlebell [12]. A simultaneous increase in the EMG activity of both muscles in the descending phase could  
Figure 5: The average values and standard deviations for the CoI (%) per phase. (**a**) AD/PD co-activation, (**b**) ESL/RA co-activation, (**c**) ESL/EO co-activation, and (**d**) GM/RF co-activation. Asterisk signs above the horizontal line represent significant differences between phases: (***) indicates \(p\) < 0.001 and (**) indicates \(p\) < 0.01.  
be explained by the need for increased stability during the deceleration of the kettlebell, which is reached by increased co-activation [13]. The core muscles (ereactor spinae (longissimus) and rectus abdominis) both showed high activity in the ascending phase of the kettlebell swing. This was expected for the erector spinae (longissimus) (<20% MVC), as it is responsible for producing the force to extend the trunk during the upward movement. The activity of the rectus abdominis (~10% MVC) as an antagonist increases core stability, which supports the transfer of the forces from the hip extensors to the arms and the kettlebell. In addition, it may stabilize and protect the trunk during the explosive rotation movement of the one-armed swing exercise ([PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], and [PERSON] [1]). In particular, the generation of flexion moments results in the activation of the extensor core muscles [14; 15]. Furthermore, the activity of the erector spinae (longissimus) (~18% MVC) in the descending phase is not surprising, since the activation of the extensor muscles is required to balance the flexion moment produced by the mass of the upper body. Generally, the activation of the erector spinae (longissimus) and the rectus abdominis is important in both phases because it mainly works to provide enough stability to prevent the spine from possible harmful twisting under load. The observed pattern of co-activation (which was greater in the ascending phase) might reflect the need to stabilize the trunk to transfer the forces from the hip extensors to the arms and kettlebell. This is in agreement with [PERSON] and [PERSON] [16] and [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], and [PERSON] [1], who stated that the muscle co-activation is often substantial during trunk exertions. The erector spinae (liicostasisis) is involved with almost the same activation in the ascending and descending phases (~20% MVC). The external oblique (which acts as antagonist eccentrically in the ascending phase) decelerates the spine extension and showed significant activation (<15% MVC) in order to support the stability of the trunk and to avoid a rotation during the ascending phase. The well-coordinated contraction of the abdominal muscles provides protection for the spine. Because of the similar function, this pair of muscles acts similarly to the erector spinae (longissimus) and the rectus abdominis, with higher CoI during the explosive ascending phase.