Comparison of Muscular Co-Activation Indexes in Karate Gyaku Tsuki Punch Under Impact and No-Impact Conditions

Muscle co-activation in karate athletes is considered a complex neuromuscular control mechanism that ensures a balance among maximal punch velocity, joint stability, and movement accuracy. The strategy of muscle co-activation during striking movements under different conditions largely determines performance outcomes but remains insufficiently studied. Therefore, we aimed to examine muscle co-activation in the punching hand of Kyokushin karate athletes during an execution of a straight Gyaku Tsuki punch. The study involved 11 karate athletes (1st Dan), aged 18–25 years, all right-handed. They performed a straight Gyaku Tsuki as a single air punch and as a single punch on the heavy bag (right hand) from the Zenkutsu-dachi stance. Surface electromyography was recorded for wrist flexors and extensors, as well as the biceps brachii and triceps brachii of the punching hand. The co-activation indexes (CI1 and CI2) were calculated using two distinct formulas adopted from existing literature. In the forearm muscles, CI1 exhibited a peak-like profile with a maximum at 0.150 s; values during the single punch on the heavy bag were 37% higher than during the single air punch. Elevated CI1 persisted between 0.075 and 0.250 s. CI2 indicated predominance of wrist extensor activity up to 0.150 s, followed by equilibrium and oscillatory variations. In the arm muscles, CI1 peaked at 0.175 s (single air punch) and at 0.200–0.225 s (single punch on the heavy bag). The maximum CI during the heavy bag punch was 60–100% higher than during the air punch, with an additional co-activation peak at 0.050–0.075 s. CI1 values for arm muscles were lower than those for forearm muscles, and the period of high co-activation was shorter. At punch onset, CI2 reflected predominance of extensor activity; equilibrium was reached at 0.125 s, followed by oscillatory changes with flexor predominance. The time course of muscle co-activation during Gyaku Tsuki demonstrates higher and more sustained CI values in the forearm muscles compared to the arm muscles. The co-activation pattern is condition-dependent: execution as a single punch on the heavy bag is characterized by greater magnitude and longer duration of muscle co-activation than the single air punch.