Results
The arm swing speeds at BI were 16.08 ± 1.35 m/s, with the value exhibiting a normal distribution. The maximum ball speeds were 27.48 ± 0.85 m/s.

Figure 3 presents the angular momentums of the whole body, attack arm, non-attack arm, head–trunk, attack leg, and non-attack leg. The angular momentum of the whole body gradually increases after TO in the counter-clockwise (+) direction, peaking at 30% normalised time, and then gradually decreases, turning clockwise (–) at 70% normalised time. The angular momentum of the attack arm rapidly increases in the counter-clockwise (+) direction from the later arm-cocking phase and peaks near 50% normalised time. The non-attack arm rapidly increases in the counter-clockwise (+) direction from TO and peaks near 20% normalised time. Then, it exhibits a small clockwise (–) angular momentum during approximately 50–70% normalised time. The head–trunk angular momentum gradually increases in the counter-clockwise (+) direction from the latter part of the arm-cocking phase and peaks near MSER. The statistical results show that the peak angular momentum of the attack arm (0.062 ± 0.017 s –1 ; r = 0.551, p = 0.024, 95% CI: 0.096 to 0.816, large positive) and non-attack arm (0.024 ± 0.008 s –1 ; r = 0.608, p = 0.011, 95% CI: [0.18–00.842], large positive) are significantly correlated with the arm swing speed at BI ( Table 1 ). The angular momentum of the attack leg increases after TO in the clockwise (–) direction, and that of the non-attack leg rapidly increases in the latter part of the arm-cocking phase. The angular momentums of both legs show similar patterns during the arm-acceleration phase, and both peak values appear at approximately 50% of the normalised time ( Figure 3 ). The peak angular momentum of the non-attack leg (–0.026 ± 0.012 s –1 ; r = –0.516, p = 0.034, 95% CI: [–0.799 to –0.048], larger negative) is significantly correlated with the hand speed at BI ( Table 1 ).

Notes:

** p < 0.01.

Bold: indicates the variable is significantly correlated with arm swing speed.

Figure 4 presents the segmental angular momentums of the upper arm, forearm, and hand of the attack arm. The segmental angular momentums of the upper arm and forearm increase in the counter-clockwise (+) direction from the latter part of the arm-cocking phase, and the peak values of the upper arm appear near MSER and of the forearm at the latter-arm-acceleration phase. The hand angular momentum increases in the counter-clockwise (+) direction from MSER, and the peak value appears near BI. The statistical results show that the peak segmental angular momentums of the forearm (0.026 ± 0.007 s –1 ; r = 0.527, p = 0.032, 95% CI: [0.062–0.804], large positive) and hand (0.022 ± 0.006 s –1 ; r = 0.824, p < 0.001, 95% CI: [0.568–0.934], very large positive) are significantly correlated with the arm swing speed at BI ( Table 1 ).

Among the nine independent variables, five correlated with arm swing speed at BI ( Table 1 ). For arm swing speed, the regression model encompassed peak non-attack leg angular momentum, peak forearm angular momentum, and peak hand angular momentum, achieving an adjusted R 2 of 0.351 ( p = 0.043). The equation for arm swing speed is represented as:

Arm swing speed = (14.10 − 3.56 × 10 × peak non-attack leg angular momentum −1.92 × 10 2 × peak forearm angular momentum + 3 × 10 2 × peak hand angular momentum).