Sensor-Based Assessment of Trunk–Lower Limb Neuromuscular Coordination During Flat Tennis Serve Using Surface Electromyography and Time–Frequency Intermuscular Coherence Analysis

Objective: To compare flat-serve performance, muscle activation, and trunk–lower limb time–frequency intermuscular coherence between professional and amateur tennis players. Methods: Twenty-four tennis players (17 male and 7 female), including 12 professional and 12 amateur players, participated in this cross-sectional group-comparison study. Each participant completed 10 valid flat serves under standardized testing conditions, and serve-placement accuracy was assessed using a predefined target area. Surface electromyographic signals from 10 dominant-side trunk and lower-limb muscles were recorded using a Delsys Trigno Wireless EMG System, while serve movements were captured using a Photron FASTCAM Mini AX200 high-speed camera. Electromyographic and video data were synchronized using a transistor–transistor logic trigger. Integrated electromyography (IEMG) was normalized to maximal voluntary contraction, and time–frequency intermuscular coherence was calculated for 21 trunk–lower limb muscle pairs across the α, β, and γ frequency bands. Between-group differences were examined using two-tailed independent-samples t-tests. The Benjamini–Hochberg false discovery rate (BH–FDR) procedure was applied to outcome families involving multiple comparisons, with statistical significance determined at FDR q < 0.05. Results: Serve-placement accuracy was significantly higher in the professional group than in the amateur group (69.2 ± 7.4% vs. 48.3 ± 8.9%; t = 6.26, raw p < 0.001, Cohen’s d = 2.55). After BH–FDR correction, professional players showed significantly higher IEMG values in the soleus (t = 3.12, raw p = 0.005, FDR q = 0.019, d = 1.27), vastus medialis (t = 2.92, raw p = 0.008, FDR q = 0.020, d = 1.19), vastus lateralis (t = 2.60, raw p = 0.016, FDR q = 0.027, d = 1.06), gluteus maximus (t = 3.17, raw p = 0.004, FDR q = 0.019, d = 1.29), and latissimus dorsi (t = 2.74, raw p = 0.012, FDR q = 0.024, d = 1.12), but significantly lower IEMG in the rectus abdominis (t = −3.07, raw p = 0.006, FDR q = 0.019, d = −1.25). Several muscle pair–frequency combinations showed nominal differences based on raw p-values and moderate-to-large effect sizes; however, none of the time–frequency intermuscular coherence comparisons remained statistically significant after BH–FDR correction (all FDR q > 0.05). Conclusion: Professional players demonstrated greater flat-serve placement accuracy and a distinct distribution of trunk and lower-limb muscle activation. The coherence findings were exploratory and did not conclusively establish skill-level differences in trunk–lower limb intermuscular coupling. Flat-serve training may emphasize lower-limb support and propulsion, hip extension, and trunk stabilization rather than focusing exclusively on arm or racket actions.