Ambient temperature fluctuations impact human physiology in general, but their direct effects on muscle activity still require further investigation. This research investigates the impact of indoor and outdoor air temperature variations on the upper trapezius muscle activity using a wearable Holter unit. Twenty-three healthy participants were exposed to six different temperature levels ranging from 31 to 40°C, and their muscle activity was recorded using surface electromyography (sEMG) signals. Statistical analysis of the acquired sEMG data, after preprocessing, included the calculation of integrated electromyography (iEMG) and Teager-Kaiser Energy Operator (TKEO) metrics. The results indicate that at 37°C and 40°C, iEMG values showed a decrease of 5.9% and 6.5% respectively, suggesting impaired muscle activation. Analysis of variance (ANOVA) revealed significant differences in both iEMG and TKEO values at 37°C (p = 0.0093 and p = 0.0130, respectively) and at 40°C (p = 0.0056 and p = 0.0229, respectively). These findings provide new insights into the detrimental effects of elevated temperatures on crucial musculoskeletal functions. Exposure to temperatures above 37°C specifically weakens trapezius activation.