Background: ThermoStim Probe (TSP) has recently joined the market as a superficial heating modality. Although there is limited research into the intramuscular heating capability of superficial heating modalities in general (moist hot pack [MHP], paraffin, warm whirlpool), no previous research has examined intramuscular heating capability of TSP. Objective: Evaluate rate and magnitude of intramuscular heating via TSP compared with hydrocollator MHP, and determine if TSP can increase tissue temperature 3°C–4°C (vigorous heating range). Design: Repeated-measures counterbalanced study. Setting: Multisite trial; 2 college/university research laboratories. Patients or Other Participants: A total of 18 healthy college-aged participants (11 females and 7 males, age: 23.0 [2.1] y, weight: 74.64 [18.64] kg, height: 168.42 [9.66] cm, subcutaneous adipose: 0.71 [0.17] cm) with calf subcutaneous adipose <1.2 cm. Interventions: MHP and TSP were applied to the posterior aspect of the nondominant calf for 20 minutes; participants underwent each treatment in a counterbalanced order. Main Outcome Measurements: Muscle temperature at a depth of 1.5 cm was measured via 21-gauge catheter thermocouple. Temperatures were recorded at baseline and during the 20-minute treatment. Results: There was a significant treatment-by-time interaction during the treatment period (F 5,85 = 14.149, P < .001), as well as significant main effects for treatment (F 1,17 = 7.264, P = .02) and for time (F 5,85 = 34.028, P < .001). Muscle temperature increased an average of 1.7° (0.9°) for the MHP and 0.6° (1.0°) for the TSP. Pairwise comparisons of the interaction (using least significant difference adjustment for multiple comparisons) indicated that MHP heated faster than TSP at minutes 12 (P = .02), 16 (P = .002), and 20 (P = .001). There was no significant correlation between subcutaneous adipose thickness and maximum temperature increase obtained with either MHP (r = −.033, P = .90) or TSP (r = −.080, P = .75). Conclusions: MHP increased intramuscular temperature significantly more than TSP; however, neither modality was capable of producing a 3°C–4°C temperature increase associated with increased tissue extensibility.