Health-Related Effects of a Short Isometric Exercise Program Integrated into School Physical Education: The Role of Biological Maturation and Baseline Functional Status

Objectives: This study examined whether integrating an isometric exercise program into physical education (PE) lessons influences functional outcomes and cardiovascular risk markers in adolescents beyond the effects of standard PE alone. Methods: Boys aged 14–15 years were randomly assigned to an experimental group (EG, n = 19) or a control group (CG, n = 21). The EG completed a 6-week isometric exercise program integrated into PE lessons, while the CG followed the regular PE curriculum only. The intervention was based on hold isometric muscle actions (HIMA) with progressively increased volume. Anthropometric measures included body height, body mass, and body mass index (BMI). Body composition was assessed using bioelectrical impedance analysis. Functional capacity was evaluated using field-based measures of lower-limb strength and power (isometric mid-thigh pull, standing broad jump, squat jump, and countermovement jump). Systolic and diastolic blood pressure were measured as indicators of cardiovascular health. Results: A mixed model ANOVA showed that no significant group × time interactions were observed for body composition, functional outcomes, or blood pressure (all p > 0.05). Lean body mass increased over time in both groups (p < 0.01). Improvements in isometric mid-thigh pull (p < 0.01) and standing broad jump (p = 0.01) occurred irrespective of group allocation. Blood pressure remained unchanged. Linear regression revealed that biological maturation did not moderate intervention effects; however, more mature participants demonstrated higher absolute strength levels independent of the intervention (p < 0.05). Conclusions: The inclusion of an isometric exercise program within PE lessons did not provide additional benefits for health-related functional outcomes beyond standard PE alone. In its current format, isometric exercise does not appear to add sufficient value to justify its implementation as a stand-alone strategy in school-based PE.