Subtle instructional changes that direct attentional focus can lead to changes in performance, potentially hindering a fitness assessment. An external attentional focus has been found to improve motor performance, however less is known about instructional effects on performance and the physiological response during an isometric endurance test. To better understand how attentional focus effects endurance fitness tests, two experiments were conducted. Experiment 1 showed that an external focus instruction (68.41 sec, SD = 34.20) led to significantly higher endurance performance compared to an internal instruction (60.22 sec, SD = 34.54). Experiment 2 revealed that heart rate during the internal condition (M = 117.51 bpm, SD = 14.19) was higher compared to the external (M = 114.17 bmp, SD = 13.19) and control (M = 115.40 bpm, SD = 16.85). Additionally, performance was greater during external instructions (M = 107.01 sec, SD = 46.35) compared to internal (M = 95.25 sec, SD = 38.19).