Studies have reported various results with respect the level of muscle activity present during the performance of exercises in stable and unstable conditions. Some of the contradictions may be due to the use of relative or absolute loading during the comparisons of the two conditions. The purpose of this investigation was to determine the effect of stable and unstable conditions on one repetition maximum strength and muscle activity during dynamic squatting using both relative and absolute loading. Ten recreationally weight trained males participated in this study (age = 24.1 ± 2.0 yrs, height = 178.0 ± 5.6 cm, body mass = 83.7 ± 13.4 kg, 1RM/body mass = 1.53 ± 0.31), which involved two laboratory sessions separated by one week. Linear position transducers were used to track bar displacement while subjects stood on a force plate for all trials. Vastus lateralis (VL), biceps femoris (BF) and erector spinae (L1) muscle activity (average integrated EMG (IEMG)) was also recorded during all trials. During the first session subjects complete a one repetition maximum test in a stable dynamic squat (S1 RM = 128.0 ± 31.4 kg) and an unstable dynamic squat (U1RM = 83.8 ± 17.3 kg) in a randomized order with a thirty minute rest period between conditions. The second session consisted of the performance of three trials each for twelve different conditions in a randomized order: 1) unstable squats using 70% of US1RM, 2) unstable squats using 80% of US1 RM, 3) unstable squats using 90% of US1RM, 4) stable squats using 70% of S1RM, 5) stable squat using 80% of S1RM 6) stable squats using 90% of S1RM, 7) unstable squats using 58.6 ± 12.1 kg, 8) unstable squats using 67.0 ± 13.9 kg, 9) unstable squats using 75.4 ± 15.6 kg, 10) stable squats using 58.6 ± 12.1 kg, 11) stable squats using 67.0 ± 13.9 kg, and 12) stable squats using 75.4 ± 15.6 kg. Results revealed a statistically significant difference between S1RM and US1RM values (p ≤ 0.05). The stable trials resulted in the same or a significantly higher value for VL, BF and L1 muscle activity in comparison to the unstable trials for all twelve conditions. It appears that an unstable condition during dynamic squatting results in the same or significantly lower values of muscle activity in comparison to stable squatting. Unstable exercises are not recommended for utilization by practitioners as they do not increase muscle activation in comparison to stable exercise modalities and may limit possible physiological adaptations.
