Interaction effects of the startle response and hormonal changes on knee stiffness
University of Delaware
Context: Growing evidence suggests the nervous system has a significant role in the high rate of non-contact anterior cruciate ligament injuries. Females are the most susceptible, but evidence for a hormonal role is conflicting. The startle reflex is universal across animals and stiffens limb muscles after sudden, unanticipated events. No studies have investigated how hormonal changes during the menstrual cycle may interact within the nervous system to alter the startle response and knee stiffness regulation among males and females. Objective: To assess if reactive knee joint stiffening strategies are affected differently in males and females during an acoustic startle. Design: Post-test only with repeated measures. Setting: University laboratory Patients or Other Participants: 18 males (21.3±2.0 yrs, 82.8±15.8 kg, 179.5±6.9 cm) and 18 females (20.9±2.0 yrs, 61.8±7.6 kg, 164.4±6.8 cm) with no previous knee injury participated in this study. All females were tested at 2 points in the menstrual cycle to form 3 groups: Males (M), Female-Follicular (FF), and Female-Ovulation (FO). Interventions: Subjects were seated on a custom stiffness device that generated a rapid 40° flexion perturbation to the knee (30° to 70° flexion arc). Subjects remained relaxed prior to the perturbation, and were instructed to react with maximal extension force as quickly as possible when the perturbation was sensed. A total of 6 trials were performed, with an acoustic startle (50ms, 1000Hz, 100dB) applied 100ms prior to the perturbation on 3 randomly selected trials. Main Outcome Measures: Normalized knee stiffness (Δtorque/Δposition, Nm/°/kg) was calculated at the short-range (0- 4°) and for the total perturbation (0-40°). The average of the control (CON) and startle trial (ST) were used for analysis. Repeated-measures analyses of variance comparing gender (M, FF, FO) and startle condition (CON, ST) were used for analysis at both short-range and total range. Results: The startle significantly increased short-range stiffness (F=4.24, p=0.04), and decreased total stiffness (F=11.25, p<0.001). Pairwise comparisons revealed that at short-range, ST displayed a significantly greater stiffness of 0.047±0.002Nm/°/kg, (p=0.003) compared to the CON (0.043±0.003Nm/°/kg). However, for total stiffness, the startle response caused significantly less total stiffness (0.028±0.002Nm/°/kg) than the CON (0.037±0.002Nm/°/kg, p=0.004) condition. No significant gender or menstrual cycle phase differences were observed for stiffness values. Conclusions: This is the first study to indicate that the startle response can significantly disrupt the normal knee stiffness regulation strategies that are required to maintain joint stability, and this effect may occur equally in males and females. Gender and menstrual cycle phase do not appear to affect knee stiffness or negatively interact with the startle response. Further studies should explore the potential role of startle responses in accidents and unintentional non-contact injuries.