The relationships between pupil size and peripheral muscle fatigue with cadence decline during all-out cycling sprints at different intensities: a preliminary analysis

Abstract

Introduction & Purpose: “Motor slowing” is described as a decline in movement speed when repetitive tasks are executed over an extended period of time and has been suggested to be influenced more by supraspinal opposed to neuromuscular mechanisms. A slowing of such tasks, such as repeated finger tapping, has been linked to an in parallel constriction of the pupil, indicating a relationship between locus coeruleus activity (segment of the brain that influences arousal) and motor slowing. Whether this relationship exists during more intense gross-motor repetitive tasks, such as cycling sprints, remains unknown. Indeed, it has been speculated that a decline in maximal cycling sprint performance is more influenceed by central rather than peripheral factors. Therefore, the present study aimed to explore the relationship between changes in cycling cadence (motor slowing) with pupil size, as well as measures of peripheral muscle fatigue.

Methods: Eight healthy participants (27±5 yrs, 5F) completed three 60-s cycling sprints on separate visits using an anaerobic test ergometer (Monark 894E) with one performed at a standard Wingate test break weight of 7.5% body weight, and the others at 2.0-3.5% and 9.5% representing a low, and high intensity, respectively (order of all sprints randomized). Within the first 10 s, participants were requested to reach their maximal cycling cadence (ramp-up phase). Then, the break weight was applied and participants were instructed to pedal at the fastest possible cadence for the entirety of the 60-s sprint. Pupil size was recorded during the entire 70 s with eye-tracking glasses (Pupil Core). Lighting was carefully controlled, and experimenters remained silent to avoid any external influence on participants. Peripheral quadricep fatigue was quantified by evaluating the drop in quadriceps muscle force after exercise compared to before. Quadriceps force was assessed in response to single magnetic femoral nerve stimulations (Q_tw), as well as to 10Hz (Q₁₀) and 100Hz (Q₁₀₀) 0.5-s trains.

Results: Cycling cadence significantly dropped during all sprints (all p£0.004). Pupils dilated during the 10-s ramp-up with no subsequent change during sprints (p=0.452), resulting in no significant correlation between the rate-of-change of cadence and pupil size (all intensities p³0.405). Quadriceps force significantly declined after exercise at low (Q_tw=-32±13%, Q₁₀=-25±12%, Q₁₀₀=-13±9%), standard (Q_tw=-33±18%, Q₁₀=-27±13%, Q₁₀₀=-15±6%) and high (Q_tw=-31±17%, Q₁₀=-18±9%) exercise intensities (all p£0.033), excluding Q₁₀₀ during the heavy intensity (-11±10%, p=0.115). During the standard exercise intensity, rate-of-change in cadence was significantly related to Q_tw decline (r=0.87, p=0.012) and Q₁₀ decline (r=0.87, p=0.024).

Discussion and Conclusion: This preliminary analysis shows a closer relationship between motor slowing during cycling sprints with peripheral quadricep fatigue than with changes in pupil size, which is contradictory to observations with smaller motor tasks. Likely both peripheral and central factors affect decrements in cadence during all-out exercise bouts. To which extent contribution of these aspects are related to the individuals’ training status and specific sporting background, needs further investigation.