Variable Foot Gearing In Jumping

The human foot can adjust gearing ratios between ground reaction force and muscle moments that can enhance muscle mechanics during walking and running, but this has never been studied during counter movement jumps (CMJ) which encompass rapid changes of direction and force. PURPOSE: To investigate differences between variable and constant gearing on Achilles muscle tendon velocity (AMTV) during jumping. METHODS: 40 active young adults participated in this IRB approved study (20 M (21.7 ± 2.2 yrs) and 20 F (23.2 ± 4.3 yrs)). 3D motion capture and force plates recorded foot and ankle motion and ground reaction forces (GRF) during CMJ. The gear ratio was determined as the GRF gear divided by the triceps surae gear. The variable gear ratio was calculated continuously throughout the task and a constant gear ratio was taken at the midpoint of each landing. AMTV was defined as the triceps surae gear multiplied by the ankle angular velocity. AMTV curves were statistically compared using statistical parametric mapping (SPM) to assess differences throughout ground contact between variable and constant gearing. RESULTS: Gear ratios in the first 15% were less than or close to 1 and in the last quarter were between 1.5 and 2. The eccentric portion of the movement resulted in negative AMTV values, thus stretching the muscles, and the concentric portion with positive AMTV values, thus shortening the muscles. Compared to constant gearing, variable gearing resulted in higher AMTV during the first 2-3% of contact (avg 21.83% higher, max 22.91% higher), and lower AMTV during the first 7-11% (avg 21.90% lower, max 24.84% lower) and 62% and later of ground contact (avg 37.35% lower, max 166.34% lower) (Fig 1). CONCLUSION: These results show that variable gearing in jumping allows for increased muscle stretch during landing and higher triceps surae force production during propulsion. Variable gearing allows humans to propel themselves more efficiently with precise musculature response.