Relationship between Joint Stiffness, Limb Stiffness and Whole–Body Center of Mass Mechanical Work across Running Speeds

The lower–extremity system acts like a spring in the running stance phase. Vertical stiffness (Kvert) and leg stiffness (Kleg) reflect the whole–body center of mass (COM) and leg–spring system loading and response in running, while joint stiffness (Kjoint) represents joint–level dynamic loading and response. This study aimed to investigate whether Kjoint is associated with Kvert and Kleg across different running speeds. Twenty healthy subjects were recruited into a treadmill running study (1.8 to 3.8 m/s, with 0.4 m/s intervals). We found that Kjoint accounted for 38.4% of the variance in Kvert (p = 0.046) and 42.4% of the variance in Kleg (p = 0.028) at 1.8 m/s; Kjoint also accounted for 49.8% of the variance in Kvert (p = 0.014) and 79.3% of the variance in Kleg (p < 0.0001) at 2.2 m/s. Kknee had the strongest unique association with Kvert and Kleg at 1.8 and 2.2 m/s. Kjoint was associated with Kleg at a wider range of speeds. These findings built a connection between joint stiffness and limb stiffness within a certain range of running speeds. Kknee may need to be considered as an important factor in future limb stiffness optimization and general running performance enhancement.