Is passive metatarsophalangeal joint stiffness related to leg stiffness, vertical stiffness and running economy during sub-maximal running?

This study examined whether passive metatarsophalangeal joints (MPJ) stiffness was associated with leg stiffness (K_leg) vertical stiffness (K_vert) and running economy (RE) during sub-maximal running. Nine male experienced runners underwent passive MPJ stiffness measurements in standing and sitting positions followed by sub-maximal running on an instrumented treadmill. With the individual foot position properly aligned, the MPJ passive stiffness in both sitting (MPJ_sit) and standing positions (MPJ_stand) were measured with a computerized dynamometer. Data were collected at a running speed of 2.78 m/s, representing a stabilized level of energy expenditure. Pedar pressure insole was used to determine the contact time (t_c) and peak reaction force for the calculation of K_leg and K_vert. A respiratory gas analysis system was used to estimate the RE. Bivariate correlation test was performed to examine the correlation among MPJ stiffness, contact time, K_leg, K_vert, and RE. The results showed that MPJ_sit and MPJ_stand were inversely correlated with RE (p = 0.04, r = −0.68 to −0.69), suggesting that stiffer MPJ improves RE. In addition, MPJ_sit was correlated positively with K_leg (p < 0.01, r = 0.87), K_vert (p = 0.03, r = 0.70) but inversely with t_c (p = 0.02, r = −0.76), while MPJ_stand was correlated positively with the K_vert (p = 0.02, r = 0.77). These findings suggested that strength of toe plantar flexors provides stability and agility in the stance phase for more effective and faster forward movement.