Is passive metatarsophalangeal joint stiffness related to leg stiffness, vertical stiffness and running economy 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 (MPJsit) and standing positions (MPJstand) 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 (tc) and peak reaction force for the calculation of Klegand Kvert. 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, Kleg, Kvert, and RE. The results showed that MPJsitand MPJstandwere inversely correlated with RE (p = 0.04, r = −0.68 to −0.69), suggesting that stiffer MPJ improves RE. In addition, MPJsitwas correlated positively with Kleg(p < 0.01, r = 0.87), Kvert(p = 0.03, r = 0.70) but inversely with tc(p = 0.02, r = −0.76), while MPJstandwas correlated positively with the Kvert(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.