The implications of time on the ground on running economy: less is not always better

Abstract

A lower duty factor (DF) reflects a greater relative contribution of leg swing versus ground contact time during the running step. Increasing time on the ground has been reported in the scientific literature to both increase and decrease the energy cost (EC) of running, with DF reported to be highly variable in runners. As increasing running speed aligns running kinematics more closely with spring-mass model behaviours and re-use of elastic energy, we compared the centre of mass (COM) displacement and EC between runners with a low (DF low ) and high (DF high ) duty factor at typical endurance running speeds. Forty well-trained runners were divided in two groups based on their mean DF measured across a range of speeds. EC was measured from 4 min treadmill runs at 10, 12 and 14 km h −1 using indirect calorimetry. Temporal characteristics and COM displacement data of the running step were recorded from 30 s treadmill runs at 10, 12, 14, 16 and 18 km h −1 . Across speeds, DF low exhibited more symmetrical patterns between braking and propulsion phases in terms of time and vertical COM displacement than DF high . DF high limited global vertical COM displacements in favour of horizontal progression during ground contact. Despite these running kinematics differences, no significant difference in EC was observed between groups. Therefore, both DF strategies seem energetically efficient at endurance running speeds.

Humans Optimize Ground Contact Time and Leg Stiffness to Minimize the Metabolic Cost of Running

“…This means they are favoring the production of rapid and high magnitudes of vertical force, generated by a stiff lower limb. Combining this understanding with previous work that identified that the majority of trained runners favored overstriding (longer stride times than optimal) (Cavanagh and Williams, 1982;de Ruiter et al, 2013), suggests trained runners favor a low duty factor indicating they rely on the storage and release of elastic energy to minimize metabolic cost (Lussiana et al, 2019). To achieve this, muscles would also be required to operate at faster shortening velocities, requiring more motor units to be recruited to produce the necessary high forces (Fletcher and MacIntosh, 2017).…”

“…A few (n = 4) runners adopted a different mechanical strategy, whereby they had longer ground contact times and a more compliant leg (less stiff) than their mathematical optimal. This would induce a higher duty factor than optimal, indicating the runners were prioritizing horizontal displacement and reducing vertical displacement (Lussiana et al, 2019). In contrast to the low duty factor strategy, muscles would operate at slower shortening velocities, needing fewer motor units to be recruited to produce lower force (Fletcher and MacIntosh, 2017).…”

“…Whilst studies have supported associations to this effect (Nummela et al, 2007;Santos-Concejero et al, 2014, conclusive causative evidence has not been forthcoming. Recently, Lussiana et al (2019) observed that short and long ground contact times may be economically beneficial depending on the type of runner you are. Runners who spend a relatively large proportion of the gait cycle in contact with the ground (high duty factor) had similar metabolic costs as those who spend a relatively small proportion of the gait cycle in contact with the ground (low duty factor) (Lussiana et al, 2019).…”

“…Recently, Lussiana et al (2019) observed that short and long ground contact times may be economically beneficial depending on the type of runner you are. Runners who spend a relatively large proportion of the gait cycle in contact with the ground (high duty factor) had similar metabolic costs as those who spend a relatively small proportion of the gait cycle in contact with the ground (low duty factor) (Lussiana et al, 2019). These findings add further support to the theory of self-optimization, as runners appeared to have subconsciously adapted both mechanically and physiologically.…”

Recognition of Foot Strike Pattern in Asian Recreational Runners

“…The frame rate was set to 50 Hz, a value which is smaller than the ones used in previous studies [1,18,19]. As the duration of the ground contact phase is reported to vary between 0.29 and 0.21 s for running speeds ranging from 10 to 14 km/h [30], we believe that a 50 Hz frame rate (0.02 s per frame) is sufficient to detect the part of the foot that first makes contact with the ground at marathon running speeds. Furthermore, another factor that was not considered in this study is footwear.…”