[4coach]

4coach on March 13, 2013 at 12:23 am #119545

Hi all

I have not seen this journal article discussed on this site, so I have placed some selected paragraphs below.
NB this article is freely available to all.
Muscular strategy shift in human running: dependence of running speed on hip and ankle muscle performance. J Exp Biol. 2012 Jun 1;215(Pt 11):1944-56. Dorn TW, Schache AG, Pandy MG.
Runners appear to use two different strategies to increase their speed. Up to ~7ms-1, running speed is increased by exerting larger support forces during ground contact, which has been shown to correlate with increases in stride length.

Larger ground forces can be generated at lower running speeds because the leg muscles have enough time to develop the forces needed to lift and accelerate the body during stance (Weyand et al., 2000). At speeds near 7ms-1, however, ground contact times become very small (Kunz and Kaufmann,1981; Mann and Herman, 1985; Mann, 1981), limiting the ability of the leg muscles to generate the ground forces needed to increase running speed still further (Weyand et al., 2000).

Above ~7ms-1, the primary strategy used to increase running speed shifts from the goal of increasing stride length to that of increasing stride frequency, which is achieved by accelerating the
legs more rapidly through the air. Peak hip-flexor, hip-extensor and knee-flexor moments all increase significantly at speeds above 7ms-1 (Belli et al., 2002; Schache et al., 2011).

The percentage increase in stride length was greater than that in stride frequency as running speed
increased from 3.5 to 7.0ms-1, but the opposite effect was observed at speeds above 7.0ms-1 (Fig.2A, Table2). Ground contact time decreased monotonically as running speed increased (P<0.01;
Fig.2B, Table2). Aerial time and effective vertical ground impulse both reached their maxima at 7.0ms-1 before decreasing at higher speeds (Fig.2B).

Across all running speeds, SOL, GAS and VAS provided roughly 75% of the total vertical support impulse needed to accelerate the body upward, with SOL contributing as much as 50%. For speeds up to 7.0ms-1, increases in the vertical ground reaction force were due almost entirely to the action of SOL. The contribution of VAS to the vertical ground force did not increase as running speed increased.

Peak forces developed by the ankle plantarflexors decreased at the higher running speeds for two
possible reasons. First, the muscles may have been operating at lengths much shorter or longer than the muscles' optimum fibre lengths (Close, 1972; Gordon et al., 1966; Woledge et al., 1985); and second,
the contractile velocities may have been too high to allow the muscles to develop high forces.

The model calculations showed that stride frequency was increased by increasing the forces generated by the hip-spanning muscles, primarily ILPSO, GMAX and HAMS, as these muscles contributed significantly to the larger hip- and knee-joint accelerations observed at higher running speeds.