Humans prefer to walk at speeds that minimise the metabolic cost of transport (COT)
We spontaneously switch from a walk to a run as speed increases. This switch is referred to as the 'preferred transition speed', and typically occurs around 2.0 m/s (7.2 km/h).
What is the rate limiting factor that causes this switch? What determines the reason for the switch?
As walking speed increases, the contractile conditions of the plantar flexors become compromised. The muscle group starts to work at an increasingly lengthened position to produce force for propulsion.
This compromised position actually results in an increase in the activation levels of the plantar flexors - a muscle working in a lengthened position on the length tension curve will have to recruit more active muscle fibres to produce the same force as a muscle working in mid-range. So we therefore see this increase in activity of the plantar flexors. However, with this increase in activity, we actually see a decrease in the ground reactive force during propulsion as the speed nears the preferred transition speed.
This suggests that the ability of these muscles to produce force diminishes as walking speed approaches the preferred transition speed. So the plantar flexors being unable to produce adequate force despite high levels of activation is the rate limiting factor that causes the body to shift state to running.
Both the force - length and force - velocity relationships directly influence a muscle’s ability to generate force, with force diminishing with increasing speeds of contraction and as the fibre length deviates from its optimal length.
The transition from walking to running improves the contractile conditions of the plantar flexors, allowing them to produce greater muscle force for a given activation level. Soleus produced 90% more peak force and gastrocnemius 86% more peak force when the transition to run occurred, even though their activation levels were relatively similar to the fast walking.
How does a race walker deal with this problem to maintain such pace whilst maintaining walking? A race walker uses specific strategies, which involve excessive pelvis rotation, knee hyperextension and pelvic shift to increase the available step length. By rotating the pelvis backwards, it's likely that this demands less lengthening of the plantar flexors, allowing them to stay in a relatively favourable length-tension position to still produce high forces.
The results of this study suggest that intrinsic muscle properties play an important role in the determination of the specific neuromotor strategies used by the nervous system in human locomotion.
Neptune, R.R., Sasaki, K. (2005). Ankle plantar flexor force production is an important determinant of the preferred walk-to-run transition speed. The Journal of Experimental Biology 208, 799-808
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