Walking downhill causes damage to human muscle

Walking downhill on a motor driven treadmill requires the leg extensor muscles to act as brakes by performing eccentric contractions, that is the muscle are activated but actually stretched so that they slow down and control the rate of descent of the body centre on mass. Eccentric contractions have been used previously to generate experimental muscle damage as evidenced by the appearance of creatine kinase in the plasma and visual disruption of the muscle fibres within biopsies. In this research Professor Sargeant reports on the progessive increase in energy cost of exercise walking downhill to the point of collapse and the consequent long term loss of normal muscle function.
European Journal of Applied Physiology
Eur J Appl Physiol Occup Physiol. 1987;56(6):704-11

4 subjects performed repeated eccentric contractions with leg extensors during prolonged downhill walking (-25% gradient) at 6.44 km.h-1 until collapse due to muscle weakness (range of exercise duration 29 to 40 min).

During the exercise oxygen uptake rose progressively from approximately 45% of the previously determined VO2max at 10 min to approximately 65% at the end of the exercise. Following the exercise there was an immediate, significant, and sustained reduction in maximal voluntary isometric contraction, and short term (anaerobic) power output measured concentrically on an isokinetic ergometer. These reductions in muscle function persisted for 96 hours post exercise, and were reflected by significant reductions in the tension generated at low frequency (20 Hz) relative to higher frequency (50 Hz) percutaneous stimulation of the quadriceps. All four subjects showed an increase in plasma levels of creatine kinase post eccentric exercise. Performing concentric contractions by walking uphill for one hour at a significantly greater metabolic cost failed to induce comparable reductions in muscle function. These results provide evidence for the consequences of prolonged eccentric work upon dynamic function which complements earlier reports of structural, enzymatic, and static function changes


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