EFFECT OF SHOE CUSHIONING ON THE DEVELOPMENT OF RETICULOCYTOSIS IN DISTANCE RUNNERS

We studied erythropoietic activity in relation to the rearfoot cushioning of shoes worn by 14 male runners before, during, and the morning after a 17-day period of increased training mileage. The percentage of reticulocytes in the red blood cell count (normal, < 0.8%) served as the marker for erythropoietic activity. Each runner was assigned to either a firm-sole group (7) or a soft-sole group (7) according to the heel impact attenuation character (Peak g) of his shoes. Peak g was 18% greater in the firm-sole group (P < 0.001). Otherwise, the groups were similar in physical characteristics, training mileage, and running ability. All subjects ran a total of 430 km, a distance that averaged 79% higher than their regular training distance for a 17-day period. Resting blood samples were obtained at baseline and on three mornings (Days 11, 13, and Day 18, which followed the completion of the increased training period). No significant differences were found between the groups in red blood cell count, hematocrit, or total hemoglobin, haptoglobin, plasma-free hemoglobin, and serum ferritin levels. The groups did not differ in percent reticulocytes at baseline (0.2% firm-sole versus 0.2% soft-sole), on Day 11 after running 280 km (0.8% firm-sole versus 0.8% soft-sole), or on Day 13 after 48 hours of rest (1.3% firm-sole versus 1.0% soft-sole). However, on Day 18 after running 430 km, reticulocyte counts were 29% higher (P < 0.05) in firm-sole than soft-sole (2.2% versus 1.7%, respectively). The overall change in percent reticulocytes was linearly correlated with Peak g (r = 0.57, P < 0.05). Although marked reticulocytosis occurred in both groups, it was significantly more pronounced in the firm-sole group. The positive correlation between percent reticulocytes and Peak g suggests that shoe rear-foot cushioning is one determinant of the erythropoietic response to increased training mileage in distance runners.