Exhausting stretch-shortening cycle (SSC) exercise causes greater impairment in SSC performance than in pure concentric performance.

The purpose of the present study was to investigate the fatigue effect of repeated exhaustive stretch-shortening cycle (SSC) exercise on concentric muscle function. Ten healthy male subjects performed SSC exercise [92 (30) jumps] on a special sledge apparatus. Exhaustion occurred on average within 3 min. A squat jump (SJ) test utilizing a concentric-only action was performed immediately before and after the SSC exercise, and then 10 min, 20 min, 2 days and 4 days later. In addition, a drop jump (DJ) test using an SSC was also performed immediately before and 20 min after the SSC exercise, and 2 days and 4 days later. During jump tests, lower limb joint moment, power, and work contributions were analyzed by using the kinetic and kinematic data. The fatigue exercise was characterized by a relatively high blood lactate concentration [7.2 (0.8) mmol x l(-1)] and a 2-day delayed increase in serum creatine kinase activity [486 (300) U x l(-1)]. SJ performance decreased markedly immediately after the SSC exercise (P<0.05) and then recovered within 10 min. In contrast, DJ performance and knee joint contribution showed a delayed decrease 2 days after the SSC exercise bout. The surface electromyographic (EMG) activity of the lower limb muscles showed no obvious change in the SJ in comparison to the DJ, although in the latter there was a delayed decrease of knee extensor EMG during the pre-activation and braking phases. The results suggest that isolated concentric muscle function is affected mainly by acute metabolic fatigue after SSC exercise. During a follow-up period after the exercise, changes in hip and knee joint contribution in SJ showed a different recovery pattern compared to those in eccentric DJ. It could be suggested that exhaustive SSC exercise would mainly influence the relative power-work balance between the hip and knee joints during the eccentric phase of SSC. Thus different motor control strategies may account for the distinctive fatigue responses observed in SJ and DJ.

Effect of hyperoxia on metabolic responses and recovery in intermittent exercise.

The aim of the present study was to investigate whether the breathing of hyperoxic gas affects hemoglobin oxygen saturation (S(a)O(2)) and blood acidosis during intense intermittent exercise and recovery in sprint runners. The hypothesis was that the breathing of hyperoxic gas prevents S(a)O(2) from decreasing, delays blood acidosis during the exercise and improves the rate of heart rate recovery after the exercise. Nine sprinters ran three sets of 300 m at different velocities on a treadmill in normoxia (NOX) and in two hyperoxic conditions (ERHOX and RHOX; F(I)O(2) 0.40) in a randomized order. In ERHOX the inspired air was hyperoxic during the entire exercise and recovery and in RHOX the hyperoxic air was only inhaled during recovery periods. Blood pH and S(a)O(2) were measured from fingertip blood samples taken after each set of runs. The mean heart rate for the final 15 s of the last run in each set (HR(work)), the mean heart rate for the final 15 s of the first minute of recovery (HR(rec)) and the difference of HR(work) and HR(rec) (HR(dec)) were determined. In NOX, S(a)O(2) decreased from 95.0 +/- 2.0% to 88.7 +/- 2.0% (p < 0.001) but S(a)O(2) did not change in ERHOX (from 95.4 +/- 1.3% to 95.9 +/- 1.8%). A significant correlation was observed between the S(a)O(2) decrease in NOX and the effect of hyperoxia on blood pH in ERHOX (r = 0.63) and on HRdec in both ERHOX (r = 0.74) and RHOX (r = 0.69). We concluded that hemoglobin oxygen de-saturation occurred during intensive intermittent exercise in normoxia but hyperoxic gas during the exercise prevents S(a)O(2) from decreasing. Furthermore, the present results suggested that the beneficial effects of hyperoxia on heart rate recovery and blood acidosis during intensive intermittent exercise were related to hemoglobin de-saturation in normoxia.

Explosive-strength training improves 5-km running time by improving running economy and muscle power.

To investigate the effects of simultaneous explosive-strength and endurance training on physical performance characteristics, 10 experimental (E) and 8 control (C) endurance athletes trained for 9 wk. The total training volume was kept the same in both groups, but 32% of training in E and 3% in C was replaced by explosive-type strength training. A 5-km time trial (5K), running economy (RE), maximal 20-m speed (V20 m), and 5-jump (5J) tests were measured on a track. Maximal anaerobic (MART) and aerobic treadmill running tests were used to determine maximal velocity in the MART (VMART) and maximal oxygen uptake (VO2 max). The 5K time, RE, and VMART improved (P < 0.05) in E, but no changes were observed in C. V20 m and 5J increased in E (P < 0.01) and decreased in C (P < 0.05). VO2 max increased in C (P < 0.05), but no changes were observed in E. In the pooled data, the changes in the 5K velocity during 9 wk of training correlated (P < 0.05) with the changes in RE [O2 uptake (r = -0.54)] and VMART (r = 0.55). In conclusion, the present simultaneous explosive-strength and endurance training improved the 5K time in well-trained endurance athletes without changes in their VO2 max. This improvement was due to improved neuromuscular characteristics that were transferred into improved VMART and running economy.