The decrease in electrically evoked force production is delayed by a previous bout of stretch-shortening cycle exercise

AIM

Unaccustomed physical exercise with a large eccentric component is accompanied by muscle damage and impaired contractile function, especially at low stimulation frequencies. A repeated bout of eccentric exercise results in less damage and improved recovery of contractile function. Here we test the hypotheses that (1) a prior stretch-shortening cycle (SSC) exercise protects against impaired muscle function during a subsequent bout of SSC exercise and (2) the protection during exercise is transient and becomes less effective as the exercise progresses.

METHODS

Healthy untrained men (n = 7) performed SSC exercise consisting of 100 maximal drop jumps at 30 s intervals. The same exercise was repeated 4 weeks later. Peak quadriceps muscle force evoked by electrical stimulation at 15 (P15) and 50 (P50) Hz was measured before exercise, after 10, 25, 50 and 100 jumps as well as 1 and 24 h after exercise.

RESULTS

P15 and P50 were higher during the initial phase of the repeated bout compared with the first exercise bout, but there was no difference between the bouts at the end of the exercise periods. P15 and P50 were again larger 24 h after the repeated bout. The P15/P50 ratio during exercise was not different between the two bouts, but it was higher after the repeated bout.

CONCLUSION

A prior bout of SSC exercise temporarily protects against impaired contractile function during a repeated exercise bout. The protection can again be seen after exercise, but the underlying mechanism then seems to be different.

Bimodal recovery of quadriceps muscle force within 24 hours after sprint cycling for 30 seconds

The aim of the study was to investigate the manifestation of potentiation and fatigue as well as the coexistence of these phenomena at different muscle lengths during a 24-hour period after a sprint cycling for 30 s.

MATERIAL AND METHODS

Twelve healthy untrained men (mean age 23.6+/-1.7 years) took part in the experiment. The contractility of quadriceps muscle was studied before (Initial) and 2, 5, 30, 60 min and 24 h after exercise via the electrically evoked contractions at 1, 15, 50 Hz and maximal voluntary contractions at short and long muscle length.

RESULTS

1) In early, fast-recovery phase (within the first 5 min), muscle force evoked by electrical stimulation of 1, 15, 50 Hz was restored at short muscle length, conversely at long length (Initial vs. 5 min: 15 Hz and 50 Hz, both P<0.05), whereas maximal voluntary contraction force was still suppressed at both muscle lengths; 2) in the second phase (from 5 min to 30-60 min), muscle force decreased at low- and high-frequency stimulations and was more expressed at low-frequency stimulation and at short muscle length than that at long length, but the maximum voluntary contraction force recovered to initial; 3) in long-lasting phase (within 24 hours), 15 Hz force was still suppressed at both muscle lengths.

CONCLUSION

A bimodal recovery of contractility of the quadriceps following sprint cycling for 30 s is determined by the concomitant complex interaction of mechanisms enhancing (potentiation) and suppressing (fatigue) contractile potential of the muscle.

Effect of age on metabolic fatigue and on indirect symptoms of skeletal muscle damage after stretch-shortening exercise

AIM

The purpose of this study was to establish the main differences between men (M) and adolescents (A) (males) in metabolic fatigue and damage induced by exercise performed at maximal intensity.

METHODS

Healthy A (age 13.4 0.6 years, n = 12) and healthy adult M (age 25.4 1.7 years, n = 12) participated in this study. To induce muscle damage and metabolic fatigue stretch-shortening exercise (SSE) (5 bouts of 20 jumps with counter-movement to 90 degrees angle in the knee with 10 s between bouts) has been chosen. The following data were measured: the force of the quadriceps muscle, aroused by electrical stimulation at different frequencies, maximal voluntary contraction force, height of jump (JH), muscle soreness, lactate (La) concentration and creatine kinase (CK) activity in the blood. All the parameters mentioned were measured before exercise and 2-5 min, 20 min, 24 h and 48 h after the SSE, except for La concentration changes in the blood measured before exercise, 2 min and 20 min after the SSE.

RESULTS

The main findings in this study are the following: 1) during SSE JH decreased significantly (P < 0.05) more in M than in A, whereas La concentration in the blood after SSE increased more in M than in A; 2) indirect symptoms of muscle damage were more evident in M than in A; 3) there was secondary decrease in electrically induced muscle force at 10-20 Hz from 3 min until 20 min after SSE but only in the muscles of M; 4) low frequency fatigue after SSE was more evident in M than A.

CONCLUSIONS

The results of the present study indicate that the muscles of adolescents are more resistant to both metabolic fatigue and exercise-induced damage than those of men.