Impellizzeri FM 1, Marcora SM 2, Sassi A 1, Mognoni P3, Rampinini E 1

1, Human Performance Lab, S.S.MAPEI, Castellanza, Italy;2, School of Sport, Health, and Exercise Sciences, University of Wales-Bangor, United Kingdom;3,  Institute of Molecular Bioimaging and Physiology, National Research Council, Segrate, Italy

Introduction Fatigue has been suggested as a risk factor for developing lower limbs injuries. In fact, higher injuries rate occurs in the last minutes of sports or recreational activities sessions. Furthermore, muscle fatigue decreases the dynamic stability of the knee and alters the neuromuscular control of the lower limb1, 2. Some evidences also suggest that lower limbs strength imbalance can increase the risk of injuries3. However, these factors have been investigated singularly, but no studies have examined the interaction between fatigue and lower limbs strength asymmetry. The aim of this study was to evaluate the acute effect of fatigue on bilateral lower limbs strength asymmetry measured using the peak of force developed during vertical jumps. Methods Twenty-six athletes (23 competitive alpine skiers and 3 tennis players) were tested during the pre-season period. Bilateral lower limbs strength asymmetry was determined from the peak of force measured during countermovement vertical jumps on a force platform. Jumps were performed using both lower limbs but with one foot on the platform and the other one on a levelled surface. A total of 10 jumps (five measures for each leg) were collected and the average peak of force of the best three jumps were used. Bilateral lower limbs strength asymmetry was expressed as the percent difference between the stronger and the weaker limbs. The sign ‘-‘ indicated a stronger left leg while the sign ‘+’ a stronger right leg. This bilateral lower limbs strength asymmetry index was found to be significantly correlated to the that determined using the peak of force measured during maximal voluntary isometric contraction on a leg press at a knee angle of 100° (r=0.84, p<0.01, n=35). The reliability determined on 60 subjects using the Bland and Altman’s limits of agreement showed a bias of 0.48% and a total error of 6.94% (Impellizzeri et al., manuscript in preparation).  The bilateral lower limbs strength asymmetry was determined before and immediately after an exercise designed to induce muscle fatigue (45 s of continuous countermovement jumps).  The Bland and Altman’s limits of agreement was used to evaluate the individual differences in the bilateral lower limbs strength asymmetry before and after fatigue. Results After the 45 s jumps test, both the right and left lower limbs peak force decreased by 16%, from 1044 ± 258 N to 877 ± 228 N (p<0.05), and from 1026 ± 225 N to 860 ± 190 N (p<0.05), respectively. The difference between changes was significant (p<0.05). Consequently, the mean bilateral lower limbs strength asymmetry before and after fatigue did not resulted significantly different (1.68 ± 8.3% vs 1.65 ± 11.4%, respectively). The peak lactate reached 5 min after the end of the 45 s jumps test was 7.45 ± 1.13 mmol·L-1. The Bland and Altman’s plot of the individual differences in the bilateral lower limbs strength asymmetry before and after fatigue (0.07 ± 11.4%), compared to the limits of agreement of the reliability. Discussion The exercise protocol used (45 s of continuous jumps) was effective in inducing fatigue, but not in determining interaction between fatigue and bilateral lower limbs strength asymmetry.  However, fatigue caused an increase of measurement variability.  In fact, after the fatiguing protocol,  some subjects demonstrated a change in lower limbs strength asymmetry larger than the one expected from the limits of agreement obtained in the reliability study. This suggests that fatigue might have had an effect in this subset of subjects.  However, the exact origin of this increase variability (physiological or chance) requires additional investigation. Consequently, in absence of a clear explanation of this higher variability, we recommend to measure lower limbs strength asymmetry in non-fatiguing condition. References 1.    Wojtys EM, Wylie BB, Huston LJ. The effects of muscle fatigue on neuromuscular function and anterior tibial translation in healthy knees. Am J Sports Med 1996; 24(5): 615-21 2.     Hiemstra LA, Lo IK, Fowler PJ. Effect of fatigue on knee proprioception: implications for dynamic stabilization. J Orthop Sports Phys Ther 2001; 31(10): 598-605 3.  Dvorak J, Junge A. Football injuries and physical symptoms. A review of the literature. Am J Sports Med 2000; 28(5 Suppl): S3-9

Comunicazione al Convegno Isokinetic 2004