MONITORING HAND FLEXOR FATIGUE IN A 24-HOUR MOTORCYCLE ENDURANCE RACE Michel Marina, Jordi Porta, Lisímaco Vallejo, Rosa Angulo INEFC Barcelona, Spain

Introduction I Fatigue can be induced by: Excessive mean muscle tension Frequent episodes of high tension Poor physical condition Fatigue can increase the risk of accidents: It makes the execution of non- or less coordinated movements more likely (Johnston et al., 1998; Parijat and Lockhart, 2008). Reduction in hand steadiness while completing tasks (Leyk et al., 2006

Introduction II Motorcycle more habitual characteristics: 1000cc Weight of up to 175 kg Engine power greater than 190 cv Gas handle and front brakes operated by the right hand. The left hand only operates the clutch. Some of the tasks of the righ hand during the race One lap at the Montmelo Circuit: 6 heavy brakes before entering a curve 15 gear changes Constant use of the gas handle. 4 pilots / team Third classified = 720 laps 4,320 brakes 10,800 gear changes

Method I 10 adult riders, aged 32.5 ± 5.5 years, Corporal Mass Index of 23.9 ± 1.7

Method II Grip strength assessment Harpenden dynamometer (British Indicators Ltd, England; precision ±1 kg ) Electromyography ME6000 electromyography system (Mega Electronics, Kuopio, Finland) Sampling frequency of 1000 Hz To compute EMG amplitude: RMS at 0.05 s interval. 20 RMS values/sec To compute EMG frequency: FFT (frame width at 1024), shift 30%, “flat-topped” windowing function For the 10 s  a total of 7 power spectrum densities All data normalised with respect to basal values

Method III: Statistics To study changes in the EMG and the force parameters throughout the 24-h race: Repeated measures ANOVAs with relay sequence (1  6) as the factor If the sphericity test to study matrix proportionality of the dependent variable was significant (p<0.001), we used the Greenhouse-Geisser’s correction If a significant effect was found, planned comparisons were conducted between relays 1-2 and 5-6 (Sidak adjustment for multiple comparisons) Eta squared (η 2 ) for effect sizes Overall paired T-Test was used to compare the differences between the normalized MF (NMF) and normalized MPF (NMPF). Level of significance = 0.05

Results: Force in MVC Significant relay main effect (F(2.18, 45)=20.36; p≤0.001) Large effect size (η 2 =0.69) Significant linear trend (F(1,9)=54.8; p≤0.001) Significant differences with planned comparisons between relays 1-2 and 5-6 (p < 0.01)

Results: EMG amplitude in MVC ns

Results: 50% MVC No significant effect for: NMF (p=0.08) NMPF (p=0.092) Significant effect for: RMS (F(1.72, 40)=4.34; p=0.039) Linear and positive trend (F(1,8)= 7,74; p=0.024), effect size (η 2 =0.35)

Discussion: MVC Fatigue is the main factor in accounting for the fall in NMVC after the second relay. In agreement with previous studies (Oksa et al., 1999) ↓ MVC combined with RMS constant ? Motor unit force twitch Number of active motor units Muscle fibres interaction De Luca, 1997 CNS is not the source of fatigue, is able to send the proper neural action potential If the source of fatigue is peripheral, the muscle tissue and/or neuromuscular junction may be responsible for ↓MVC De Luca, 1984

Discussion: 50% MVC EMG amplitude increment ? Muscle fatigue (De Luca, 1997; Kuroda et al., 1970; Lindstrom et al., 1977; Lloyd, 1971; Merletti and Lo Conte, 1995; Merletti et al., 1990; Moritani et al., 1986; Stephens and Usherwood, 1975) ↓MVC combined with ↑EMG amplitude ? RECRUITMENT of additional motor units to maintain the same sub-maximal level of force (Maton, 1981) of fibres with higher action potential and greater peak tension, as replacement of fatigued fibres with lower action potential (Moritani, Muro, & Nagata, 1986) Glycogen depletion could induce ↓recruitment threshold force of these fibres (Vollestad et al.,1984) ↑Stiffness, because the muscle analysed would increase its force output to maintain the external force (De Luca, 1984) ↑Synchronisation (Lippold, Redfearn, & Vuco, 1957; Furness, Jessop, & Lippold, 1977)

Discussion: 50% MVC NO conclusive trend about the EMG frequency values during 50% MVC VARIABILITY: Riders from the different teams did not all complete the same sequence of relays (Gap between measurements: from 3 h to 2 h) Typical incidents of a race of this nature (battling for position involving stressful overtaking manoeuvres, falls, etc.) Time elapsed from the end of the rider’s relay to the beginning of the assessment was too long (5 min as max), allowing an excessive recovery of the EMG frequency (De Luca, 1984)

Discussion: 50% MVC NO conclusive trend about the EMG frequency values during 50% MVC Methodological questions  what kind of protocol? Intermittent versus continuous protocol Duration Continuous  from 1’ to 5’ (Blackwell et al., 1999; Clancy et al., 2008; Kuroda et al., 1970; Lindstrom et al., 1977; Lloyd, 1971; Mamaghani et al., 2002; Merletti and Lo Conte, 1995; Merletti et al., 1990; Stephens and Usherwood, 1975) Intermittent  25’ (Eksioglu, 2006) Intensity of the contraction 15% - 30% of MVC (Eksioglu, 2006) 40-50% of MVC (Clancy, Bertolina, Merletti, & Farina, 2008) Intermittent NO: Eksioglu et al. (2006) Clancy et al. (2008) Intermittent YES: (Oksa et al., 1999)

Conclusion 1.All riders answered negatively when asked if the handgrip protocol had increased fatigue to a point at which their riding was prejudiced. Thus, we believe that this protocol is valid for assessing local muscle fatigue during a motorcycle endurance race BUT: 2.More motorcycle racers and during more races 3.Field and well controlled laboratory experiments should be carried out to gain a deeper understanding of true fatigue in long duration motorcycle riders

Thank you very much for your attention