Maximal voluntary torque during lengthening contractions may be limited by a reduced neural drive, either by lower activation of all recruited fibres and/or by selective recruitment of type II fibres and concomitant de-recruitment of type I fibres.

Following ethic committee approval, ten subjects (6 males) signed informed consent forms. Voluntary activation levels during maximal lengthening, isometric and shortening contractions was investigated using superimposed electrical stimulation of the femoral nerve (triplet, 300 Hz). Recruitment of type I, IIA and IIAX fibre populations was assessed during these three modes of contraction (n = 5, 4 males) by analysis of needle muscle biopsies obtained at rest and immediately after exercise (10 contractions, 1 s on/1 s off). Single fibre fragments were isolated from freeze-dried samples and characterized using mATPase stainings. Subsequently, remaining fragments of these fibres were measured for phosphocreatine (PCr) and creatine (Cr) content. A reduction in the ratio of PCr to Cr (PCr/Cr) was used as a measure of fibre activation (Beltman et al. 2003).

Voluntary activation (mean ± S.D) was significantly lower during lengthening (79 ± 8 %) than during isometric (93 ± 5 %) and shortening contractions (92 ± 3 %)(P < 0.05; ANOVA for repeated measures). Maximal voluntary torque during lengthening (270 ± 55 Nm) was not significantly different from isometric contractions (252 ± 47 Nm, P > 0.05) but significantly higher compared with shortening contractions (199 ± 47 Nm, P < 0.05).At rest, PCr/Cr ratios (mean ± S.D) were 2.5 ± 0.6, 2.0 ± 0.7 and 2.0 ± 0.7, respectively for type I, IIA and IIAX fibres. After 10 lengthening, isometric or shortening contractions, the mean PCr/Cr ratios for grouped fibre populations were significantly different from rest and between contraction modes; 1.3 ± 0.2, 0.7 ± 0.3, 0.8 ± 0.6, respectively (P < 0.05, Kruskall Wallis followed by Mann-Whitney U-test). The calculated cumulative frequency distributions for the PCr/Cr ratio of each fibre type were significantly different from rest (P < 0.05, Kolmogorov-Smirnov two-sample test), suggesting activation of all fibre types. Compared to rest the shift in distributions was smallest for lengthening contractions (P < 0.05).

The present results show that there was a reduced voluntary drive during lengthening contractions. Changes in PCr/Cr ratios of single fibres suggest that this reduced drive is not caused by a selective recruitment of type II fibres but that all fibre populations were recruited at a lower rate.