Nemaline myopathy is a congenital neuromuscular disorder, characterised by severe muscle weakness, predominantly of proximal limb muscles (Wallgren-Pettersson et al. 1999), which seems not directly linked to muscle atrophy. The nature of this muscle weakness is presently unclear but may be related to peripheral as well as central aspects of neuromuscular control. This study seeks to improve the understanding of underlying processes that may be responsible for the muscle weakness found in these patients. The aim of the present study was to investigate whether voluntary activation and/or activation frequency contribute to the weakness experienced in the quadriceps muscle.

The local ethical committee approved this study. Voluntary isometric knee extension torque (MVC) was obtained at 60 deg knee flexion angle in ten patients of one family with nemaline myopathy and eight healthy controls (4 from the same family). A superimposed stimulation technique (supramaximal triplet at 300 Hz applied via surface electrodes) was used to assess the degree of voluntary activation and electrically evoked twitch and tetanic (10, 20, 50, 150 Hz) torque responses were obtained. Further, MVC (with superimposed stimulation) was measured at different angles of knee flexion (30, 40, 50, 60 and 70 deg).

At 60 deg voluntary torque was significantly lower (Student’s unpaired t test; P < 0.05) in patients (mean ± S.E.M.; 125 ± 10 Nm) compared with controls (194 ± 20 Nm). This was not due to an impairment of voluntary activation, since activation was even higher (P < 0.05) in the patient group (95 vs. 85 %). The patients showed greater relative torques at low stimulation frequencies (P < 0.05). For example, with 10 Hz stimulation relative torques were 51 ± 5 and 25 ± 3 % in patients and controls, respectively. Maximal voluntary torque was found, as expected, at ~60 deg knee flexion in the controls but at a significantly lower (P < 0.05) angle in the patient group (~45 deg).

In conclusion, nemaline myopathy seems to lead to changes at the peripheral site of neuromuscular performance, as indicated by the higher relative forces observed for the patients at low frequencies of activation. However, this finding is probably not directly related to the disease but can be attributed to adaptations in muscle morphology in the patients, i.e. the observed shift in the knee angleÐtorque relationship and the reported relatively large proportion of type I fibres (Gommans et al. 2002).

All procedures accord with current local guidelines and the Declaration of Helsinki.