1. Growth hormone-deficient hypopituitary adults often complain of weakness and fatigue. The cause of the fatigue is unknown but could be an increased proportion of fast, fatiguable, type 2 fibres in the muscle. The aim of this study was to examine the contractile properties of the quadriceps muscle in a group of these patients compared with healthy controls. Changes in these properties were also examined in a small subset of the patients following growth hormone replacement.
2. Isometric strength, half-relaxation time from a twitch (t1/2) and the force—frequency relationship were measured using electrically evoked contractions in 14 growth hormone-deficient patients and 14 age- and sex-matched controls. Six patients were restudied following 6–24 month's replacement therapy with growth hormone (daily dose 0.04 ± 0.01 i.u./kg).
3. The growth hormone-deficient patients had a significantly lower t1/2 than the controls (46.1 ± 6.1 ms versus 56.1 ± 10.5 ms respectively; P = 0.0072; mean ± SD). The 10/100% ratio was also significantly lower in growth-hormone-deficient patients (38.6 ± 9.9% versus 52.3 ± 8.0%; P = 0.0005), as was muscle strength (349 ± 99 N versus 493 ± 215 N; P = 0.036). Following growth hormone replacement, muscle strength increased significantly (P < 0.05). The 10/100% ratio also increased towards control values, but this change was not significant.
4. These results demonstrate that the relaxation times of the quadriceps are significantly shorter and that the force—frequency relationship shifted to the right in growth hormone-deficient patients, which is consistent with a greater proportion of type 2 fibres within the muscle. Preliminary data on a small group suggest that replacement with growth hormone results in a change in these properties towards that of controls.