weakness and asymmetry is prevalent in women aged 65 and over, with and without a history of falls. Poor lower limb explosive power combined with asymmetry between limbs may be more predictive of future falls than more traditional measurements of strength in older women who live independently.

1. The effect of the different phases of the menstrual cycle on skeletal muscle strength, contractile properties and fatiguability was investigated in ten young, healthy females.Results were compared with a similar group on the combined (non-phasic) oral contraceptive pill (OC). Cycle phases were divided into the early and mid-follicular, midcycle (ovulatory) and mid-and late luteal. Cycle phases were estimated from the first day of the menstrual bleed. 2. Subjects were studied weekly through two complete cycles. Previous studies of muscular performance during the menstrual cycle have concentrated mainly on changes in endurance performance, rather than strength, and have usually compared the early follicular and luteal phases (for review see Lebrun, 1994). Those studies which have looked at explosive power events have mainly measured performance levels, which are influenced by many variables other than muscle strength or power output. One study investigated the changes in handgrip strength and standing long jumps during the menstrual, ovulatory and luteal phases of the cycle (Davies, Elfors & Jamieson, 1991). The only significant difference found was a stronger handgrip strength during the menstrual phase, which the authors attributed to the lower oestrogen and progesterone levels. These results therefore conflict with those of Phillips et al.(1993a).Despite the high level of interest in the effect of the menstrual cycle on athletic performance and fitness, there remains considerable controversy in the literature. The purpose of this study was to investigate the effect of the menstrual cycle phase on simple tests of muscle strength in large muscle groups which are important in many sporting and everyday tasks (quadriceps and handgrip). In order to

The central changes associated with a period of strength training have been investigated in a group of 32 young healthy volunteers. Subjects participated in one of three 12 week training programmes, which required different degrees of skill and coordination. Study 1 consisted of unilateral isometric training of the quadriceps with the contralateral leg acting as a control, the apparatus providing firm back support and a lap strap. In Study 2 training consisted of unilateral concentric leg-extension with back support and hand-grips. In Study 3 subjects performed bilateral leg-extension with no back support. Measurements of maximum voluntary isometric strength were made at 2-3 week intervals and a continual record was kept of the weights lifted in Studies 2 and 3. The largest increase in isometric force was seen for the trained leg in Study 1 (approximately 40%). There was no significant change in strength in the contralateral untrained leg. In Studies 2 and 3 there was a large increase in training weights (about 200%) associated with smaller increase in isometric force (15-20%). It is concluded that a large part of the improvement in the ability to lift weights was due to an increased ability to coordinate other muscle groups involved in the movement such as those used to stabilise the body. The importance of these findings for athletic training and rehabilitation is discussed.

1. Increases in strength and size of the quadriceps muscle have been compared during 12 weeks of either isometric or dynamic strength training. 2. Isometric training of one leg resulted in a significant increase in force (35 +/- 19%, mean +/- S.D., n = 6) with no change in the contralateral untrained control leg. 3. Quadriceps cross-sectional area was measured from mid-thigh X-ray computerized tomography (c.t.) scans before and after training. The increase in area (5 +/- 4.6%, mean +/- S.D., n = 6) was smaller than, and not correlated with, the increase in strength. 4. The possibility that the stimulus for gain in strength is the high force developed in the muscle was examined by comparing two training regimes, one where the muscle shortened (concentric) and the other where the muscle was stretched (eccentric) during the training exercise. Forces generated during eccentric training were 45% higher than during concentric training. 5. Similar changes in strength and muscle cross-sectional area were found after the two forms of exercise. Eccentric exercise increased isometric force by 11 +/- 3.6% (mean +/- S.D., n = 6), and concentric training by 15 +/- 8.0% (mean +/- S.D., n = 6). In both cases there was an approximate 5% increase in cross-sectional area. 6. It is concluded that as a result of strength training the main change in the first 12 weeks is an increase in the force generated per unit cross-sectional area of muscle. The stimulus for this is unknown but comparison of the effects of eccentric and concentric training suggest it is unlikely to be solely mechanical stress or metabolic fluxes in the muscle.

SUMMARY The use of percutaneous stimulation for twitch superimposition in the human quadriceps has been compared with stimulation via the femoral nerve. It was found that the relationship between the extra force generated by the twitch and the level of voluntary contraction was independent of the percentage of the muscle stimulated. The technique was used to study activation of the quadriceps and biceps in a group of young healthy adults, and quadriceps activation in patients with musculo-skeletal disorders. With small muscle groups such as the adductor pollicis it is possible to measure the maximum tetanic force by stimulating the motor nerve. However stimulation of the femoral nerve innervating the quadriceps is only practicable with co-operative subjects as the nerve is relatively inaccessible and the generation of maximum tetanic force is rather an alarming proAddress for reprint requests: OM Rutherford, Department of Medicine, University College, London, The Rayne Institute, University St, London WCIE 6JJ, UK.Received 17 January 1986. Accepted 1 March 1986 cedure. For this reason we have compared percutaneous stimulation of part of the quadricepsS with supramaximal stimulation of the femoral nerve as a way of testing for maximum activation. Subsequently percutaneous stimulation has been used to investigate muscle activation in the quadriceps and biceps in normal subjects and for the quadriceps in a group of patients with muscle or bone disorders. Methods SubjectsQuadriceps study A total of 70 subjects were studied for quadriceps activation during maximal voluntary isometric contractions. These comprised 50 young healthy volunteers (40 male, 10 female; age range 17-45 years) of average athletic prowess, eight competing athletes, and 12 patients with a variety of muscle or joint complaints. Details of the patients are given in the table. Biceps study A total of 18 young healthy volunteers were investigated (10 female, eight male; age range 17-33 years). ProceduresQuadriceps study Voluntary isometric contraction force of the quadriceps was recorded with the subject seated in a testing chair.5 For percutaneous stimulation two large moistened pad electrodes were placed proximally and distally on the anterolateral thigh. Stimulation was with unidirectional square wave pulses of 50 is duration and up to 400 V. The femoral nerve was stimulated at the groin 1288