The effect of ageing in spinal cord injured humans on the blood pressure and heart rate responses during fatiguing isometric exercise

Groups of 50 healthy male controls and 50 subjects suffering from paraplegia (aged 20-65 years) were examined as to the inter-relationships between age, paraplegia and the strength, endurance, blood pressure and heart rate responses to fatiguing isometric exercise. Contractions were maintained in both groups under voluntary effort and through a contraction induced by electrical stimulation in the paraplegic group. All contractions were maintained to fatigue at a tension of 40% of the maximal muscle strength in either the handgrip or quadriceps muscles. Muscle strength of the handgrip was higher in the paraplegic subjects than in the controls, averaging 589 N and 463 N, respectively for the two groups. In contrast, quadriceps leg extension strength averaged 696 N in the controls and 190 N in the paraplegic groups; for both groups, ageing was associated with a reduction in muscle strength. While leg endurance was less in the paraplegic group than the control group, handgrip endurance was similar in the two groups, endurance increasing with ageing in both the controls and paraplegics. Both systolic and diastolic blood pressures increased at rest in paraplegic and control subjects with age. The magnitude of the pressor response to exercise also increased with age. This was true during both voluntary exercise and exercise induced through electrical stimulation in the paraplegic groups. The heart rate response (change in heart rate during exercise) to a fatiguing isometric handgrip contraction decreased by about 50% between the ages of 20 and 60 years in both the controls and paraplegics for isometric handgrip exercise. In contrast, heart rate changed little with age during contractions of the quadriceps muscle in paraplegics which were induced by electrical stimulation.