Dynapenia

Dynapenia is the age associated loss of muscle strength and power that is not explained by neurological or muscle disease. Strength declines faster than muscle mass, so mass alone does not predict weakness.

Weak grip affects a substantial minority of people over 65, and the proportion rises sharply after 80. It predicts falls, disability and mortality independently of muscle mass.

Pathophysiology. Neural factors dominate: loss of motor units, reduced central drive, lower firing rates and slower conduction. Muscle factors add type II fibre atrophy, fat infiltration of muscle, and reduced force for a given muscle size from impaired excitation contraction coupling.

Slow chair rises, difficulty with stairs and carrying, reduced walking speed, poor balance and falls. Grip, knee extension and hip flexion are weak, with normal sensation and reflexes. The fall in muscle power (strength times speed) is often more disabling than loss of strength alone.

Measure handgrip with a calibrated dynamometer (under 27 kg in men and under 16 kg in women is low) and the five time chair stand. Assess lower limb strength and gait speed. Exclude neurological and muscle disease by examination, creatine kinase, thyroid function, calcium, vitamin D and vitamin B12. Body composition scanning shows whether muscle mass is also low.

Differential diagnosis.

• Sarcopenia with low muscle mass.
• Myopathy (drug induced, endocrine, inflammatory) and neuropathy.
• Myasthenia gravis, motor neuron disease and cervical or lumbar myeloradiculopathy.
• Depression, pain and fear of falling limiting effort.

Prescribe progressive resistance training at high intensity (70% to 80% of the one repetition maximum, 2 to 3 sessions weekly), including fast velocity power exercises, with balance training. Ensure protein 1.0 to 1.2 g/kg daily and correct vitamin D deficiency. Review medicines that cause weakness or sedation.

Monitoring. Repeat grip strength and chair stand every 3 to 6 months.