Motor unit magnetic resonance imaging to assess muscle twitch dynamics in mitochondrial disease after an exercise programme.

Motivation: Changes to muscle twitch dynamics are overlooked in trials assessing resistance exercise in primary mitochondrial myopathies (PMM). Goal(s): Motor unit MRI to measure twitch dynamics in PMM participants before and after a 12-week exercise programme. Approach: Voxel-wise measurements of rise time (Trise), contraction time (Tcontract) and half-relaxation time (Thalf-relax) in the tibialis anterior in 10 controls and 9 PMM participants. PMM participants scanned twice, before and after a 12-week exercise programme. Results: Tcontract of the tibialis anterior was significantly longer in PMM participants post exercise; Trise, Thalf-relax demonstrated no change. In participants who had the highest adherence to exercise Tcontract increased the most. Impact: Motor unit MRI (MUMRI) detected slower muscle contraction times in primary mitochondrial myopathies post resistance exercise programme. This may evidence increased numbers of type-I fibres post-exercise. MUMRI could be used to measure changes in muscle twitch dynamics in neuromuscular diseases.