Metabolic and Mitochondrial Myopathy as a Fatigue Perturbation

Metabolic and mitochondrial myopathy is a direct perturbation of the fatigue dynamics (§6). Impaired ATP resynthesis makes the slow fatigue variable load FASTER and recover INCOMPLETELY: the fitted time constant shortens from 60 s (normal) to 25 s, and recovered force after rest falls from 0.974 to 0.835 — below the 0.95 reversibility floor that defines healthy fatigue. Both DIRECTIONS are verified [V].

T5's reversible exponential decline is re-run with a shortened τ and a non-recovering residual term. Force then declines faster (τ 60→25 s) and rest no longer restores it (recovered fraction 0.835 vs 0.974 normal, vs the ≥0.95 healthy threshold) — the exercise-intolerance phenotype. Recovery is monotone in the residual deficit. Direction [V]; anchor [L]; absolute magnitude [O].

Fatigue that comes early and does not lift

T5 established muscle fatigue as a reversible exponential force decline to a plateau, fully recovered on rest, with a cited time constant. Metabolic / mitochondrial myopathy perturbs exactly that variable: defective oxidative ATP supply means the fatigue process charges faster and its recovery is incomplete. Two cited features of the phenotype map onto two changes to T5 — a shorter τ and a residual, non-recovering deficit — with no new dynamics.

Faster onset, incomplete recovery

The fitted fatigue time constant shortens from 60 s to 25 s, so force falls earlier into exertion. After the standard rest period the normal muscle recovers a fraction 0.974 of its force (effectively full), while the metabolic muscle recovers only 0.835 — below the 0.95 floor that characterises healthy, fully reversible fatigue. That persistent deficit is the modelled correlate of exercise intolerance and post-exertional weakness. Deepening the residual deficit monotonically lowers the recovered fraction.

What is NOT claimed

The DIRECTIONS — faster decline and sub-threshold recovery — are the result, and the fatigue time-constant band is cited [L]. The absolute magnitude of the force loss and the precise recovery percentage for any specific enzyme defect are not fixed by the substrate and stay [O].