RESUMEN
Fatigue is a common feature of paralysed skeletal muscle, hindering performance when subjected to functional electrical stimulation (ES) for movement. We asked whether (1) 20 Hz ES for 5% of each day (2.5 s on and 2.5 s off for 3 h) increases tibialis anterior and medial gastrocnemius muscle and motor unit (MU) endurance after paralysis by hemisection and deafferentation (HSDA), and (2) muscle length or loading affects their isometric contractile properties. The daily 5% ES increased muscle endurance, largely independent of muscle length or loading, but to a lesser extent than the daily 50% ES (2.5 s on and 2.5 s off for 24 h). The former was effective in counteracting the decline and slowing of muscle force promoted by the 50% ES. The altered muscle properties were confirmed at the MU level in final experiments once the properties had plateaued. Fast-fatigable MUs were converted to fatigue-intermediate and -resistant MUs that finally comprised â¼80% as compared to â¼10% of the total MU number in the daily 5% ES and the control normal groups, respectively. We conclude that the daily 5% ES regimen counteracts the fatigue of paralysed muscle without compromising contractile force, and thereby, is effective in conditioning muscle for effective movement. KEY POINTS: We asked whether 20 Hz electrical stimulation (ES) for 5% of each day (2.5 s on and 2.5 s off for 3 h; 5% ES) preserves medial gastrocnemius and tibialis anterior muscle and MU isometric contractile forces and increases their endurance after paralysis. Daily 5% ES promoted increased muscle endurance irrespective of the muscle length or loading but to a lesser extent than daily 50% ES (20 Hz ES 2.5 s on and 2.5 s off for 24 h). 5% ES was effective in counteracting decline and slowing of muscle force that resulted from 50% ES. Motor units (MUs) were converted from fast fatigable to fatigue intermediate and resistant MUs, comprising â¼80% as compared to â¼10% in the control normal groups. We conclude that the 5% ES regimen counteracts the fatigue of paralysed muscle without compromising contractile force, and thereby is effective in conditioning the muscle for effective movement.
Asunto(s)
Neuronas Motoras , Traumatismos de la Médula Espinal , Humanos , Neuronas Motoras/fisiología , Músculo Esquelético/fisiología , Contracción Muscular/fisiología , Traumatismos de la Médula Espinal/terapia , Parálisis/terapia , Estimulación Eléctrica/métodos , Fatiga Muscular/fisiologíaRESUMEN
This study of medial gastrocnemius (MG) muscle and motor units (MUs) after spinal cord hemisection and deafferentation (HSDA) in adult cats, asked 1) whether the absence of muscle atrophy and unaltered contractile speed demonstrated previously in HSDA-paralyzed peroneus longus (PerL) muscles, was apparent in the unloaded HSDA-paralyzed MG muscle, and 2) how ankle unloading impacts MG muscle and MUs after dorsal root sparing (HSDA-SP) with foot placement during standing and locomotion. Chronic isometric contractile forces and speeds were maintained for up to 12 months in all conditions, but fatigability increased exponentially. MU recordings at 8-11½ months corroborated the unchanged muscle force and speed with significantly increased fatigability; normal weights of MG muscle confirmed the lack of disuse atrophy. Fast MUs transitioned from fatigue resistant and intermediate to fatigable accompanied by corresponding fiber type conversion to fast oxidative (FOG) and fast glycolytic (FG) accompanied by increased GAPDH enzyme activity in absolute terms and relative to oxidative citrate synthase enzyme activity. Myosin heavy chain composition, however, was unaffected. MG muscle behaved like the PerL muscle after HSDA with maintained muscle and MU contractile force and speed but with a dramatic increase in fatigability, irrespective of whether all the dorsal roots were transected. We conclude that reduced neuromuscular activity accounts for increased fatigability but is not, in of itself, sufficient to promote atrophy and slow to fast conversion. Position and relative movements of hindlimb muscles are likely contributors to sustained MG muscle and MU contractile force and speed after HSDA and HSDA-SP surgeries.