Contraction-induced potentiation of human motor unit discharge and surface EMG activity.

In quadrupeds, an electrically induced, moderate to high intensity brief muscle contraction potentiates autogenetic excitation and leads to enhanced recruitment and/or tonic firing frequency of alpha-motor neurons. To determine if similar adaptations occur in humans, single motor units (SMUs) and surface electromyographic activity (EMG) were recorded from the right biceps brachii before and immediately after a 5-s 25% or 50% maximum voluntary contraction (MVC), while subjects held a handle (0-1% MVC) attached to a force transducer or maintained a 2% MVC for 30-60 s. Of 26 SMUs recorded, 15 increased, 4 decreased, and 7 showed no change in firing frequency (mean increase: 5 imp/s, P less than 0.01). Twelve SMUs had lower recruitment force thresholds after contraction. There was no significant treatment effect for the % MVC intensity. The postcontraction surface EMG power spectrum broadened, increased in amplitude, and contained a higher frequency component than the control contraction power spectrum. Changes in recruitment and/or frequency coding were reflected in the raw EMG records. Findings agree with previous reports in animals of contraction-induced potentiation of subsequent submaximal muscle contractions. Such acute adaptations in spinal neuromuscular pathways would function to optimize force output to a submaximal range of neural input frequencies.

Post-contraction errors in human force production are reduced by muscle stretch.

1. Based on findings from a previous study of plantar-flexor muscles, the effect of a conditioning 25, 50 or 100% maximum voluntary contraction (m.v.c.) of elbow flexor muscles on the accuracy of reproducing a learned criterion muscle force (2% m.v.c.) was investigated. 2. Each conditioning contraction induced a significant error in reproducing the criterion muscle force under conditions of no visual feed-back of force. As with plantar-flexor muscles, the error was consistently in the direction of a positive bias. The magnitude of the error co-varied with the magnitude of the previous contraction and, in all cases, decayed toward criterion force values over a 35 s period. 3. A brief muscle stretch, induced before subjects attempted the criterion force, reduced the size of the error but did not completely eliminate the bias. 4. The findings provide indirect evidence of post-contraction potentiation of stretch reflex pathways. Residual post-contraction errors in force production after muscle stretch may be attributed to other central or peripheral neural factors such as, for example, potentiating effects of prior activation on submaximal tension production in skeletal muscle.