Motor unit recruitment and rate coding in response to fatiguing shoulder abductions and subsequent recovery

The purpose of the present study was to investigate motor unit (MU) recruit ment and firing rate, and the MU action potential (MUAP) characteristics of the human supraspinatus muscle during prolonged static contraction and sub sequent recovery. Eight female subjects sustained a 30 degrees shoulder abd uction, requiring 11-12% of maximal voluntary contraction (MVC), for 30 min . At 10 and 30 min into the recovery period, the shoulder abduction was rep eated for 1 min. The rating of perceived exertion for the shoulder region i ncreased to "close to exhaustion" during the prolonged contraction, and the surface electrompography (EMG) recorded from the deltoid and trapezius mus cles showed signs of local muscle fatigue. From the supraspinatus muscle, a total of 23,830 MU firings from 265 MUs were identified using needle elect rodes. Of the identified MUs, 95% were continuously active during the 8-s r ecordings, indicating a low degree of MU rotation. The mean (range) MU firi ng rate was 11.2 (5.7-14.5) Hz, indicating the relative force contribution of individual MUs to be larger than the overall mean shoulder muscle load. The average MU firing rate remained stable throughout the prolonged abducti on, although firing rate variability increased in response to fatigue. The average concentric MUAP amplitude increased by 38% from the beginning (0-6 min) to the end (24-29 min) of the contraction period, indicating recruitme nt of larger MUs in response to fatigue. In contrast, after 10 min of recov ery the average MU amplitude was smaller than seen initially in the prolong ed contraction, but not different after 30 min, while the MU firing rate wa s higher during both tests. In conclusion, MU recruitment plays a significa nt role during fatigue, whereas rate coding has a major priority during rec overy. Furthermore, a low degree of MU rotation in combination with a high relative load at the MU level may imply a risk of overloading certain MUs d uring prolonged contractions.