(14) The Effects of Short-Term Resistance Exercise Training on the Motor Unit Firing Rates of the First Dorsal Interosseus.

Purpose: This study investigated the effects of a short-term resistance exercise training (RET) of the first dorsal interosseus muscle (FDI) on strength and motor unit (MU) firing rates. Methods: Nineteen females (age = 20.4±1.1 yrs, height = 170.4±7.9 cm, body mass = 70.4 ±16.7 kg) completed 2 pre-testing visits (pre-1 and pre-2) and were then placed into a control (CON, n=10) or RET (n=9) group. The RET group completed 9 training sessions entailing isometric and isotonic index finger abductions followed by a post-testing visit. Isometric training included 3 maximal voluntary contractions (MVC) and 3 triangular tracings at 3 different specified intensities (25%, 50%, and 75% MVC) in randomized order. The isotonic training included 3 sets of 8-12 repetitions. Intensity started at 80%, then increased by 30g when a subject was able to complete 3 sets of 12 repetitions during a training session. During pre-1, pre-2, and post-testing, a 5-pin electromyographic (EMG) decomposition sensor was placed over the belly of the FDI to record MUs from the triangular 75% MVC. Recruitment thresholds and mean discharge rates for each identified MU from the 75% MVC was calculated. The recruitment thresholds relative to MVC (%MVC) and mean discharge rate were linearly regressed for each subject and contraction to obtain the y-intercept and slope. Two-way repeated measure ANOVAs (group [CON vs. RET] x time [pre-1 vs. pre-2 vs. post) were performed to examine potential changes in the y-intercepts and slopes from MU firing rates vs recruitment threshold relationships along with MVC strength. Results: There was no significant group*time interaction for the y-intercepts (p=0.089) or main effects for time (p=0.512) and group (p=0.760) (Table 1). For the slopes, there was no significant group*time interaction (p=0.155) or main effects for time (p=0.755) and group (p=0.563) (Table 1). There was a significant group*time interaction for MVC strength (p=0.028) (Table 1). A repeated measure ANOVA revealed a significant main effect of time the RET group (p=-0.006) (Table 1). Isometric maximal strength was greater at post in comparison to pre-2 (p=0.038) with no other significant differences between time points or groups. Conclusions: Increases in strength following short-term RET of the FDI may not be the result of increased MU activation. Practical Applications: Early improvements in strength of the hand during a RET program may be primarily mechanical in origin. Acknowledgements: None