A number of investigations have used surface electromyographic (EMG) procedures to identify the power output associated with the onset of neuromuscular fatigue (NMF) during cycle ergometry [10-12]. NMF is typically characterized by an increase in the electrical activity of the working muscles over time [11,13]. Moritani et al. [11] suggested that the fatigue-induced increase in EMG amplitude is a result of progressive recruitment of additional motor units (MU) and/or an increase in the firing frequency of MUs that have already been recruited. De Vries et al. [13] developed a submaximal discontinuous cycle ergometer test utilizing this concept of progressive muscle activation and introduced the concept of "the physical working capacity at the fatigue threshold" (PWC_FT). This utilized the EMG fatigue curves to identify the power output that corresponded to the onset of the NMF threshold. PWC_FT represents the highest power output that does not result in a significant increase in the electrical activity of the thigh muscle over time (p > 0.05). PWC_FT has been shown to be reliable [10], valid [10] and sensitive to changes in fitness levels and creatine supplementation [14] in older men and women (62 – 73 years). Furthermore, de Vries et al. [10] suggested that PWC_FT in elderly men and women may be more appropriate than the assessment of physical work capacity by measurement of maximal oxygen uptake capacity (i.e., V0_{2 max}). In addition, de Vries et al. [10] reported PWC_FT was a more sensitive measure of monitoring changes, in comparison to fitness tests that depend upon heart rate/power output relationships and/or maximal cardiac output.

The concept of physical working capacity (PWC), a measure of aerobic power, muscular endurance and efficiency is typically measured by oxygen consumption rate (VO₂) during a maximal graded exercise test (GXT) [19]. Recently, several studies have reported on the effects of BA supplementation on PWC_FT during incremental cycle ergometry tests in young men and women [16,17]. Stout et al. [16,17] reported that 28 days of BA supplementation in a younger population (21–27 years) resulted in a significant increase in PWC_FT by 12 – 15%, respectively. In agreement, the current study demonstrated a 28.5% increase in PWC_FTafter ninety days of BA supplementation. The two-fold increase in PWC_FT in the elderly compared to young men and women in previous studies may be due to differences in initial skeletal muscle carnosine levels or supplementing duration. In support, Tallon et al. [4] reported that in Type II skeletal muscle, carnosine concentration was 47% lower in elderly (70.4 ± 5.0 yrs) compared to younger (23.8 ± 4.6 yrs) men and women, although Kim [20] found normal muscle carnosine levels in elderly Korean subjects with impaired glucose tolerance. Harris et al. [2] demonstrated that 28 days of BA supplementation significantly (60%) increased skeletal muscle carnosine levels while Hill et al. [15] demonstrated a further 20% increase when BA supplementation was continued for an additional 35 days. In light of these reports, the greater change in PWC_FT in elderly participants in the present study, compared to previous studies in young men and women, was most likely due to both initial carnosine levels and length of time of BA supplementation.

De Vries et al. [13] suggested the PWC_FT test may be more appropriate and sensitive to training effects on PWC in elderly, compared to other methods (i.e. VO_{2 max}) that require maximal effort and may be ill-advised or possibly hazardous in this population. To our knowledge, this is the first study to examine the effects of BA supplementation using the PWC_FT test in elderly men and women. However, de Vries et al. [13] did examine the effects of 10 weeks of moderate intensity (70% PWC_FT) endurance training three times per week on PWC_FT in elderly men and women (67.9 ± 5.3 years). The results showed a significant (p < 0.05), 30% increase in PWC_FT as a result of the endurance training. While the results by de Vries et al. [13] support and recommend endurance training as a means to significantly improve physical working capacity, it should be noted that the elderly subjects in the current study were untrained, and the 28.5% increase in PWC_FT occurred without any type of additional training during the ninety days of supplementation.

It has been proposed that exercise-induced decreases in intramuscular pH may interfere with the excitation-contraction coupling process of skeletal muscle, which, in turn may lead to decreases in power output and fatigue [21]. Maintaining the intracellular pH during exercise could therefore be important for normal muscle function in the elderly [4]. In order to maintain pH homeostasis, various buffering systems are involved, including active H⁺ export from muscle [2]. However, the immediate line of defense remains the buffering of H⁺ by intracellular physico-chemical buffers, principally phosphates and carnosine. Marsh et al. [22] demonstrated that there was a significant delay in the onset of intracellular acidosis during progressive exercise after six weeks of moderate intensity training, resulting in an increased capacity for submaximal work in a similar cohort of elderly individuals. Improving the ability to buffer intramuscular H⁺ accumulation, therefore, appears to be an important factor for delaying the onset of fatigue and increasing exercise capacity in older men and women.