A series of EST studies suggest that neural adaptations mediate increases in MVC force because the increases in force occur without significant muscle hypertrophy (Brocherie et al.
2005; McMiken et al.
1983; Pichon et al.
1995). Although none of these studies measured directly muscle size before and after the training program, the brevity of the protocols, which sharply contrasts with early studies using extreme stimulation conditions that are not adaptable to clinical settings (e.g., stimulation for 3 h per day, 6 days a week for 6 weeks, Rutherford, 1988) (Jarvis et al.
1996; Pette and Vrbova
1992; Schiaffino et al.
1989; Scott et al.
1985), and the advanced athletic status of the participants indirectly suggest neural and not muscle adaptations are the core moderators of increases in MVC force. For example, after 3 weeks of EST, isokinetic eccentric and concentric torque increased significantly in nine ice-hockey players whose 10 and 30 m sprint skating times also improved (Brocherie et al.
2005). Likewise, EST of the latissimus dorsi increased the strength and swimming performances of a group of competitive swimmers (Pichon et al.
1995). These data must be interpreted cautiously because of a lack of direct measurement of muscle fiber size. However, results from studies that did directly measure changes in muscle fiber size, reviewed next, are still compatible with the idea that neural adaptations contributed to the gains in motor performance.
Although there are numerous studies that reported changes in muscle metabolism, none provide compelling evidence that EST, similar to the paradigms used in clinical settings, produces early muscle hypertrophy (i.e., in less than 6 weeks). While there were significant increases in quadriceps MVC force from 1,295 N (±135) to 1,530 N (±131) after a 4 week training period in one study that used 6 min effective EST in each of 15 sessions, these changes were similar to the gains produced by VST without significant changes in enzyme activities, muscle fiber characteristics, or mitochondrial properties (Eriksson et al.
1981; Kim et al.
1995; Martin et al.
1993,
1994; St Pierre et al.
1986). Twenty-one days of EST of the gastrocnemius muscle reduced skinfold thickness by 20% and it increased MVC force by 42%, nuclear number per unit area by 37%, mean length of muscle cell nuclei by 14%, and nuclear density by 21% (all
p < 0.05), muscle fiber size increased moderately (16%,
p < 0.05) in the 50 Hz stimulation group and by 10% (
p > 0.05) in the 2,000 Hz stimulation group (Cabric and Appell
1987; Cabric et al.
1987). A study using nuclear magnetic resonance spectroscopy after 13–15 min of low-intensity (10% MVC) voluntary or electrical stimulation-evoked contractions of the gastrocnemius revealed that the muscle was more acidic and the muscle cytoplasm was more oxygenated during stimulated than voluntary exercise, suggesting higher levels of glycolysis and oxygen demand (Vanderthommen et al.
2003). Thus, the data confirmed that electrical muscle stimulation is fatiguing but there was no hint for changes in metabolic pathways linked to muscle hypertrophy. Six weeks of quadriceps EST at 8 Hz, a rate lower than most EST studies, in 20 sedentary, 10 active, and five endurance-trained subjects using 25 consecutive 10 s isometric contractions increased citrate synthase activity, capillary number per type IIA and IIB fibers, and the percentage of type IIA muscle fibers but, again, failed to increase the size of type I, IIA, and IIB muscle fibers (Theriault et al.
1996). In an EST study similar to the protocols most often used in clinical settings to strengthen healthy and an orthopedic injury-weakened muscle (Bax et al.
2005), vastus lateralis biopsy samples from ten healthy volunteers who received EST at 45–60 Hz, with 12 s of stimulation followed by 8 s of rest for a total of 30 min per day, 3 days/week for 6 weeks revealed an increased expression of MHC isoform IIA through a shift of MHC-I and MHC-IIX isoforms to the MHC-IIA isoform (Perez et al.
2002), but these fiber type conversions occurred without muscle fiber hypertrophy. Unlike in previous studies (Brocherie et al.
2005; McMiken et al.
1983; Pichon et al.
1995), these EST-induced shifts in the biochemical characteristics were not associated with improvements in whole-body aerobic performance or neuromuscular function produced by muscles of the entire limb (Perez et al.
2002). EST over an initial 4 week period failed to increase quadriceps size measured with ultrasound, but muscle size did increase a modest 4% (±2,
p < 0.001) from week 4 to 8 (Gondin et al.
2005) and increased up to 12% after 8–9 weeks (Gondin et al.
2011; Ruther et al.
1995; Stevenson and Dudley
2001), suggesting muscle hypertrophy occurred in the late phase of the program. Taken together, these results suggest that EST administered with a clinical dose and parameters for up to about 6 weeks does bring about changes in muscle metabolism and promotes subtle isoform shifts, but the increased MVC force is not the result of overt muscle hypertrophy, but instead, it is mediated by changes in some elements of the nervous system. Therefore, these studies seem to support the idea that short-term EST increases MVC force through neural mechanisms.