Most studies involving NMES and strength evaluated this in individuals with complete or motor complete SCI (Hjeltnes and Lannem, 1990; Kagaya et al. 1996; Shields and Dudley-Javoroski, 2006). In general, all studies produced beneficial results on muscle size (i.e., reduced muscle atrophy). In addition to enhancing muscle bulk, most interventions also focused on improving muscle function, most notably strength and endurance, as well as contractile speed and muscle fatigue.
Studies with the strongest research design and supporting the efficacy of NMES were conducted by Harvey et al. (2010) and Shields and Dudley-Javorski (2006). Harvey et al. (2010) used an RCT design in persons with both complete and incomplete SCI and found that NMES-assisted exercise increased voluntary quadriceps strength over those that received no intervention. The increase in strength was statistically higher in the experimental group, but it was uncertain if the increase had a clinically important effect. Shields and Dudley-Javorski (2006) employed an experimental non-RCT design to examine the effect of long-term (up to 3 years) NMES exercise to unilateral ankle plantarflexor muscles with the untrained leg serving as a control. This study examined 7 males with complete and relatively recent injuries (~6 weeks post-injury). Peak stimulated ankle torque (i.e, non-voluntary) was found to be significantly greater in the stimulated leg as compared to the untrained leg. The trained side also generated significantly higher torque-time integrals than the untrained side. Other pre-post study designs of NMES-assisted exercise also found increased stimulated muscle forces or torques following training although the participants involved in these studies were generally more chronic (Sabatier et al., 2006; Kagaya et al., 1996; Hjeltnes and Lannem, 1990).
There is level 1b evidence (Harvey et al., 2010) that PES-assisted exercise may increase voluntary muscle strength, but the increase may not have a clinically important treatment effect.
There is level 2 evidence (Baldi et al., 1998) that PES-assisted isometric exercise reduces the degree of lower limb muscle atrophy in individuals with recent (~10 weeks post-injury) motor complete SCI, but not to the same extent as a comparable program of FES-assisted cycling exercise.
There is level 4 evidence (Sabatier et al., 2006) that PES-assisted exercise may partially reverse the lower limb muscle atrophy found in individuals with long-standing (>1 year post-injury) motor complete SCI.