Superimposed single impulse and pulse train electrical stimulation: A quantitative assessment during submaximal isometric knee extension in young, healthy men

Assessing Voluntary Muscle Activation with the Twitch Interpolation Technique

“…It has been suggested that twitch interpolation involving a single superimposed stimulus may be incapable of detecting sub-optimal motor unit firing rates [42,43] and that the evoked-voluntary force relationship becomes less steep or even asymptotic at forces at which recruitment is complete and rated coding becomes the sole means of enhancing force [31] . This view is contradicted by two observations.…”

“…Recently it has become common for 2 or more stimuli (50-100 Hz) to be employed [17,18,34,38,51] because the evoked force increments are larger and more readily detected [15,32,43,45,54,55,56,57,58] . There is also evidence that supramaximal twin, triple and quadruple stimuli (at 125 Hz) evoke less variable force increments than single stimuli when superimposed over voluntary isometric contractions at 50% of maximum [45] .…”

“…Consequently submaximal stimuli are often employed [43,56,58,59] and this introduces the potential problems outlined previously [see 60] . The use of multiple stimuli has also been criticised on the grounds that spinal reflexes have more time to influence the superimposed response [33,60] .…”

“…Regardless of the potential problems involved, trains of 10 to 100 stimuli are quite commonly employed to assess the maximality of voluntary activation [32,43,55,56,57,58,62] . As with interpolated twitches, activation is then sometimes quantified by scaling superimposed force increments to control responses evoked in relaxed muscles [32,56] .…”

Assessment of Neuromuscular Function Using Percutaneous Electrical Nerve Stimulation

“…Indeed, some authors suggested that VAL overestimates maximal muscle activation [61][62][63] . It may not be sensitive enough to detect variations in activation levels during contractions above 90 % MVC 62 .…”