Plotting a Strength duration curve (SDC) requires stimulating a muscle at its motor point with fixed pulse duration, ranging from 0.01 to 300 milliseconds (ms), recording the current strength in milliampere mA required to elicit a threshold twitch contraction. The mA values obtained and the pulse duration utilized is plotted on the X and Y axis respectively to obtained SDC graph [1
]. The SDC was historically performed from the 1930s-1960s to assess nerve injuries prior to the common recent use of electromyography and nerve conduction testing [1
]. SDC was a mean of evaluating the severity and subsequent recovery of a nerve injury. It provides graphic representation of the integrity of the muscle-nerve complex. The SDC remains useful even though electrophysiologic evaluation techniques have become more sophisticated in the past three decades, it still remains a reliable index of muscle/nerve functional integrity [1
]. The response of a nerve and a muscle to electrical stimulus depend on three variable excitation factors: strength of the stimulus, period of time for which the current flows and the rate of change of the stimulus. In SDC testing, the stimulus consist of rectangular impulses of interrupted direct current of varying strength and duration. SDC of a denervated muscle will be shifted to the right compared to normal innervated muscle; subsequently shifting to the left occur during re-innervation [1
]. Thus, SDC can be used to demonstrate or confirm normal innervation of a muscle, presence or progress of lower motor neuron lesion disorders [1
For many years, traditional electrodiagnosis has been the main means of testing for radiculopathies caused by lumbar disc herniation. Historically, the use of electrodiagnosis in the study of radiculopathies with disc compression evolved in three stages. It began in 1868 with Erb’s method of electrodiagnosis by faradic and galvanic currents. The second stage, first through Lapigue in 1926 and then in 194, saw the introduction of the SDC obtained with rectangular or triangular-wave electrical pulses of variable duration and the three typical types of curve corresponding to normality, partial denervation, and total denervation [2
]. The introduction of the needle-concentric electrode by Adrian and Bronk in 1929 marked the start of the last stage, the electromyographic one, which expanded in the 1960s. Electromyography (EMG) still constitutes the instrumental methodology of reference in the central and peripheral nerve and muscle pathologies [3
In patients with chronic low back pain, it is important to establish whether or not radiculopathy is present. This is not difficult when clinical, radiological and electromyographic abnormalities consistent with focal nerve root involvement are found [4
]. However, a high percentage of the patients referred to back pain clinics presents with leg pain only. The neurological examination may be normal or confusing showing non-radicular sensory changes [5
]. Imaging studies may lack diagnostic specificity [6
]. Needle EMG, which tests only ventral root function, may be normal in the absence of motor symptoms [6
]. The clinical presentations of lumbosacral radiculopathy vary according the level of nerve root or roots involved. The most frequent are the L5 and S1 radiculopathies [6
]. Patients present with pain, sensory loss, weakness, and reflex changes consistent with the nerve root involved. However, in radiculopathies, when the level of radicular compression must be sought, there appears to be no reason to abandon traditional electrodiagnosis i.e. SDC testing, which, compared to EMG, is easy to perform, is clearly better tolerated by patients and less costly [3
]. It appears that SDC is not used as often as it could be, which may be due to lack of appreciation of its possible uses or its proven accuracy. The possibility of disc herniation with compression on peripheral nerves leading to reduction in the conduction velocity and alteration in SDC parameters in terms of an increase in rheobase and/or chronaxie of the affected nerve root segments has been suggested [9
]. Effects of graded compression on nerve function were analysed by Dahlin et al. [8
] in order to evaluate the relative importance of pressure level and duration of compression for functional deterioration. In Dahlin etal’s study, the pressure was applied by means of a small inflatable cuff and the effects of two pressure levels, i.e., 80 mm Hg applied for 2 hr or 400 mm Hg applied for 15 min, were studied in rabbit tibial nerves. Their findings indicated that mechanical pressure on peripheral nerve could cause ischemia of the compressed nerve segment and some degree of mechanical deformation of the nerve trunk, which in turn could lead to incomplete recovery following pressure release. They also concluded that duration of compression is of importance for the degree of nerve injury.
The correlation of the SDC with clinical and electrodiagnostic findings in normal individuals and in patients with subnormal peripheral nerve conduction and with EMG equivalents of axonal lesion was reported by [10
]. In a similar vein, Nardin et al. [4
] reported the accuracy of SDC parameters in evaluating chronic entrapment neuropathies in proportion to the incidence of axonal lesion.
Analysis of literature, reveals a scarcity of data on characteristics pattern of SDC in patients who suffer low back pain (LBP) with radiculopathy as a results of disc herniation. The present study was designed to compare SDC characteristics pattern and to compare rheobase and chronaxie between patients with low back pain lumbar spine disc herniation at L4/L5 and apparently healthy adults. We hypothesize that chronaxie and rheobase values will be significantly higher in LBP patients with radiculopathy than in apparently healthy individuals.