Article Figures & Data
Figures
- Figure 1.
Area of pain and location of fine-wire electrodes. The injection site, approximate area of reported pain for each subject (gray), and location of fine-wire EMG electrodes are shown. Hypertonic saline (5% NaCl) was injected into the patella–femoral fat pad in study A, and FPL in study B.
- Figure 2.
Number of discriminated motor units in quadriceps (A) and FPL (B) during the three contraction conditions. A similar pattern of recruitment between conditions is seen in both muscles.
- Figure 3.
The discharge rate of motor units recruited in the quadriceps (n = 18) and FPL (n = 8) in trials both with and without pain decreased significantly with pain (closed circles). When the discharge rate of the whole population of units identified in either condition (open circles) was compared, a reduction in discharge rate was also identified in both muscles. The discharge rate of the units recruited during pain at the lower force level (closed squares) is also shown. *p < 0.05; **p < 0.001. Mean and SD are shown.
- Figure 4.
Simultaneous recordings of force and motor unit activity from two fine-wire electrodes inserted into the quadriceps of one subject. The top trace shows the force level during the three experimental conditions and the second and third traces show the muscle activity (EMG) recorded throughout the ∼15 s constant force contractions. These traces show the consistency in force held and motor unit firing within, and variability of motor unit firing between conditions. An example of a low-force contraction after the pain was completely resolved (lower force post pain) is also shown (far right). Below the two EMG traces, 1 s of data has been shown in greater detail from both electrodes in each condition. The discharge rate and spike-trigger averaged electrical profile of each discriminated motor unit are shown. Data show the reduction in discharge rate during pain of motor units identified in the no-pain and pain low-force conditions (A, D, and E). The figure also shows a population of units that were recruited during the no-pain condition that were not recruited during pain (B and C). Force is maintained during pain by the recruitment of new units (F and G) that, in this case, are the next ones expected given orderly recruitment. Identification of units B and C before and after the pain condition confirms that shape had not changed with condition and therefore that these two units were not missed for that reason. The residual EMG may contain some very small spikes (X) that are not classified or used for analysis purposes, but are not the units of interest.
- Figure 5.
Recordings of motor unit activity from a fine-wire electrode inserted into the FPL of one subject. Single motor units are shown with their respective discharge rates [mean (SD)] and spike-trigger averaged electrical profile. Once these units were identified and removed from the EMG trace, there was very little residual left (top trace). An exception of two small spikes in the residual of the painful contraction is highlighted (X and Y). Neither spike reflects a unit that regularly discharges, and therefore they do not meet the criteria as discussed in Materials and Methods. The small residual trace clearly indicates that all reliable units were identified from this file. Three motor units were recruited during both the no-pain and pain lower-force trials (A, C, and E). These units all decreased in discharge rate during pain. Three new units (B, F, and G) that were not seen in the lower-force no-pain conditions were recruited during pain. Of these units, only B is expected given orderly recruitment of the motoneuron pool as determined from those additional units recruited without pain at a slightly higher force level (B and D). Unit D is clearly not recruited during pain. The derecruitment of unit D, which coincides with the recruitment of B, F, and G during pain, demonstrates a change in recruitment during pain.
- Figure 6.
Recordings of motor unit activity from a fine-wire electrode inserted into the quadriceps of one subject. Single motor units are shown with their respective discharge rates and spike-trigger averaged electrical profile. The small residual EMG trace clearly indicates that all reliable units were identified from this file. Two motor units were recruited during both the no-pain and pain lower-force trials (B and C); their motor unit firing rate decreased during pain. Three new units (D–F) that were not seen in the low-force no-pain conditions were recruited during pain. Unit A is not recruited during pain. The derecruitment of unit A, which coincides with the recruitment of D–F during pain, demonstrates a change in recruitment during pain.
