Table 1.
Paired motor unit validity criteria and ramp characteristics for seated and standing measures
| Posture |
|||
|---|---|---|---|
| Standing | Seated | Return to Standing | |
| Requirements for motor unit pairs | |||
| Time between reference and test unit recruitment, s (minimum accepted value: >1 s,b >2 sa,c) |
1.83 ± 0.81d | 2.79 ± 1.20d | 2.56 ± 1.24 |
| Rate modulation (minimum difference of 0.5 ppsa,c) |
3.11 ± 1.90 | 3.21 ± 1.44 | 2.44 ± 1.15 |
| Rate-rate correlation coefficient (minimum accepted value: r ≥ 0.7a,c) |
0.84 ± 0.13 | 0.84 ± 0.11 | 0.80 ± 0.15 |
| Ramp and firing rate characteristics | |||
| Duration of ramp rise, s | 5.51 ± 0.37 | 5.91 ± 0.73 | 5.57 ± 0.48 |
| Duration of ramp decline, s | 5.96 ± 0.61 | 5.97 ± 0.58 | 5.60 ± 0.81 |
| Instantaneous firing frequency of reference unit at recruitment, pps | |||
| Only first ISI | 5.14 ± 0.84 | 5.36 ± 1.37 | 5.45 ± 2.79 |
| Average of first 3 ISIs | 6.23 ± 0.99 | 6.01 ± 1.12 | 5.98 ± 1.07 |
| Instantaneous firing frequency of test unit at recruitment, pps | |||
| Only first ISI | 5.39 ± 1.61 | 5.92 ± 2.36 | 5.28 ± 1.35 |
| Average of first 3 ISIs | 6.21 ± 1.43 | 6.41 ± 1.20 | 7.21 ± 2.68 |
| Control unit firing frequency at test unit recruitment, pps | 8.59 ± 1.67 | 9.50 ± 1.68 | 9.68 ± 1.25 |
| Control unit firing frequency at test unit derecruitment, pps | 5.95 ± 1.49 | 6.35 ± 1.77 | 6.63 ± 1.56 |
| Antagonist and soleus muscle activation | |||
| Relative antagonist muscle activation at reference unit recruitment, %baseline TA sEMG RMS | 69.32 ± 26.41d,e | 14.70 ± 6.79d,e | 83.15 ± 48.14e |
| Relative antagonist muscle activation at test unit recruitment, %baseline TA sEMG RMS | 34.92 ± 25.35d | 14.98 ± 8.16d | 31.96 ± 31.88 |
| Relative soleus muscle activation at reference unit recruitment, %MVC soleus sEMG RMS | 5.51 ± 3.40d | 3.27 ± 2.79d | 5.46 ± 5.62 |
| Relative soleus muscle activation at test unit recruitment, %MVC soleus sEMG RMS | 17.70 ± 15.04d | 7.46 ± 4.43d | 20.45 ± 10.68 |
Values are group means ± SD in participants at standing (n = 15), seated (n = 15), and return to standing (n = 9) positions. All motor unit pairs accepted for analysis in this experiment exceeded the minimum accepted value for published validation criteria:
Instantaneous firing frequencies (pulses/s, pps) at recruitment of test or reference units were quite low when only the first interspike interval (ISI) was isolated.
Significant difference between seated and standing posture (paired-samples t test; n = 15).
Main effect of posture (RM ANOVA with least significant difference pairwise comparisons; n = 9).
MVC, maximum voluntary contraction; sEMG RMS, root mean square of surface electromyography; TA, tibialis anterior.