Quadriceps performance under activation of foot dorsal extension in healthy volunteers: an interventional cohort study

In this monocentric cross-sectional pilot study, healthy volunteers were recruited at the rehabilitation clinic (“RehaClinic”), Bad Zurzach, Switzerland (orthopedic and neurological rehabilitation). First, all employees of the clinic were informed and asked to participate by e-mail. Recruiting was stopped when the predetermined minimum sample size was reached (see Analysis). Inclusion criteria were healthy subjects between 18 and 60 years and agreement to participate by informed consent. Excluded were persons having musculoskeletal, neurological, or joint disease. No financial or other recompense was given. The project was approved by the local ethics committee, the “Kantonale Ethikkommission (KEK) des Kanton Aargau (AG)” (cantonal ethic committee of the canton Aargau, Switzerland), KEK AG 2010/063.

The test procedure was performed as described in the flowchart of Fig. 1. EMG testing on the leg press was performed at angles of 45°and 90° of knee flexion. Between 0° and 45°, stress on the femoropatellar joint is minimized and safety for the subject is high [3, 4]. At 45° knee flexion, rectus femoris and biceps femoralis do not only act as antagonists but also as synergists [15]. Leg press at 90° knee flexion has been performed in several previous studies that provide comparative data [7, 14]. For pre-activation of dorsal foot extension, the subjects had to walk 10 steps on the heel. On the leg press, dorsal foot extension was performed by each individual on the command of the examiner without any supports or assistance. It was ensured that only the heel and not the forefoot contacted the plate on the leg-press. To control that dorsal foot extension was really performed and to quantify it during testing (with compared to without foot dorsal extension), the EMG was derived from the m. tibialis anterior. Since detailed EMG analysis of the single leg hop was not a main focus of the study (the main focus being on the quadriceps, the most important knee extensor), and the m. gastrocnemius (antagonist) is relaxed under dorsal foot extension, this muscle was not examined by EMG.

The 12-repetition maximum (12RM) weight was identified per individual and defined as the load on the leg press so that the current findings could be compared with previous EMG recordings and training settings [3, 16]. It is used in most of the comparable studies. The 12RM is the preinstalled weight that the subject is able to press 12 times at maximum. The second reason to prefer the 12RM was to be closer to realistic muscle training that generally recommends 10–20 repetitions rather than 1 repetition (1RM), for example [3]. The 12RM corresponded to 67 % of the 1RM and was determined by standardized protocol [17]. After initial estimation, it was tested in 2–3 passes where the weight was successively changed by 10-20 % with recovery pauses of 2 min each between the tests.

After holding the correct knee flexion angle and the correct foot position on the plate, the proband had to press the isometric 12RM on the leg press. During this activity, EMG was recorded twice in 2 s. The average of the two EMG measurements was taken for further analysis.

The EMG recorder was rented from Prophysics, Switzerland (registered trade mark). It had the capacity to measure all four muscles simultaneously, the m. tibialis anterior, m. biceps femoris, m. vastus medialis, and m. rectus femoris. The sum of all activated muscle fibers, i.e. motoneural units by depolarization and repolarization, results in a recordable, electric surface tension. It is assumed to be proportional to the effort of the measured muscle [11]. This particular recorder had a sampling frequency of 4000Hz and an amplifier bandwidth frequency of 5-1000Hz. The EMG signal above 400Hz (high pass filter) and below 10Hz (low pass filter) was filtered off and the frequencies in between were smoothed by the software (“proEMG”) of the recorder to the root mean square (RMS) as the unit being used in most EMG studies [3, 16]. As its name says, the RMS is the square root of the mean of the squared single signals. The electrode “Ambu Blue Sensor N” (20×30mm) provided a sensitivity of 600Ω -AC-impedance at an internal random noise of <15 μV.

The single leg hop has high validity and test-retest reliability in the quantification of strength and performance of the leg [18, 19]. Arm position was not prescribed [18]. Hop distance was always measured by the same person (M.K.) using the same measuring tape from the top of the hallux to the most posterior point of the heel. After one training hop, the second hop was measured and taken for further analysis.

EMG data were quantified by root mean squares (RMS) in microvolt (μV), single hop distance in centimeters (cm), and were described by their arithmetic means and standard deviations. Since intra-individual RMS differences were calculated, any possible systematic bias (e.g. by variable muscle activation patterns and quality of the skin between subjects) has been eliminated. To illustrate the activity of the agonist compared to that of the antagonist, the RMS ratio of the quadriceps (m. rectus femoris) to the antagonist (m. biceps femoris) was determined.

Intra-individual differences between two examinations were described and expressed as standardized response means (SRM). The SRM equals the group mean of the differences (between the tests with and without dorsal foot extension) derived from RMS-values or single leg hop distances for each individual divided by the standard deviation of those intra-individual differences of the group. Derived from the original Glass’s delta, the SRM is part of the family of effect sizes and quantifies the change of the whole group of participants (with minus without dorsal foot extension for each individual) in number of standard deviations of those changes [20‐22]. It can be related to standardized mean differences of randomized controlled studies and is, in this sense, a modern standard of parametrizing effect sizes [22]. A positive SRM means improvement between two tests (with dorsal foot extension versus without dorsal foot extension). An SRM of 0.50-0.79 reflects moderate effects, an SRM ≥ 0.80 large effects.

To statistically detect at least a moderate effect, i.e. ≥half of the standard deviation of the differences, a minimum sample size of n ≥ 32 for intra-individual change data with an a priori type I error of p = 0.05 (5 %) and a power of 0.80 (type II error of 0.20) was required [21]. Since the changes of EMG and hop distances were symmetrically but only approximately normally distributed, the score changes were statistically compared by application of the Wilcoxon rank sum test. The non-parametric Wilcoxon test tests more conservatively than the t-test, i.e. significant results by the Wilcoxon test will also be significant by the t-test, but not vice versa. All analyses were performed using the statistical software SPSS 22.0 for Windows (SPSS Inc., Chicago, IL, USA).

The first 35 subjects who replied to the invitation and who met the inclusion criteria were recruited, whereby 3 of these were included in addition to the required 32 as a reserve. Twenty-three (66 %) of the 35 subjects were women. Mean age was 34.5 years (range 21-57years). Mean height was 170.7 cm (158-188 cm) and mean body weight was 65.2 kg (47-88 kg). They pressed a mean 12RM weight of 79.7 kg (40-160 kg) on the leg press.

The RMS (μV) without dorsal foot extension was relatively low and comparable for all 4 muscles at 45° knee flexion (Table 1). As indicated by the RMS increase for the tibialis anterior from mean 7.9 μV to 170.8 μV (SRM = 2.02), the activity of the m. vastus medialis with foot dorsal extension significantly grew by SRM = 1.39 and that of the m. rectus femoris by SRM = 0.71 (all p < 0.001).

At 90° knee flexion, all RMS levels were higher for all measurements (Table 1). Foot dorsal extension – proven by SRM = 2.18 for the m. tibialis anterior, which was comparable to that at 45° – resulted in a significant increase of SRM = 1.08 for the m. vastus medialis and of SRM = 0.89 for the m. rectus femoris (all p < 0.001).

The activity of the m. biceps did not increase at 45° (SRM = −0.20) but at 90° (SRM = 0.97). The mean RMS ratio of the rectus femoris to the biceps femoris increased from 0.66 without to 1.52 with foot dorsal extension at 45° knee flexion (computed from the values of Table 1). At 90° knee flexion, the ratio increased from 3.38 to 4.68.

After dorsal foot extension on the leg press (see Fig. 1), mean single leg hop distance increased from a mean of 169.8 cm (at hop 2, before dorsal foot extension) to 178.9 cm (at hop 3, after dorsal foot extension) yielding an increase of 9.1 cm, SRM = 1.09 (p < 0.001) (Table 2). Between hop 1 and hop 2, leg press without dorsal foot extension was performed, which did not significantly alter the average hop distance (168.2 cm to 169.8 cm).