Muscle activity and strength in maximum isokinetic legpress testing with unstable footplates in active individuals

Abstract

BACKGROUND:

Compensating unstable situations is an important functional capability to maintain joint stability, to compensate perturbations and to prevent (re-)injury. Therefore, reduced maximum strength and altered neuromuscular activity are expected by inducing instability to load test situations. Possible effects are not clear for induced instability during maximum legpress tests in healthy individuals.

OBJECTIVE:

To compare isokinetic legpress (LP) strength and lower-leg muscle activity using stable (S) and unstable (UN) footplates.

METHODS:

16 males (28 $\pm$ 4 yrs, 179 $\pm$ 7 cm, 75 $\pm$ 8 kg) performed five maximum LP in concentric (CON) and eccentric (ECC) mode. The maximum force (Fmax) and muscle activity were measured under conditions of S and UN footplates. The tested muscles comprised of the tibialis anterior (TA), peroneus longus (PL) and soleus (SOL) and their activity were quantified against the MVIC of each muscle respectively.

RESULTS:

The main finding revealed a significant reduction in Fmax under UN condition: 11.9 $\pm$ 11.3% in CON and 23.5 $\pm$ 47.8% in ECC ( $P<$ 0.05). Significant findings were also noted regarding the RMS derived values of the EMG of PL and TA.

CONCLUSION:

Unstable LP reduced force generation and increased the activity of PL and TA muscles which confirmed greater neuromuscular effort to compensate instability. This may have some implications for resistance testing and training coupled with an unstable base in the prevention and rehabilitation of injury to the neuromusculoskeletal system.

1. Introduction

Table 1

Subject characteristics (mean $\pm$ SD)

	$N$	Age (years)	Height (cm)	Weight (kg)	Physical activity (h/wk)	FAAM score (%)
						ADL	Sport
Mean $\pm$ SD	16	28 $\pm$ 4	179 $\pm$ 7	75 $\pm$ 8	7 $\pm$ 6	99 $\pm$ 2	98 $\pm$ 3

The use of unstable surfaces like balance balls and wobble boards as an element in resistance training has in recent years gained a special niche in rehabilitation and sport programs [5]. Resistance training with unstable surfaces (RTU) is alleged to enhance training adaptation and the neuromuscular control which reduce the risk of ankle sprain recurrence after a major ankle sprain by providing greater stimuli [4, 5, 6, 23] but in terms of outcome measures such as the force output and muscle activity, the effect of RTU is not clear. Several studies considered the acute effect of RTU on force generation and muscle activity with different stability devices to investigate the functional mechanisms of these exercises [7, 8, 9, 12, 14, 15]. Saeterbakken and Filmland reported that compared with a stable squat the force output measured by force cells attached to an Olympic bar was higher than unstable supporting surfaces with no differences in muscle activity [22]. However, no difference in one repetition maximum (RM) and EMG activity in the chest press in stable vs. unstable (UN) situations were reported by Goodman et al. [9]. These contradictory effects of RTU might be due to different methodological approaches used in the above studies. RTU is considered as one of the important parts of rehabilitation in recurrent ankle sprain however, the evidence is not consistence that RTU is more effective than strengthening using resistance [23]. Furthermore, it is important to apply relative loads instead of an absolute one to have a standard comparison as resistance training is prescribed as a repetition maximum load or a percentage of it. Therefore, more standardized test methods for analyzing and defining the RTU are necessary.

The unstable squat has been the most utilized testing situation for evaluating RTU by measuring force output and muscle activity of the lower limbs [22]. Unstable surface caused 34% reduction of 1RM in McBride et al. study while only a 7% loss of 6-RM was reported by Andersen et al. whilst squatting on a foam cushion [1, 17]. In the McBride et al. study [17], the type of the unstable condition was not specified, therefore; using different surfaces might be the reason behind these differences. Consequently, more standard unstable surfaces are needed in order to have comparable results. Isokinetic leg press (LP) is a closed kinetic chain method which uses a dedicated device for measuring total leg strength, in either bi-or unilateral mode [2, 8, 13, 15, 18]. No effect of gender, bilateral deficit and leg alignment on force and muscle activity of medial gastrocnemius, peroneus longus, biceps femoris, vastus lateralis, semitendinosus, biceps femoris during eccentric legpress exercise have found [15]. A difference was found between S and UN footplate in peak and mean force values [15]. Isokinetic legpress could get equipped with stable and unstable footplates Although the foot is fixed in this footplate, the footplate has some degree of freedom of inversion/eversion movement (unstable footplate characteristics can be found in the methods section). Therefore, the maximum dynamic strength under stable and UN conditions can be measured.

Given the unclear nature of some of the previous findings, the purpose of this study was to comparatively explore the effect of UN vs. stable support in performing maximum isokinetic legpress tests where the outcome measures include the maximum force output as recorded by the dynamometer and the EMG activity of the lower leg muscles. We hypothesized that UN support would have a detrimental effect on the force while enhancing the muscular activity of the ankle stabilizing muscles.

2. Methods

2.1 Participants

Sixteen men between 20 to 40 years of age participated in this study (Table 1). Subjects were included if they were healthy, without any lower limb complains and physically active for more than 2 hours per week (h/w). Participants with a history of previous surgeries to the musculoskeletal structures (i.e. bones, joint structures and nerves) in either lower extremity as well as a history of a fracture in either lower extremity requiring realignment were excluded. Further, subjects with acute injury to the musculoskeletal structures of other joints of the lower extremity in the previous 3 months were excluded [10]. In addition, ankle integrity was evaluated by the Foot and Ankle measure (FAAM) questionnaire [19]. Subjects were excluded if the ADL scale was $<$ 90% ( $<$ 76) and sport scale $<$ 80% ( $<$ 26) of the FAAM questionnaire. The study was approved by the institutional review board of Potsdam University (24.11.2016). Written informed consent was obtained for all participants before evaluation.

2.2 Instrumentation

Muscle activity of peroneus longus (PL), tibialis anterior (TA) and soleus (SOL) of the right leg were analyzed using bipolar surface EMG (Myon, Switzerland, bandpass filter: 5 $\pm$ 500 Hz; sampling frequency: 4000 Hz). The examiner prepared the skin of participants including shaving, sweeping and cleaning with disinfectant. Electrodes (AMBU Medicotest, Denmark, and Type N-00-S) were attached to the muscle belly of the PL, TA, and SOL. In addition, skin resistance was controlled by measuring skin impedance ( $<$ 5 k $\mathrm{\Omega }$ ) . To test lower limb maximum strength capacity in extension an isokinetic legpress (CON-TREX ${}^{\mathrm{\circledR }}$ LP, Germany) was used. Data were acquired from the right leg by use of a PC equipped with dedicated software which calculated and displayed force and joint displacement values. The footplates were covered with an anti-slip material and three fixation points (heel, instep and toe) to fix the foot. In contrast to UN (flexible) footplate which has 15 ${}^{\circ }$ freedom in each eversion and inversion, S (rigid) footplate movements were blocked in the longitudinal and mediolateral axis (Fig. 1).

Figure 1.

Isokinetic legpress device from frontal view and the flexible (unstable) footplate with 15 ${}^{\circ }$ freedom in each eversion and inversion.

Figure 2.

Testing procedures.

2.3 Testing procedures

Initially, height and weight were measured. Subjects were prepared for recording muscle activity and maximum isokinetic strength. Standard shoes were given to all subjects. The experimental procedures are depicted in Fig. 2.

Figure 3.

Subject fixed with right leg in the isokinetic legpress device in neutral ankle position (100 ${}^{\circ }$ of knee angle).

Figure 4.

Normalized muscle activity [%] of A) Tibialis anterior (TA), B) Peroneus longus (PL) and C) Soleus (SOL) muscle. RMS of TA in concentric mode and RMS of PL in both contractions were significant. *Significant difference between conditions ( $P<$ 0.006).

Subjects were secured in a seat with a harness across the trunk to limit compensatory total body movements (Fig. 3). The range of motion (ROM) was set to approximately 70 ${}^{\circ }$ (45 ${}^{\circ }$ to 115 ${}^{\circ }$ ) excluding a full extension. One-minute submaximal trials with a stable (S) footplate to familiarize the subjects with the isokinetic testing procedure were performed as warm-ups. All movements were performed at a velocity of 0.3 m/s and participants were given a minimum of 60 seconds break before the next trial commenced. Practice trials were performed until subjects were familiarized with the test set-up. The procedure was followed by 5-s of isometric contraction of the leg during dorsiflexion, plantar flexion and plantarflexion with eversion at 100 ${}^{\circ }$ of knee angle (neutral position) to obtain isometric maximum voluntary contraction (IMVC) of SOL, TA and PL muscles (Fig. 3). Test trials always began with five maximal concentric (CON) extensions, followed by eccentric (ECC) extension. Subjects were asked to keep their knee in medio-lateral direction straight while doing each trial. If participants did not understand the test and if they moved the knee into valgus or varus position the trial was repeated. The order of the two conditions S and UN footplate was randomized. The same examiner conducted the tests for all subjects.

2.4 Data reduction and statistical analysis

IBM SPSS Statistics 22 and Excel 2013 were used to analyze the data. The maximum force (Fmax) in N, was obtained from the average of the five trials in both CON and ECC contraction in the S and UN conditions. Root mean square (RMS) of the amplitudes of each trial were analyzed for all three muscles (TA, PL and SOL) (IMAGO process master, LabView ${}^{\mathrm{\circledR }}$ -based, Pfitec, biomedical systems, Endingen, Germany). Mean RMS of each muscle in S and UN condition was normalized to RMS of 50 ms before and after constant activity during isometric contraction for both CON and ECC extension. The Mean and SD of the values were calculated for all outcome parameters. Shapiro-Wilk-Test was applied to check the normality of the data. Paired t-tests were used to compare RMS and Fmax changes between S and UN conditions for both CON and ECC mode ( $P<$ 0.05). Bonferroni correction applied for rectifying multiple testing ( $P<$ 0.006).

Table 2

Mean and SD of legpress concentric and eccentric force [%] in two condition of stable and unstable footplates

Contraction	Condition	$F_{\max }$ [N]	%	$P$ value
		Mean $\pm$ SD
Concentric	Stable	1310 $\pm$ 228	11.9 $\pm$ 11.3	$<$ 0.001*
	Unstable	1171 $\pm$ 205
Eccentric	Stable	1605 $\pm$ 466	23.5 $\pm$ 47.8	$<$ 0.001*
	Unstable	1300 $\pm$ 315

*Significant differences between conditions, $P<$ 0.006.

3. Results

3.1 Variations in Fmax

The values obtained in the S and UN conditions in the concentric and eccentric mode are displayed in Table 2. A significant decline in force production was found using UN footplate in both dynamic contractions (concentric: 11.9 $\pm$ 11.3% and eccentric: 23.5 $\pm$ 47.8%). ECC contraction produced more force than CON in both stable (18.7 $\pm$ 50.9%) and unstable (9.9 $\pm$ 34.9%) footplate conditions.

3.2 Variations in normalized RMS

There was a significant difference in this variable between S and UN condition in PL muscle in ECC mode ( $S=$ 66.1 $\pm$ 28.3%, UN $=$ 88 $\pm$ 39.1%, $P<$ 0.006) as well as CON mode ( $S=$ 77.3 $\pm$ 27.5%, UN $=$ 62.8 $\pm$ 22.5%, $P<$ 0.006). Data from TA activity in CON contraction was also significantly different between S and UN footplates ( $S=$ 12.6 $\pm$ 5.4%, UN $=$ 30.2 $\pm$ 15.8%, $P<$ 0.006) but not in ECC extension ( $S=$ 15.8 $\pm$ 9.8%, $U=$ 24.8 $\pm$ 16.3%, $P>$ 0.006). However, there was no significant difference in SOL muscle activity in either CON ( $S=$ 94.6 $\pm$ 55.6%, UN $=$ 70.1 $\pm$ 39.3%, $P>$ 0.006) or ECC contraction ( $S=$ 76.8 $\pm$ 36.1%, UN $=$ 68.7 $\pm$ 39.7%, $P>$ 0.006) (Fig. 4).

4. Discussion

The main findings of this study were that maximum isokinetic force of lower limb reduced, and changes happened in muscle activity of tested muscles. Using unstable footplates during maximal isokinetic lower limb testing reduced maximum force which is in line with previous studies [3, 16, 17, 20]. The reduction in Fmax can be attributed to the need to stabilize the ankle joint at the expense of extending the knee.

Amount of force reduction in this study was different from previous experiments (12% CON, 24% ECC). McBride et al. in 2006 reported a 46% reduction in Fmax in isometric squatting on inflated balance disks [16]. Similar results were found in another study which indicated that maximal strength during UN squatting was 35% to 46% less than during a stable one [17]. Performing 6-RM of normal squat and Bulgarian squat on Swiss ball also reduced force by 7% and 10% respectively [1]. However, differences in the testing procedures could explain the variations in the rate of reduction. In the McBride et al. 2006 study, Fmax was measured statically while in the two other studies the squat was tested dynamically [1, 16, 17]. Furthermore, where the UN surface was provided by an inflated balance disk [16] dynamic normal and Bulgarian squat was performed using a Swiss ball [1, 16]. It, therefore, seems that not only different UN conditions but also the type of muscle contraction and strength tests can affect the amount of force production.

The present findings indicate that the reduction of force in ECC was almost twice as high as in the CON mode. Eccentric contractions generally produce the greater mechanical output [8] but the observed reduction could be due to different inhibitory mechanisms which are located at both pre-and postsynaptic sides of the motorneurons whose function it is to prevent potential damage to the muscle [8].

Muscle activity differences (35 $\pm$ 41.5% and 11.8 $\pm$ 9% reduction) of SOL as the agonist muscle in LP movement might be relevant but could not be shown significantly different in UN condition in both CON and ECC contractions in agreement with previous studies [1, 22]. However, it seems that the activity of the SOL muscle is affected by the nature of the exercise namely based on open or closed kinetic chains. In Anderson et al. [1], there was a significant increase in SOL activity in all conditions (squatting, free weights and Smith machine). Similar results were reported with respect to back squat during S and UN weight bearing [14]. Therefore, SOL activity was more noticeable under UN open kinetic chain exercises.

Unstable LP caused significant increases in TA activities as one of the antagonist muscles but only in CON contraction (58.3 $\pm$ 66%). Performing on unstable supporting surfaces seems to enhance antagonist activity which might provide more stability for the joint [3]. More TA activity was observed in our study than in Behm et al.’s where TA experienced a 30% increase in activity [3]. Furthermore, the EMG activity of the PL was significantly different in the UN condition. Stable concentric contraction showed higher muscle activity than its UN counterpart while UN eccentric contraction had higher EMG activity than under stable eccentric contraction. Ferreira et al. indicated that the muscle activity of TA and PL increased with increased instability [7, 11]. It seems that in CON contraction instability compensates mostly through co-contraction of agonist and antagonist muscles but in ECC contraction the role of the PL is more prominent.

In the present study, EMG activity of TA and SOL muscles during CON effort was higher than in ECC effort but not significantly for the small sample. Furthermore, PL activity in LP showed significantly higher RMS in ECC (64.5% increase) compared to the CON (16.9% decrease), supporting the role of PL muscle in controlling ankle movements, in the face of substantial reductions in SOL and TA activity during UN condition. The activity of SOL muscle in unstable condition decreased 35% and 11.8% in concentric and eccentric mode respectively, however, the lack of statistic difference might be due to high variability among subjects in this exercise. This change was also seen in TA muscle where activity increased 36.5% in ECC contraction.

The ability to measure force and muscle activity during eccentric contraction is especially meaningful since it is under eccentric conditions that injuries most often occur. The significant relative reduction in the ECC Fmax compared to its CON counterpart serves to highlight its important feature. Admittedly, omitting measurement of the quadriceps activity denies a stronger position on the issue of Fmax reduction and this certainly deserves another study. In addition, the sample consisted of male subjects only and given some pertinent inter-gender differences, a study involving women is needed. ACL injury in female athletes is reported to be 3 times higher in basketball and soccer than male athletes [21]. At last, a more homogeneous group in terms of sports discipline and the amount of weekly exercise need to be recruited to reduce the variability among the results.

In conclusion, reduction of maximum force and increased muscle activity of TA and PL as antagonist and stabilizers of the ankle joint are expected variations during the performance of isokinetic LP equipped with unstable footplates. This observation adds support to the notion that exercising under unstable support conditions could functionally enhance the strength of the ankle stabilizers thus serving in the prevention of ankle injuries and/or their rehabilitation.