The effect of knee angle and subject position on plantar flexors isokinetic performance and muscular activity

Abstract

BACKGROUND:

Assessment of the plantar flexion (PF) isokinetic performance has been greatly diverse and based on personal preferences rather than standardized guidelines.

OBJECTIVE:

To examine the performance of the plantar flexors under different settings including knee joint angle and subject position.

METHODS:

Thirteen women and 20 men took part in this study. The isokinetic protocol (60 ${}^{\circ}$ /s) was set to ankle movement between 10 ${}^{\circ}$ dorsiflexion to 30 ${}^{\circ}$ PF. Participants performed three repetitions of concentric PF in randomly-ordered knee angles; 15 ${}^{\circ}$ , 45 ${}^{\circ}$ and 90 ${}^{\circ}$ , and in seated and supine positions. Surface electromyography (EMG) data were collected from the Soleus (SOL) and Gastrocnemius.

RESULTS:

Knee angle impacted the PF moment ( $P\leqslant$ 0.001–0.026) and work ( $P\leqslant$ 0.05) measures in both genders. The moment and work measures were significantly less in the 90 ${}^{\circ}$ than those in the 45 ${}^{\circ}$ and 15 ${}^{\circ}$ positions. The 45 ${}^{\circ}$ position had the highest values, particularly in sitting in the male participants. Only the GL EMG data was significantly impacted ( $P=$ 0.017) by the subject position. However, the difference was trivial (1.6%). The SOL muscle showed a consistent pattern of increased activity when the knee was in flexion.

CONCLUSION:

The 45 ${}^{\circ}$ position seems to be optimal for obtaining the highest isokinetic PF scores.

Keywords

Knee position Triceps Surae moment electromyography

1. Introduction

Assessment of muscular performance is an essential element of the physiotherapeutic practice, especially in the musculoskeletal domain. A valid muscle performance has to ensure that that the tested muscle/s produce maximal moment. This necessitates placing the joints and surrounding muscle/s in optimum position and length, respectively, for maximum exertion. Achieving such optimization could be a challenge, particularly with structurally intricate muscle groups such as the Triceps Surae (TS), which is comprised of two main muscles, the Gastrocnemius (GC) and the Soleus (SOL). Traditionally, both the GC and SOL muscles have been treated as one unit although distinctive functions of these muscles have been reported, particularly during walking and landing activities [1, 2].

However, the assessment of plantarflexion (PF) strength has to a great extent not been standardized. A subject could be assessed in a supine [3, 4], sitting [5, 6, 7], prone [8, 9, 10, 11], or even side lying position [12, 13]. The knee position in these studies ranged from full extension to 90 ${}^{\circ}$ flexion, which affects the activity of the TS group (mainly the GC) by changing its length. In one study, knee angle was not controlled across the different subject positions [14, 15]. Such procedural diversity limits comparability, results in questionable conclusions and can undermine the achievement of the highest moment productions.

Isokinetic assessment/rehabilitation of the TS group may be performed in different positions. The sitting position forces the subject to bend the hip joint, which would elongate the Hamstrings that overlaps the origin of the GC muscle. Thus, hip joint position might indirectly affect the activity of the plantarflexors. Additionally, the sitting position is less stable than lying position due to the vertical orientation of the trunk, which might impact the performance of the distal muscles such as the TS. Accordingly, it seems that the subject position during isokinetic assessment/rehabilitation of the TS muscles is significant.

Therefore, the objectives of this study were to examine the isokinetic evaluation of PF strength under different procedural settings including different knee joint angles and subject positions, and explore the positional settings that would result in maximal performance.

2. Methods

2.1 Participants

Thirteen women and 20 men, aged 18–45 years with a BMI of 18–25, took part in the study (Table 1). Participants were excluded if they suffered any musculoskeletal or neurological disorders of the trunk and/or lower extremities, participated in athletic activity on a regular basis for at least a year before joining the study, or were employed in jobs imposing extensive physical exertion on the lower extremities. The study conformed with The Code of Ethics of the World Medical Association (Declaration of Helsinki). All participants signed a consent form approved by the institutional review board (Prince Sultan Military College of Health Sciences, IRB-2018-PT-006) after being informed of the procedures and risks of the investigation.

Table 1
Participants’ demographic data

Parameter	Males ( $n=$ 20)	Females ( $n=$ 13)	Total ( $n=$ 33)
	Mean (SD)	Mean (SD)	Mean (SD)
	(95% CI)	(95% CI)	(95% CI)
Age (y)	21.2 (1.1)	24.4 (4.4)	22.4 (3.2)
	(18.8–22.4)	(22.8–25.9)	(21.3–23.6)
Weight (Kg)	64.4 (9.5)	55.2 (8.7)	60.8 (10.2)
	(60.2–68.6)	(50.0–60.4)	(57.2–64.4)
Height (cm)	170.8 (4.7)	155.7 (6.2)	164.9 (9.2)
	(168.4–173.3)	(152.7–158.8)	(161.6–168.1)
BMI	22.1 (2.8)	22.6 (2.2)	22.3 (2.6)
	(20.8–23.2)	(21.1–24.1)	(21.3–23.2)

BMI: Body mass index, SD: Standard deviation, CI: Confidence interval.

2.2 Study design

This study employed a repeated-measures design. The knee joint angles examined were 15 ${}^{\circ}$ , 45 ${}^{\circ}$ and 90 ${}^{\circ}$ . The subjects were tested in supine lying and sitting positions. The dependent variables were PF moment, power, work, ankle angle of peak moment, and EMG activity of the SOL and the medial (GM) and lateral (GL) heads of the GC. Data was collected from the dominant lower extremity. Three trials were performed and averaged for each test condition. The order of test conditions was randomized beforehand to avoid the learning effect.

2.3 Instrumentation

In this study, a Biodex 4 pro multi-joint dynamometer (Biodex Medical Systems Inc, Shirley, NY, USA) was used. System calibration was performed each session prior to data collection. During the sitting test condition, participants were seated in the Biodex chair with the back support inclined to 70 ${}^{\circ}$ . During the supine test condition, the dynamometer’s back support was lowered to a flat horizontal alignment. The dynamometer supporting pads and straps, and adjustable height were utilized to position the knee in the targeted angle. The movement velocity was set at 60 ${}^{\circ}$ /s. The range of ankle movement was set between 10 ${}^{\circ}$ dorsiflexion and 30 ${}^{\circ}$ plantarflexion.

An eight-channel Delsys Trigno wireless EMG system (Delsys Inc., Natick, Massachusetts, USA) was used to collect the EMG data from the GM, GL and SOL muscles. The recording unit comprised two parallel Silver bar electrodes with a 1 cm inter-electrode distance. Surface EMG data was collected at a sampling rate of 2000 Hz using EMGworks ${}^{\text{\textregistered}}$ software. This system applied a band-pass filter of 10–500 Hz; had a common mode rejection ratio of $>$ 80 dB; a baseline noise of $<$ 0.75 uV RMS and a gain of 1000. Full wave rectification was performed to calculate the root mean square (rms) amplitudes. The greatest rms amplitudes recorded during maximum voluntary isometric contraction (MVIC) trials was utilized for normalization.

2.4 Procedures

After signing the consent form, the participant’s skin was prepared for placement of EMG electrodes according to the SENIAM recommendations [16]. Electrodes were placed over the SOL, GM, and GL as described before [17]. Signal quality check was performed prior to data collection to detect baseline noise or artifacts.

Participants were instructed to assume the testing position. The trunk and pelvis were stabilized by Velcro straps. The knee joint was placed in the predetermined angle. Finally, the foot was secured to the footplate. The anatomical axis of the ankle joint (Lateral Malleolus) movement was aligned with that of the dynamometer.

The participant assumed the first testing position (supine or sitting) and performed 3–5 familiarization trials. Participants performed a three-second MVIC of the PFs in each test position with the ankle locked in neutral position. This was followed by a five-minute rest. Then, the participant performed the data collection trials. Three repetitions were performed for each test condition with a two-minute rest in-between. Data collection for the different knee angles was completed in the respective participant’s position before moving to the next one. This was followed by a five-minute rest, during which the participant assumed the other testing position and the same procedures were repeated.

2.5 Statistical analysis

Data was analysed using SPSS version 21 (SPSS, Chicago, IL). The mean $\pm$ standard deviation (SD) values were computed for descriptive statistics. A two-way analysis of variance (ANOVA) was employed to analyze the main and interaction effects of the independent variables (subject position $\times$ knee joint angle). When a significant effect is detected, pairwise comparisons using Tukey’s HSD post hoc analysis was performed to explore the source of significance. The significance level ( $\alpha$ ) was set at $P\leqslant$ 0.05.

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Table 2
Mean (SE) and 95% CI of the isokinetic data and normalized EMG of male participants as a function of subject position and knee angle main effect

	Position		Knee angle
	SUP	SIT	15 ${}^{\circ}$	45 ${}^{\circ}$	90 ${}^{\circ}$
Pk-M	84.6 (4.5)	86.6 (5.3)	85.1 (4.9)	90.7 (5.8)*	81.1 (4.2)
	(75.2–93.9)	(75.5–97.7)	(74.8–95.4)	(78.5–102.8)	(72.2–89.9)
Avg. Pk-M	80.7 (4.4)	81.4 (5.1)	80.3 (4.9)	85.3 (5.6)*	77.4 (4.2)
	(71.5–89.8)	(70.7–92.1)	(70.2–90.5)	(73.7–96.9)	(68.5–86.3)
Pk-M/BW	132.1 (6.1)	134.6 (7.1)	132.3 (6.6)	140.9 (8.1)*	126.6 (5.6)
	(119.4–144.6)	(119.7–149.4)	(118.5–146.1)	(124.1–157.8)	(114.9–138.3)
Avg. Pk-M/BW	125.7 (6.1)	126.3 (6.8)	124.8 (6.5)	132.6 (7.8)*	120.7 (5.8)
	(113.2–138.3)	(111.9–140.7)	(111.1–138.5)	(116.3–148.9)	(108.6–132.8)
Avg. Power	35.8 (3.2)	36.5 (3.6)	35.2 (3.1)	37.9 (4.1)	35.3 (3.3)
	(29.1–42.6)	(29.1–43.9)	(28.8–41.6)	(29.5–46.5)	(28.4–42.2)
Work	128.6 (7.8)	131.4 (8.8)	127.9 (8.2)	136.7 (9.5)*	125.3 (8.2)
	(112.2–144.9)	(112.9–149.9)	(110.8–145.1)	(116.9–156.6)	(108.2–142.5)
SOL	90.8 (1.4)	89.08 (1.5)	89.1 (2.2)	90.8 (1.2)	91.1 (1.4)
	(87.8–93.9)	(86.6–92.8)	(84.4–93.6)	(88.3–93.3)	(88.2–94.1)
GM	88.8 (2.4)	88.2 (2.3)	88.7 (2.3)	88.2 (2.3)	88.5 (2.6)
	(83.9–93.5)	(83.3–93.1)	(83.9–93.5)	(83.4–92.9)	(83.1–93.9)
GL	92.3 (0.8)	90.7 (0.7)	91.6 (0.7)	91.6 (0.8)	91.2 (1.1)
	(90.7–93.8)	(89.2–92.2)	(90.2–93.1)	(89.9–93.4)	(88.8–93.6)

Bold numbers indicate significant difference at $p<$ 0.05. *Indicates significant difference between 90 ${}^{\circ}$ and 45 ${}^{\circ}$ . SE: Standard Error, CI: Confidence Interval, SUP: Supine position, SIT: Sitting position, Pk-M: Peak Moment, Avg.: Average, BW: Body Weight, SOL: Soleus, GM: Medial Gastrocnemius, GL: Lateral Gastrocnemius.

Table 3

Mean (SE) and 95% CI of the isokinetic data and normalized EMG of female participants as a function of subject position and knee angle main effect

	Position		Knee angle
	SUP	SIT	15 ${}^{\circ}$	45 ${}^{\circ}$	90 ${}^{\circ}$
Pk-M	41.3 (3.5)	39.9 (3.1)	39.1 (3.3)	45.4 (4.3),*	37.4 (2.6)
	(33.7–48.8)	(33.3–46.7)	(31.9–46.1)	(36.1–54.9)	(31.8–43.1)
Avg. Pk-M	37.4 (3.3)	35.9 (3.2)	35.5 (3.3)	41.2 (4.4),*	33.3 (2.4)
	(30.3–44.6)	(28.8–42.9)	(28.4–42.6)	(31.6–50.8)	(28.1–38.4)
Pk-M/BW	75.5 (6.1)	72.6 (4.7)	71.5 (5.7)	82.2 (6.8),*	68.3 (4.3)
	(62.3–88.7)	(62.3–82.7)	(59.1–83.9)	(67.4–97.1)	(58.9–77.8)
Avg. Pk-M/BW	68.4 (5.6)	65.2 (5.1)	65.1 (5.8)	74.6 (7.1),*	60.6 (3.8)
	(56.2–80.5)	(54.1–76.2)	(52.4–77.8)	(59.3–89.9)	(59.3–68.9)
Avg. Power	14.1 (1.8)	12.7 (1.6)	13.4 (1.8)	15.4 (2.3)*	11.4 (1.1)
	(10.2–18.1)	(9.3–16.2)	(9.5–17.3)	(10.4–20.5)	(9.1–13.8)
Work	58.6 (6.1)	54.9 (6.5)	56.6 (6.5)	62.9 (8.9)*	50.8 (4.2)
	(45.3–71.9)	(40.7–69.2)	(42.2–70.9)	(43.6–82.2)	(41.8–59.9)
SOL	88.9 (2.1)	87.7 (2.6)	85.1 (3.7)	89.1 (2.5)	90.7 (1.1)
	(84.3–93.4)	(81.8–93.7)	(76.8–93.4)	(83.5–94.7)	(88.3–93.1)
GM	92.3 (1.3)	91.7 (1.8)	91.2 (1.7)	93.1 (1.4)	91.8 (1.5)
	(89.5–95.2)	(87.6–95.7)	(87.4–95.1)	(89.9–96.1)	(88.5–95.1)
GL	91.4 (2.2)	90.9 (2.8)	90.8 (2.8)	91.7 (2.1)	90.9 (2.7)
	(86.4–96.3)	(84.7–97.2)	(84.4–97.1)	(86.9–96.5)	(84.8–97.1)

Bold numbers indicate significant difference at $p<$ 0.05. *Indicates significant difference between 90 ${}^{\circ}$ and 45 ${}^{\circ}$ . **Indicates significant difference between 15 ${}^{\circ}$ and 90 ${}^{\circ}$ . SE: Standard Error, CI: Confidence Interval, SUP: Supine position, SIT: Sitting position, Pk-M: Peak Moment, Avg.: Average, BW: Body Weight, SOL: Soleus, GM: Medial Gastrocnemius, GL: Lateral Gastrocnemius.

3. Results

3.1 Isokinetic measures

Participants’ demographic data are presented in Table 1. A sample of 33 (13 females) volunteers was recruited for the study. Considering the reported gender-based difference in muscle strength, the data was stratified by gender and reported accordingly. Results revealed that subject position did not significantly influence the isokinetic measures of the PFs in both males and females. However, knee joint angle had a significant main effect on most of the measures including all the moment ( $P\leqslant$ 0.001–0.026) and work ( $P\leqslant$ 0.05) variables for both genders, and the average power ( $P=$ 0.023) for females only (Tables 2 and 3). Post-hoc analysis showed that the 45 ${}^{\circ}$ knee angle was significantly different than the 90 ${}^{\circ}$ position ( $P=$ 0.006–0.047) for both genders for all the moment and work variables. Similarly, the moment variables in the 15 ${}^{\circ}$ position were different ( $P=$ 0.013–0.031) than the 45 ${}^{\circ}$ position, but for females only. Furthermore, the 45 ${}^{\circ}$ knee angle had the highest values for all the measured isokinetic variables, while the 90 ${}^{\circ}$ had the lowest. The average difference for both genders between the values recorded in the 90 ${}^{\circ}$ and 45 ${}^{\circ}$ knee joint angles was 8.8 Nm, 7.9 Nm, 14.1 Nm/Kg, 12.9 Nm/Kg, and 11.7 Joules for peak moment, average peak moment, peak moment/bodyweight, average peak moment/bodyweight and total work, respectively.

The subject position and knee joint angles did not have an interaction effect on any of the outcome variables ( $P>$ 0.05) (Fig. 1). However, a thorough analysis of the data revealed that the 45 ${}^{\circ}$ knee angle in the sitting position had the highest scores for all the isokinetic measures in male participants. Similarly, in female participants the 45 ${}^{\circ}$ knee angle had the highest magnitude for all the measured variables, regardless of the position.

Figure 1.

Isokinetic data for both genders as a function of position and knee angle interaction. The horizontal axis represents knee angles. BW: Body Weight.

3.2 Electromyography

The results showed that the subject position and knee angle main effect and its interaction with knee joint angle did not significantly affect the myoelectric activity of the PF muscles. Only the GL muscle in male participants was significantly affected by the subject position ( $P=$ 0.02). However, the difference between the sitting and supine positions was trivial (1.6%).

Interestingly, the SOL activity showed a consistent pattern for both genders where its activity increased with flexing the knee. This was particularly more noticed among female participants where a difference of 7% was measured between the 15 ${}^{\circ}$ and 90 knee angles in the sitting position (Fig. 2).

Figure 2.

Normalized EMG data for both genders as a function of position and knee angle interaction. The horizontal axis represents knee angles. GL: Lateral Gastrocnemius, GM: Medial Gastrocnemius, SOL: Soleus.

4. Discussion

4.1 Isokinetic performance measures

Overall, the results showed that the isokinetic moment and work measures of the PFs were mainly affected by the knee angle. The moment and work measures decreased in knee flexion compared with extension. This is in agreement with several previous studies that reported increased PF moment in extended knee position, and vice versa [5, 18, 19, 20, 21, 22, 23]. However, it is important to notice that this relationship was not consistent across all knee angles investigated in the current study. The moment and work values recorded in the 45 ${}^{\circ}$ knee angle were higher than those recorded in the 90 ${}^{\circ}$ (flexed position) and 15 ${}^{\circ}$ (extended position) knee angles. Few studies examined the PF moment in the 45 ${}^{\circ}$ knee angle, and they reported findings that are in accordance with ours [3, 4, 18].

The length-tension relationship has been postulated as a key relationship influencing the performance of the PFs. A shorter muscle length, as in flexed knee, would make the PFs mechanically disadvantaged limiting its force productivity. This is further supported by the fact that the two-joint GM and GL of the TS group have been reported to work on the ascending limb of the length-tension relationship curve, which is attributable to the anatomical constraints of the knee and ankle joints [21]. Furthermore, architectural changes including fascicular length and pennation angles could manipulate the muscular performance. Placing the muscle-tendon unit of the PFs in a shortened condition would decrease the fascicle length and increase the pennation angle, which was postulated to decrease the overall force transmitted through the muscle-tendon unit to the calcaneous [24]. Shorter PFs length was found to be associated with decreased moment productivity [22, 25] and inhibited neural drive to the Gastrocnemius [13, 20, 26]. However, evidence showed that the SOL fascicle length is not influenced by knee position as the Gastrocnemius does, despite the changes in moment productivity [12, 24] and muscular activity of the PFs [27]. Accordingly, the reduction of PF moment and work in the flexed (90 ${}^{\circ}$ ) compared with the extended position (15 ${}^{\circ}$ ) observed in our study could be attributed to the architectural characteristics and contractile behavior of the PFs.

To examine the impact of knee position on PF performance, previous studies typically compared flexed (mostly 90 ${}^{\circ}$ ) versus extended (mostly 0 ${}^{\circ}$ ) knee positions and made conclusions considering two knee positions only [5, 8, 19, 23, 28]. This led to support the generalized notion that PF moment decreases with knee flexion. However, very few studies examined the PF moment in other knee positions, particularly the 45 ${}^{\circ}$ position [4, 18]. Interestingly and in accordance with our findings, the 45 ${}^{\circ}$ knee position resulted in greater peak PF isokinetic moment and power than the extended position [4]. Additionally, Landin et al. [18] reported that the greatest PF simulated moment was at 45 ${}^{\circ}$ knee position.

To explain the discordant findings associated with the 45 ${}^{\circ}$ knee angle, the following explanations may be considered. Procedural differences could potentially be a cause for the inconsistent findings. The current study implemented an isokinetic testing protocol, while previous studies mostly utilized an isometric protocol at predefined fixed knee or ankle positions. The mechanism of actin-myosin interaction and the formation of cross-bridges could be different between the two modes of contraction, and it may have likely impacted the PF moment development. During isometric contraction the muscle fascicle length is fixed and, accordingly, the actin-myosin interaction can occur in a way that corresponds well with the principles of the length-tension relationship. On the other hand, during isokinetic contraction the muscle fascicle length continuously changes at one end during joint movement. It appears that the length-tension relationship may not work optimally in such case [29]. Additionally, the testing procedures involved ankle movement starting at 10 ${}^{\circ}$ dorsiflexion, which elongates the GM fascicles [22] and may have compensated for the potential fascicular shortening that could have occurred by the 45 ${}^{\circ}$ knee position, and hence, increasing its force exertion capacity [18]. Taken together, it is reasonable to assume that during an isokinetic performance, the 45 ${}^{\circ}$ knee position may not have significantly impacted the GM fascicular length similarly as the 90 ${}^{\circ}$ position, which was reflected on the PF moment and work measures in each test condition of the current study.

In the current study, participants were instructed to actively return the foot, as fast as possible, to the starting position (10 ${}^{\circ}$ dorsiflexion) at a resistance-free velocity (360 ${}^{\circ}$ /s) after each PF. This could be considered as an active lengthening process of the TS muscle that could have developed a residual force enhancement (RFE) phenomenon and, thus, enhanced the moment productivity of the PFs [30]. A recent study confirmed the occurrence of RFE in the human PFs and reported a significantly greater RFE (greater PF moment) in long (15 ${}^{\circ}$ dorsiflexion) compared with short (0 ${}^{\circ}$ ) conditions [31]. Additionally, Dalton et al. [6] reported a significantly greater PF RFE in flexed (90 ${}^{\circ}$ ) than extended (180 ${}^{\circ}$ ) knee position, suggesting that the PF RFE is mainly driven by the SOL. Thus, recent evidence supports greater PF RFE with dorsiflexed ankle and bent knee, which may resemble the 45 ${}^{\circ}$ test condition in our study.

Our results showed that the PF isokinetic measures were not influenced by the subject position. However, sitting had slightly higher measures than supine for all the variables in the male participants whereas the findings of the female participants showed the opposite pattern. Previous studies reported variable findings about the influence of position on the PF performance. The PF moment was shown to be greater in the sitting than the prone position [14]. The isometric PF moment was reported to be significantly higher in the supine and prone than the sitting position [11], which disagrees with the current results. Additionally, the superiority of the measured PF performance in the supine and prone positions was instruction-dependent. Previous reports have shown that coactivity of other lower extremity muscles, such as the knee extensors, during a PF exertion could significantly influence the PF moment and work measures [9, 10]. Thus, it seems that several factors could modulate the PFs mechanical performance in different subject positions including instructions, muscular co-activation and joint angle, and probably gender, which could explain the inconsistent findings across the literature.

4.2 Plantar flexors electromyography

The current results show that the PFs muscular exertion was not affected by the knee angle or subject position, except the GL muscle in male participants only. Additionally, The SOL activity apparently increased in flexed (45 ${}^{\circ}$ and 90 ${}^{\circ}$ ) more than extended knee position, which agrees with previous reports [13, 17, 26]. On the other hand, other studies reported inconsistent findings regarding the SOL activity in response to variable knee positions. The SOL activity was reported to either be unaffected by knee position [19, 20, 22, 27] or increase with knee flexion [13, 26] whereas in disagreement with our results, Gastrocnemius activity was reported to decrease with knee flexion [20, 22, 27]. The reciprocal activity of the TS muscles with different knee positions was attributed to various factors. These included selective activation of the GM and SOL muscles in different knee positions [13], inhibitory neural input to the GM motoneuron pool in flexed knee position [20, 22, 26], and reduced firing rate and increased recruitment threshold of the GM motor units with knee flexion [19]. Other factors as greater activation reduction of the SOL with knee flexion during a force-enhanced isometric contraction [6], and spinal and supraspinal inhibitory mechanism were also presented [28].

Some study limitation ought to be considered, though. We implemented an isokinetic assessment protocol only. Other muscle contraction approaches, e.g. eccentric, may provide further valuable insights to illustrate the PF performance under similar assessment procedures. The study sample comprised young and healthy participants, which may limit the generalizability of the findings to other populations as the elderly and those with musculoskeletal conditions of the lower limb. Finally, the detection of the EMG activity of the PF muscle using needle electrodes may have yielded a more distinguished activity of the motor units of each muscle distinctively. Unfortunately, such invasive procedures were inapplicable in the current study.

5. Conclusion

Considering the above-mentioned elaboration, the 45 ${}^{\circ}$ knee position used in the current study may provide the optimum condition for an isokinetic test of PF moment and work productivity.

Author contributions

CONCEPTION: Ahmed Farrag, Walaa Elsayed and Eidan Alzahrani

PERFORMANCE OF WORK: Walaa Elsayed, Moath Almusallam and Nora Almulhim

INTERPRETATION OR ANALYSIS OF DATA: Ahmed Farrag, Walaa Elsayed and Zaenab Alowa

PREPARATION OF THE MANUSCRIPT: Ahmed Farrag and Walaa Elsayed

REVISION FOR IMPORTANT INTELLECTUAL CONTENT: Ahmed Farrag, Walaa Elsayed, Eidan Alzahrani, Moath Almusallam, Nora Almulhim and Zaenab Alowa

SUPERVISION: Ahmed Farrag

Ethical considerations

This study was approved by the institutional review board of the Prince Sultan Military College of Health Sciences (IRB-2018-PT-006). All participants signed a written informed consent.

Funding

This work was supported by the Prince Sultan Military College of Health Sciences, Saudi Arabia (grant number 2017-PT-003).

Footnotes

Acknowledgments

The authors would like to thank all the participants for their time and effort.

Conflict of interest

The authors report no conflict of interest.

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The effect of knee angle and subject position on plantar flexors isokinetic performance and muscular activity

Abstract

BACKGROUND:

OBJECTIVE:

METHODS:

RESULTS:

CONCLUSION:

Keywords

1. Introduction

2. Methods

2.1 Participants

Table 1 Participants’ demographic data

2.3 Instrumentation

2.4 Procedures

2.5 Statistical analysis

Table 2 Mean (SE) and 95% CI of the isokinetic data and normalized EMG of male participants as a function of subject position and knee angle main effect

3.1 Isokinetic measures

4.1 Isokinetic performance measures

4.2 Plantar flexors electromyography

5. Conclusion

Author contributions

Ethical considerations

Funding

Footnotes

Acknowledgments

Conflict of interest

References

Table 1
Participants’ demographic data

Table 2
Mean (SE) and 95% CI of the isokinetic data and normalized EMG of male participants as a function of subject position and knee angle main effect