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Abstract

BACKGROUND:

There is insufficient knowledge about the rate of force development (RFD) characteristics over both single and multiple joint movements and the electromechanical delay (EMD) values obtained in athletes and untrained individuals.

OBJECTIVE:

To compare single and multiple joint functions and the neural drive of trained athletes and untrained individuals.

METHODS:

Eight trained athletes and 10 untrained individuals voluntarily participated to the study. The neuromuscular performance was assessed during explosive and maximum voluntary isometric contractions during leg press and knee extension related to single and multiple joint. Explosive force and surface electromyography of eight superficial lower limb muscles were measured in five 50-ms time windows from their onset, and normalized to peak force and electromyography activity at maximum voluntary force, respectively. The EMD was determined from explosive voluntary contractions (EVC’s).

RESULTS:

The results showed that there were significant differences in absolute forces during knee extension maximum voluntary force and EVC’s ( $p<$ 0.01) while trained athletes achieved greater relative forces than untrained individuals of EVC at all five time points ( $p<$ 0.05).

CONCLUSIONS:

The differences in explosive performance between trained athletes and untrained individuals in both movements may be explained by different levels of muscle activation within groups, attributed to variation in biarticular muscle function over both activities.

Keywords

Maximum voluntary force rate of force development electromechanical delay neural activation

1. Introduction

Dynamic exercise involves multiple rapid joint movements and rhythmic contractions, where movements occur about several joints simultaneously [1]. Success in intense dynamic sports such as sprinting and jumping demands large explosive muscular contractions, which last approximately 50–250 ms [2, 3]. However, the time required to reach maximum force in some human muscles may exceed 300-ms [4]. Therefore, individuals with a large contractile rate of force development (RFD) (the slope of the force-time curve) will achieve greater force production during the initial phase of dynamic movement possibly leading to enhance performance during explosive contractions. Electromechanical delay (EMD), the time interval between the change in electrical activity at the muscle and the generation of force [5] is also an indicator of rapid neuromuscular performance. Theoretically, shorter EMD latencies would improve explosive performance by reducing muscle activation response time.

RFD and EMD are descriptors of performance in explosive contractions [5] however, studies comparing these physiological characteristics in trained athletes (TA) and untrained individuals (UTI) are limited and predominantly centred on single joint activities; yet information from multi-joint assessment is more related to dynamic performance [6]. It could be expected that differences, if any, between the groups may be more pronounced in more complex multi-joint tasks, as multiple joint movements are believed to require greater coordination and rely more heavily on muscle activation [7].

RFD, which describes the ability of the neuromuscular system to develop rapid action velocities, has been suggested to be the most important variable determining performance in explosive movements [8, 9, 10]. It is likely that the enhanced RFD in athletic groups is due to physiological adaptations to their training. Indeed, RFD can be improved specifically through resistance exercises designed to improve explosive performance [8]. Dynamic strength training regimens can enhance the afferent neural drive (the sum of the spiking activities of motor neurons) to muscles [11] leading to improved efferent neural responses [12], resulting to a shift of the force-time curve to the left [13] and subsequent increase in development of explosive strength [14] and thus, RFD. The link between neural activation and RFD is further evidenced by the strong positive relationships between both factors, documented by de Ruiter et. al. [9]. The degree of synchronicity in activation onset of agonist muscles may further influence RFD [5].

EMD depicts the time required for propagation of an action potential, the excitation contraction coupling process, initial tension development and stretching of the series elastic components (SEC) to remove the natural slack, thereby facilitating force transmission [15]. Explosive sporting activities such as sprinting, involve repetitive muscle stretch-shortening actions resulting in high muscle-tendon stiffness [16]. Stiffer tendons remove series elastic components slack at a faster rate by allowing increased tension development for each unit change in length of the muscle fibres, as validated by Blackburn et. al. [17], resulting in shorter EMD.

The insufficient knowledge concerning RFD characteristics over both single and multiple joint movements and the equivocal EMD results obtained with TA and UTI warrants further investigation. The purpose of this study was to compare the single and multiple joint functions and the neural drive of TA and UTI. Specifically, to compare the RFD and EMD in both groups during voluntary isometric contractions of the knee extensors and muscles involved in the leg press activity. Comparison of neural activation via analysis of electromyography (EMG) recordings may assist in delineating any observed differences in muscular performance between the groups over both tasks.

2. Methods

2.1 Partcipants

Eighteen healthy men [(TA): $n=$ 8, age $=$ 23.3 $\pm$ 3.3 years, height $=$ 182.5 $\pm$ 9.1 cm, weight $=$ 75.7 $\pm$ 10.6 kg, body fat (%): 6.03 $\pm$ 2.5 and (UTI): $n=$ 10, age $=$ 25.3 $\pm$ 5.7 years, height $=$ 179.4 $\pm$ 7.7 cm, weight $=$ 84.8 $\pm$ 11.6 kg, body fat (%): 14.5 $\pm$ 4.4] volunteered to participate in the study. The TA consisted of trained athletes regularly involved in strength/power training [ $\geqslant$ 8.5 hours training sessions per week (hour/day): calisthenics: 1.3 $\pm$ 0.8, plyometrics: 1.0 $\pm$ 0.6, sprints: 1.9 $\pm$ 0.5, weight training: 1.8 $\pm$ 0.4] for 24 months. The UTI consisted of light to moderately active [performing $\leqslant$ three sessions aerobic activity per week (hour/day): 0.8 $\pm$ 0.5] but were not engaged in strength/power training. All subjects were informed of the nature and purpose of the study and possible risks involved. A written informed consent was obtained from each player in accordance with the ethical standard of Helsinki Declaration. The study was approved by Loughborough University Ethical Advisory Committee (R10-P127). The participants were instructed to avoid strenuous physical activity for the 24-hour period prior to testing.

2.2 Procedure

The partcipants visited the laboratory for 60–120 min on two separate occasions separated by 7–10 days. Visits consisted of a familiarization and a test trial (conducted between the hours of 12.00 and 18.00). During both tests, subjects completed two seated exercises with their dominant legs. A modified leg press device (Precor, Woodinville, WA) and an isometric strength testing chair [18] were used to perform isometric maximum voluntary contractions (MVC) and explosive voluntary contractions (EVC) of the muscles. Measurements were taken in the following order: 1) leg press MVC’s, 2) leg press EVC’s, 3) knee extension MVC’s, and 4) knee extension EVC’s. Surface EMG of the Gluteus Maximus (GMAX), Vastus Medialis (VM), Vastus Lateralis (VL), Rectus Femoris (RF), long head of the Biceps Femoris (BF), Semitendinosus (ST), Soleus (SOL) and Gastrocnemius Lateralis (GL) were recorded throughout these contractions.

2.3 Surface EMG measurements

Surface EMG was obtained from the dominant leg muscles in the leg press and the knee extension, using two Delsys Bagnoli-4 EMG systems (Delsys, Boston, MA). After the skin was made ready by shaving, lightly abrading, and cleansing with 70% ethanol, double differential surface electrode configurations (10 mm inter-contact distance, model DE-3.1; Delsys, Boston, MA) were positioned at standardized percentages of the length measured from sacral vertebrae to greater trochanter (GMAX, 50%), from the anterior spina iliaca superior to the joint space in front of the anterior border of the medial ligament (VM, 80%), from the anterior spina iliaca superior to the lateral side of the patella (VL, 65%), from the anterior spina iliaca superior to the superior part of the patella (RF, 50%), from ischial tuberosity to medial epicondyle of the tibia (BF, 50%; ST, 50%), from medial condyle of femur to medial malleus (SOL, 66%), and head of the fibula to calcaneus (GL, 33%) [19]. The reference electrode was placed on the patella of the same limb for each subject. The same investigator completed skin preparation and surface electrode placement across all subjects. EMG signals were amplified (x100 using a differential amplifier, 20–450 Hz) and sampled at 2000 Hz with the aforementioned analogue to digital converter (Micro 1401; CED, Cambridge, UK) and PC software (Spike2, CED, Cambridge, UK). All EMG signals underwent a band-pass–filtered in both directions between 6 and 500 Hz using a second-order Butterworth digital filter.

2.4 Isometric leg press force measurements

Participants were firmly secured to the leg press with a waist belt. A 20 cm $\times$ 10 cm $\times$ 1.5 cm piece of cushioning was placed at the subject’s lumbar curvature to minimize discomfort. Hip and knee joint angles were fixed at $\approx$ 135 ${}^{\circ}$ and $\approx$ 130 ${}^{\circ}$ , respectively. This was achieved using a goniometer to measure the respective angles and subsequently fixing the leg length using the ratchet and lever attached to the device. The same investigator performed this procedure across all participants. The calcaneus was rested on a metal frame, which was attached to the calibrated force plate (92868A, Kistler Instruments Ltd, London, UK). The ankle was secured to this frame using Velcro strapping. A wedge was placed between the subjects heal and force plate to minimize discomfort. Subjects were instructed to hold on to the available handlebars and refrain from using their non-dominant leg, which was positioned at a knee angle of $\approx$ 75 ${}^{\circ}$ with the toes in contact with the ground. The force signal was amplified (x1000), input into an analogue-to-digital converter (CED Micro 1401; CED Cambridge, UK) and sampled at 2000 Hz using Spike 2 software (CED, Cambridge, UK) by a personal computer (PC). Summation of the four force channels enabled the total vertical force (total Fz) recording to be obtained. This force signal was filtered using a second-order Butterworth lowpass filter in both directions with an edge frequency of 100 Hz to remove any harmonics of the mains frequency. Real-time total Fz (fz1 $+$ fz2 $+$ fz3 $+$ fz4) was displayed on the PC monitor to provide performance feedback.

2.5 Isometric knee extension force measurements

Subjects were tightly secured to the strength-testing chair using a waist belt and shoulder straps. Hip and knee angles were fixed at $\approx$ 100 ${}^{\circ}$ and $\approx$ 90 ${}^{\circ}$ , respectively. The depth of the chair was modified to ensure a gap of $\approx$ 5-cm was palpable between the popliteal fossa and edge of apparatus. Subjects were instructed to hold on to similarly available handlebars and refrain from using their non-dominant leg. An ankle strap was placed 2-cm proximal to the medial malleolus and was attached to a calibrated U-shaped aluminium strain gauge, which was aligned perpendicular to the movement of the tibia during the contraction [20]. The force signal provided by the strain gauge was amplified (x1000), integrated with the same external analogue to digital converter (Micro 1401; CED, Cambridge, UK) and analysed at sampling rate of 2000 Hz using aforementioned PC software (Spike2, CED, Cambridge, UK). Real-time force was presented on the PC monitor for performance feedback.

2.6 Maximal Voluntary Contractions (MVC’s) and data analysis (force and EMG)

The participants completed a two-minute warm up of submaximal contractions before three isometric MVC’s separated by $\geqslant$ 30 s rest. They were instructed to remain still and in response to an auditory signal (3-2-1-go) were asked to push as hard as possible against the force plate/ extend their knee as hard as possible for $\geqslant$ 2 s. Verbal encouragement throughout each contraction accompanied real time visual feedback of the total vertical force (Fz)/force signal on the PC monitor. The MVF of the muscles was defined as the highest peak voluntary force obtained during any MVC. The difference between baseline and absolute MVF was determined for each individual and averaged across the groups. The root mean square (RMS) of the EMG signal for each muscle was measured over a 0.5 second period at rest and at MVF and the relative change was determined. The average RMS EMG value for each muscle group was also calculated.

2.7 Explosive Voluntary Contractions and data analysis (force, EMG, and EMD)

A series of 10 EVC’s with 20-s rest between efforts was performed. Subjects were asked to relax in order to avoid any pretension or countermovement, and respond as rapidly as possible to an auditory signal (3-2-1-go) by attempting to extend their dominant leg/knee as “fast and hard” as ossible for approximately one second. During all contractions emphasis was placed on “fast” as described with Maffiuletti et al. [21]. The baseline force was displayed on a high-resolution scale on the PC monitor, enabling fluctuations to be identified. Also displayed was the slope of the force-time curve (5-ms time constant), providing direct feedback on the RFD via the peak slope of each EVC. The RFD was mainly determined by the capacity to produce maximal voluntary activation in the early phase of an explosive contraction (first 50–75 ms), particularly as a result of increased motor unit discharge rate [21].

Figure 1.

Maximal Voluntary Force of trained athletes (TA) ( $n=$ 8) and untrained individuals (UTI) ( $n=$ 10) during isometric contractions of the muscles involved in the leg press (A) and knee extension (B). Data are means $\pm$ SD. ${}^{**}p<$ 0.01.

Figure 2.

Absolute (A) and normalized (B) force of trained athletes (black squares; $n=$ 8) and untrained individuals (white squares; $n=$ 10) during explosive isometric contractions of the muscles involved in the leg press (Fig. 2.1) and knee extension (Fig. 2.2). Data are means $\pm$ SD. ${}^{**}p<$ 0.01.

Figure 3.

Absolute (A) and normalized (B) rate of force development (RFD) of trained athletes (light bars; $n=$ 8) and untrained individuals (dark bars; $n=$ 10) over five time periods (0–50, 50–100, 100–150, 200–250 ms, represented with the numbers 1, 2, 3, 4 and 5, respectively) during explosive isometric contractions of the muscles involved in the leg press (Fig. 3.1) and knee extension (Fig. 3.2). Data are means $\pm$ SD. ${}^{*}p<$ 0.05.

The three contractions with the highest peak of the force-time curve for each individual meeting the following criteria were analysed: $\geqslant$ 80% of their MVF and no significant countermovement or pretension, defined as $<$ 2.8 N and $<$ 0.8 N change in baseline over the 100 ms before contraction onset, for leg press and knee extension, respectively. Force was measured every 50-ms from contraction onset to 250-ms and expressed in absolute terms and relative to each individual’s MVF. The RFD ( $\Delta$ force/0.05 s) for all consecutive five-time periods (1–50, 50–100, 100–150, 150–200, and 200–250 ms) was calculated and described in absolute terms and normalized to each individual’s MVF (i.e. RFD/MVF). Absolute force at each 50 ms time point was expressed relative to body mass (relative strength), as this ratio had potentially greater functional application [21, 22]. The difference between baseline and RMS amplitude of the EMG signal for each muscle was determined for each 50 ms time window from activation onset of the first muscle, and normalized to peak activation of the same muscle during MVC. Absolute and normalized EMG was determined for each muscle group throughout each time period. In addition, EMG at MVF and during each time period of the EVC’s was examined within each group over both tests. To ascertain the synchronicity of muscle activation at onset, the time delay between the first and last muscle (leg press) and first and last agonist (knee extension) to fire was calculated. The order of muscle group activation was determined. The time difference between EMG and force onset was calculated for each muscle for each contraction. The maximal EMD (EMD ${}_{\text{max}}$ ) for the leg press and knee extensor contractions were determined as the time difference between activation of the first muscle/first superficial quadriceps and force onset, respectively. All measured variables were averaged across the three explosive contractions for individual participant results, which were subsequently analysed.

Table 1

Absolute and relative strength values for trained athletes (TA) ( $n=$ 8) and untrained individuals (UTI) ( $n=$ 10) at each 50 ms time point during knee extension explosive voluntary contractions

Time point (ms)	Strength	TA		UTI		$t$
50	Absolute (N)	119.6	$\pm$ 41.63	75.5	$\pm$ 55.97 ${}^{*}$	$t$ (16) $=$ 2.61, $p<$ 0.05
	Relative (N.kg ${}^{-1}$ )	1.58	$\pm$ 0.55	0.89	$\pm$ 0.66 ${}^{*}$	$t$ (16) $=$ 2.39, $p<$ 0.05
100	Absolute (N)	352.8	$\pm$ 43.9	252.1	$\pm$ 79.1 ${}^{**}$	$t$ (16) $=$ 3.21, $p<$ 0.01
	Relative (N.kg ${}^{-1}$ )	4.69	$\pm$ 0.45	3.00	$\pm$ 0.97 ${}^{*}$	$t$ (13.18) $=$ 4.91, $p<$ 0.001
150	Absolute (N)	474.1	$\pm$ 44.2	369.8	$\pm$ 77.6 ${}^{**}$	$t$ (16) $=$ 3.39, $p<$ 0.01
	Relative (N.kg ${}^{-1}$ )	6.33	$\pm$ 0.62	4.38	$\pm$ 0.87 ${}^{***}$	$t$ (16) $=$ 5.32, $p<$ 0.001
200	Absolute (N)	538.7	$\pm$ 66.3	422.5	$\pm$ 64.0 ${}^{**}$	$t$ (16) $=$ 3.77, $p<$ 0.01
	Relative (N.kg ${}^{-1}$ )	7.15	$\pm$ 0.50	5.00	$\pm$ 0.67 ${}^{***}$	$t$ (16) $=$ 7.58, $p<$ 0.001
250	Absolute (N)	558.4	$\pm$ 79.7	427.7	$\pm$ 72.3 ${}^{**}$	$t$ (16) $=$ 3.76, $p<$ 0.01
	Relative (N.kg ${}^{-1}$ )	7.39	$\pm$ 0.42	5.05	$\pm$ 0.67 ${}^{***}$	$t$ (16) $=$ 8.60, $p<$ 0.001

Data are means $\pm$ SD, ${}^{*}p<$ 0.05; ${}^{**}p<$ 0.01; ${}^{***}p<$ 0.01: significance level TA vs. UTI.

Figure 4.

Absolute force relative to body mass (relative strength) for trained athletes ( $n=$ 8) (black squares; $n=$ 8) and untrained individuals (white squares; $n=$ 10) during explosive isometric contractions of the muscles involved in the leg press (A) and knee extension (B). Data are means $\pm$ SD. ${}^{*}p<$ 0.05, ${}^{***}p<$ 0.001.

2.8 Detection of signal onset

Identification of force and EMG onsets for all contractions was done manually (visually), considered to be the optimum method, having greater sensitivity, precision and accuracy compared to automated mathematical models [5]. Visual inspection can detect onsets approximately 60-ms earlier compared to computerized algorithms [23, 24]. All signal onsets were determined and analysed by the same investigator. Each signal recording was viewed on a consistent scale to allow onset detection to be differentiated from the noise pattern. The EMG standard view was the same for both activities (x-axis scale of 500-ms and y-axis scale of 10 mV). The standard view for force consisted of an x-axis scale of 500-ms and a y-axis scale of $\approx$ 22 N and $\approx$ 1 N, for leg press and knee extension, respectively. Contraction onset was defined as the last peak or trough before the signal deviated from baseline noise. Investigation of this area at a greater resolution ensured the vertical cursor was placed on the apex of the peak/trough.

2.9 Statistical analyses

Results are presented as means $\pm$ and standard deviation (Sd). Statistical analysis was performed using SPSS 20.0 (SPSS Inc., Chicago, IL.), with the alpha level set at 5%. The influence of time (within subject factor) and group (between subject factor) on force, RFD and EMG throughout the explosive contractions were assessed using two-way repeated-measures mixed ANOVA. If a significant main effect of group was identified, independent samples t-tests with Holm’s sequential Bonferroni procedure applied to the alpha level, were performed to establish differences at specific time points. The remaining dependent variables (MVF and EMD) were analysed using independent samples $t$ -tests.

3. Results

3.1 Voluntary force and RFD

No statistical difference between groups in leg press MVF was detected (Fig. 1A). TA produced 18.3% greater MVF compared to UTI during isometric contraction of the knee extensors ( $p<$ 0.01; Fig. 1B).

There was no significant effect of group on absolute force (Fig. 2.1A), and subsequently levels of absolute RFD (Fig. 3.1A) were similar between the groups during leg press EVC’s. Normalized force (Fig. 2.1B) and RFD (Fig. 3.1B) were comparable. TA produced a greater absolute force at 100, 150, 200 and 250 ms during EVC’s of the knee extensors ( $p<$ 0.01, Table 1; Fig. 2.2A). However, their absolute RFD was only significantly different during the 50–100 ms time period ( $p<$ 0.05, Fig. 3.2A). Normalized force between TA and UTI at all time points was comparable (Fig. 2.2B) and there was no main effect of group on normalized RFD (Fig. 3.2B).

While there was no statistically significant effect of group on relative strength during leg press EVC’s (Fig. 4A), TA attained significant higher relative strength values at all five time periods ( $p<$ 0.05 for 50 ms time point and $p<$ 0.001 for the other four time points) during EVC’s of the knee extensors (Table 1; Fig. 4B).

3.2 Neural activation during explosive contractions

The order of muscle activation onset during the EVC’s was not consistent within or between subjects of either group over both tests. TA and UTI achieved comparable time differences between the activation onset of the first and last muscle and first and last agonist muscle during the leg press and knee extensor contractions.

When RMS of the EMG signal was normalized to RMS EMG at MVF, no differences between groups for any muscle or muscle group in any time window was observed in the leg press activity. In relation to knee extension, normalized EMG of the mean superficial quadriceps over all five-time periods was similar for TA and UTI (Fig. 5). There was a significant main effect of group on normalized EMG of the main antagonists, the mean hamstrings ( $p<$ 0.05). However, when Holm’s sequential Bonferroni adjustment procedure was performed, the difference during all time periods was above the new alpha criteria level ( $p<$ 0.013).

Table 2
Electromyography amplitude of the Vastus Medialis (VM), Vastus Lateralis (VL), Rectus Femoris (RF), mean hamstrings and mean superficial quadriceps at maximum voluntary force of trained athletes ( $n=$ 8) over both activities

Muscle/muscle group	Leg press (mV)		Knee extension (mV)		$t$
VM	196.33	$\pm$ 186.99	465.36	$\pm$ 154.63 ${}^{**}$	$t$ (14) $=$ 3.14, $p<$ 0.01
VL	111.24	$\pm$ 58.25	349.22	$\pm$ 101.49 ${}^{***}$	$t$ (14) $=$ 5.75, $p<$ 0.001
RF	52.27	$\pm$ 68.78	303.96	$\pm$ 145.95 ${}^{**}$	$t$ (9.96) $=$ 4.41, $p<$ 0.01
Mean hamstrings	50.80	$\pm$ 44.44	8.16	$\pm$ 12.42 ${}^{*}$	$t$ (8.09) $=$ $-$ 2.61, $p<$ 0.05
Mean quadriceps	119.95	$\pm$ 96.66	372.85	$\pm$ 102.07 ${}^{***}$	$t$ (14) $=$ 5.09, $p<$ 0.001

Data are means $\pm$ SD, ${}^{*}p<$ 0.05; ${}^{**}p<$ 0.01; ${}^{***}p<$ 0.001: significance level leg press vs. knee extension.

Figure 5.

Root mean square (RMS) electromyography (EMG) of the mean superficial quadriceps for trained athletes (black squares; $n=$ 8) and untrained individuals (white squares; $n=$ 10) during explosive isometric knee extensor contractions when normalized to RMS EMG of the same muscle group at maximum voluntary force (MVF) (RMS EMG as a % of RMS EMG at MVF). Data are means $\pm$ SD.

Comparison of the RMS EMG amplitude at MVF within groups revealed differences over both activities. Significant differences in the EMG activity levels of the VM, VL, RF, mean hamstrings and mean superficial quadriceps were identified in the TA ( $p<$ 0.05; Table 2). UTI elicited statistically different EMG activity levels of all muscles and muscle groups ( $p<$ 0.05; Table 3) except the ST, BF and mean hamstrings.

Table 3

Electromyography amplitude of the soleus (SOL), Gastrocnemius Lateralis (GL), Gluteus Maximus (GMAX), Vastus Medialis (VM), Vastus Lateralis (VL), Rectus Femoris (RF), mean calves and mean superficial quadriceps at MVF of untrained individuals ( $n=$ 10) over both activities

Muscle/muscle group	Leg press (mV)		Knee extension (mV)		$t$
SOL	22.79	$\pm$ 8.61	8.20	$\pm$ 11.75 ${}^{**}$	$t$ (18) $=$ $-$ 3.17, $p<$ 0.01
GL	24.44	$\pm$ 9.29	5.26	$\pm$ 10.63 ${}^{***}$	$t$ (18) $=$ $-$ 4.30, $p<$ 0.001
GMAX	10.22	$\pm$ 8.75	3.39	$\pm$ 5.08 ${}^{*}$	$t$ (18) $=$ $-$ 2.14, $p<$ 0.05
VM	71.08	$\pm$ 52.78	186.26	$\pm$ 112.18 ${}^{*}$	$t$ (12.80) $=$ 2.94, $p<$ 0.05
VL	57.47	$\pm$ 56.35	150.62	$\pm$ 79.84 ${}^{*}$	$t$ (18) $=$ 3.01, $p<$ 0.05
RF	12.71	$\pm$ 13.33	90.04	$\pm$ 85.44 ${}^{*}$	$t$ (9.44) $=$ 2.83, $p<$ 0.05
Mean calves	23.62	$\pm$ 7.53	6.73	$\pm$ 10.93 ${}^{***}$	$t$ (18) $=$ $-$ 4.03, $p<$ 0.001
Mean quadriceps	47.09	$\pm$ 37.73	142.31	$\pm$ 83.64 ${}^{**}$	$t$ (12.52) $=$ 3.28, $p<$ 0.01

Data are means $\pm$ SD, ${}^{*}p<$ 0.05; ${}^{**}p<$ 0.01; ${}^{***}p<$ 0.001: significance level leg press vs. knee extension.

Furthermore, differences in EMG levels during EVC’s within groups over both tests were identified. TA had significantly different RMS EMG amplitudes of the VM, VL, RF, mean hamstrings and mean superficial quadriceps ( $p<$ 0.05), while UTI had different EMG levels of all muscles (SOL, GL, GMAX, VM, VL, RF) ( $p<$ 0.05) and muscle groups (mean calves and mean quadriceps) ( $p<$ 0.05) except the BF and mean hamstrings.

3.3 EMD

TA and UTI had similar EMD ${}_{\text{max}}$ during both EVC’s of the muscles involved in the leg press (24.20 $\pm$ 7.99 ms vs 28.52 $\pm$ 14.42 ms, respectively) and the knee extension (18.58 $\pm$ 3.99 ms vs 17.18 $\pm$ 3.37 ms, respectively).

4. Discussion

The present study compared the RFD, neural activation and EMD of TA and UTI during EVC’s of the muscles involved in the leg press and the knee extensors. During leg press EVC’s, TA and UTI achieved similar levels of absolute and normalised force and RFD. Correspondingly, normalised EMG of all muscles and muscle groups and relative strength were comparable. TA had a greater absolute force at 100, 150, 200 and 250 ms during EVC’s of the knee extensors, resulting from higher absolute RFD during the 50–100 ms time period and greater absolute force relative to body mass at all five time points. Surprisingly, normalised force and normalised RFD did not differ between the groups and no discrepancies in normalised EMG of the mean superficial quadriceps were noted. EMD levels were comparable between groups however TA and UTI had significantly different EMG activity levels of a number of muscles over both activities. This is the first study to show that differences between TA and UTI in single and multiple joint movements exist, and appear to be related to discrepancies in muscle activation within the groups over both activities.

The leg press EVC’s, TA and UTI achieved similar levels of absolute force at all time points (Fig. 2.1A); consequently, no difference in levels of absolute RFD (Fig. 3.1A) was reported. The force produced at specific time points and absolute RFDs are related to muscle strength and MVF [3, 25]. Therefore, the similarities in MVF obtained between TA and UTI (Fig. 1A) may explain the comparable force and RFD levels reported, as normalized force and normalized RFD (Figs 2.1B and 3.1B) were similar. The study results of Tillin et. al. [5] are the most comprehensive to date, reporting higher absolute and normalised RFD in explosive power athletes, attributed to greater agonist muscle neural activation. Comparisons of RFD within the athletic population has revealed greater absolute RFD in strength-trained athletes (different weight categories) and power-athletes (sprinters) compared to endurance-trained individuals [26]. When RFD is normalized to maximum voluntary force (MVF), the difference between explosive, strength and endurance athletes and untrained individuals is equivocal [26, 27]. In these studies, RFD was calculated at time points that corresponded to varying percentages of MVF, possibly explaining the contrasting results. In addition, possible causes for the variation of RFD between groups were not elucidated. Neural activation, as measured by EMG amplitude, influences RFD [5, 8, 9, 12]. Indeed, de Ruiter et. al. [9] concluded that RFD capacity may be dependent on the level of muscle activation (as measured by preceding surface EMG) at the contraction onset. When RMS of the EMG amplitude for each muscle and muscle group over each time period was normalized to RMS of the EMG signal at MVF, no difference between groups was reported. This is not surprising as both absolute and normalized RFD levels were comparable. Furthermore, the similarities in synchrony of activation onset and EMD ${}_{\text{max}}$ of the muscles may account for the inseparable initial RFD of the groups, as both factors are related to the later [12, 28, 29].

While absolute RFD is imperative in explosive powerful performances [9] and information on the physiological mechanisms of this characteristic are somewhat provided by examining normalized RFD, it is probable that the relative strength ratio is of greater functional relevance. Dynamic explosive activities require individuals to maintain joint stabilization whilst propelling their body weight. Accordingly, relative strength may provide a better indication of explosive performance. In contrast to the study results of Viitasalo and Komi [27], TA and UTI in the present study had similar levels of relative strength at all time points (Fig. 4A), possibly contributing to the similar levels of absolute force obtained.

TA achieved greater absolute force at 100, 150, 200, and 250 ms (Fig. 2.2B) during knee extension EVC’s, owing to a combination of their greater absolute RFD during the 50–100 ms time period (Fig. 3.2B) and relative strength values at all five time points (Fig. 4B). MVF was 18.3% greater in the same group, implying that this difference in strength could account for the 32% higher absolute RFD attained as no difference in normalized force or normalized RFD were reported (Figs 2.2B and 3.2B). Tillin et al. [5] also reported greater levels of absolute force and RFD in TA compared to UTI during EVC’s of the knee extensors. However, while the authors reported greater normalized mean quadriceps EMG amplitude, this study found similar levels between the groups, which, coupled with the similar normalized antagonist EMG levels, are possibly the main factors contributing to the comparable normalized RFD levels between TA and UTI. The positive correlation between neural activation and voluntary isometric knee extensor RFD [9] appear to support this neurologic explanation. The synchrony in activation onset and EMD ${}_{\text{max}}$ of the agonist muscles were comparable between the groups, which may further explain the similar initial normalized RFD. TA had greater relative strength values at all five time points (Fig. 4B), consistent with the results of Viitasalo and Komi [27]. This result, in combination with the greater absolute RFD during the 100–150 ms time period, are the most probable factors differentiating explosive performance between the groups.

One limitation to the present study was the normalisation of RMS EMG to MVF, as normalising to a supramaximal compound muscle action potential (M-wave) may be more appropriate [30]. EMD ${}_{\text{max}}$ values (17–29 ms) were slightly lower than those reported in the majority of studies $\approx$ 30–60 ms [10, 31, 32, 33, 34, 35] and were similar in both groups over both tests. The small sample size was the possible limitation for these slightly lower values. The similarities obtained emphasize the impact of numerous muscle-tendon morphologic e.g., fibre type [27, 36, 37] and mechanical e.g. tendon stiffness [32, 38] properties on EMD. Zhou [31] reported shorter EMD latencies in weightlifters versus endurance-trained athletes and untrained individuals. However, Tillin et al. [5] failed to identify a difference between athletic and non-athletic groups. The equivocal results highlight the dependence of EMD on many further aspects of muscle-tendon morphology and mechanics, including muscle fibre-type composition [36], type of muscle contraction (shorter EMD associated with eccentric contractions) [15], fatigue (can increase EMD latency by up to 70%) Zhou [31] and neural activation [5]. Techniques including muscle biopsy and ultrasonography may elucidate the relationship between these factors and are recommended for future investigations.

The present study results indicate that while discrepancies in the physiological characteristics were evident in the single joint knee extension, no differences between groups in the multi-joint leg press activity were found. This is surprising considering the anticipated greater difference between groups in the multi-joint movement due to a higher degree of muscle co-ordination and activation [7, 39]. This phenomenon may be related to muscle function during both tasks rather than the inherent physiological characteristics of the groups. The quadriceps are the only contributors to force generation during knee extension [40], while force production during leg press is dependent on a wider range of muscles and muscle groups [40, 41]. Reports have indicated that the EMG activity of individual muscles in the knee extension is not consistent with that of the same muscles during leg press [42, 43]. The results of this study would concur. TA elicited significantly different EMG activity levels of the VM, VL, RF, mean hamstrings and mean superficial quadriceps at both MVF (Table 2) and during the EVC’s in both tests. UTI had different EMG levels of all muscles and muscle groups except the ST, BF and mean hamstrings at MVF, while activity levels of the BF and mean hamstrings were the only similarities found during the EVC’s (Table 3). Due to the relationship between neural activation and RFD [9], it is likely that the discrepancies in muscle activation within groups over both tasks may account for the differences in RFD and thus explosive performance between groups, during single and multi-joint movements.

Whether a muscle is mono- or biarticular is most likely the primary factor contributing to discrepancies in muscle activation in single and multiple joint movements, as these muscle types exhibit different roles in such activities [44]. It has been suggested that the RF, a knee extensor agonist, may act as an antagonist during the leg press by flexing around the hip joint, which would reduce the activity level of this superficial quadricep [43]. The decrement in RF activity in the leg press compared to the knee extension in both groups (Tables 2 and 3) would appear to support this proposal. In addition, the bi-articular ST, BF (mean hamstrings) and GL act as agonists during the leg press but perform antagonistic roles during knee extension [2, 45], reinforced by the different EMG levels of these muscles in the groups during both activities (Tables 2 and 3). Force production is inherently related to muscle activation. Due to different operational mechanisms of several of the muscles tested over both single and multiple joint tasks, one may appreciate that differences between TA and UTI in the knee extension does not translate to a difference between the same groups in the leg press.

The results of the present study have practical implications for improving sport performance in all populations. The greater force produced by TA at $\geqslant$ 100 ms during the knee extension contractions; owing to their greater absolute RFD during the 50–100 ms time period and relative strength values at all five time points, indicate that single joint tasks requiring force production during this period may benefit from developing maximal strength. However, the potential benefit of increasing maximal strength in the aforementioned time period during multi-joint movement is questionable, due to similarities in absolute force and RFD between the groups. Nevertheless, improving maximal strength may also be advocated for tasks requiring immediate force production (0–100 ms), including joint stabilisation, as initial normalised RFD and neural activation were similar for both groups over both tests.

5. Conclusion

The results of this study indicated that RFD, neural activation and EMD were similar between TA and UTI during EVC’s of the muscles involved in the leg press. In contrast, while EMD was comparable between the groups during knee extension EVC’s, athletes had greater absolute RFD after 50 ms, attributed to their greater strength. The normalised RFD of both groups was similar, owing to comparable levels of neural activation. The similarities in the explosive performance between TA and UTI in the leg press and discrepancies during the knee extension may be explained by differences in neuromuscular activation within groups over both activities, most likely due to the different roles of biarticular muscles in single and multi-joint tasks. Furthermore, future studies should attempt to ascertain whether differences in neuromuscular performance between TA and UTI exist under dynamic conditions.

Ethical considerations

A written informed consent was obtained from each player in accordance with the ethical standard of Helsinki Declaration. The study was approved by Loughborough University Ethical Advisory Committee (R10-P127).

Funding

This paper was supported by The Scientific and Technological Research Council of Turkey (TUBITAK grant B.02.1.TBT.0.06.01-219-37-110).

Footnotes

Acknowledgments

The author would like to thank the students Katherine Maria Lydon, Mikaela Harrison from Loughborough University School of Sport, Exercise and Health Sciences, and trained athletes from Loughborough University Athletics for their assistance in the study. Special thanks are extended to Dr. J P. Folland for sharing his knowledge and experience and his support for studying in his laboratory.

Conflict of interest

The author has no conflicts of interest to report.

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Comparison of single and multiple joint muscle functions and neural drive of trained athletes and untrained individuals

Abstract

BACKGROUND:

OBJECTIVE:

METHODS:

RESULTS:

CONCLUSIONS:

Keywords

1. Introduction

2. Methods

2.1 Partcipants

2.2 Procedure

2.3 Surface EMG measurements

2.4 Isometric leg press force measurements

2.5 Isometric knee extension force measurements

2.6 Maximal Voluntary Contractions (MVC’s) and data analysis (force and EMG)

2.7 Explosive Voluntary Contractions and data analysis (force, EMG, and EMD)

2.9 Statistical analyses

3. Results

3.1 Voluntary force and RFD

3.2 Neural activation during explosive contractions

Table 2 Electromyography amplitude of the Vastus Medialis (VM), Vastus Lateralis (VL), Rectus Femoris (RF), mean hamstrings and mean superficial quadriceps at maximum voluntary force of trained athletes ( n = 8) over both activities

4. Discussion

5. Conclusion

Ethical considerations

Funding

Footnotes

Acknowledgments

Conflict of interest

References

Table 2
Electromyography amplitude of the Vastus Medialis (VM), Vastus Lateralis (VL), Rectus Femoris (RF), mean hamstrings and mean superficial quadriceps at maximum voluntary force of trained athletes ( $n=$ 8) over both activities