Do the Quadriceps Work Harder When the Knee is Near Full Flexion Compared to Near Full Extension?

A question is proposed to decide if quadriceps work harder and whether more hypertrophy is stimulated by working the quadriceps in a position of relative flexion or relative extension. The research articles cited below do not make conclusions regarding hypertrophy, so that will have to remain an assumption, for now. However, the studies cited below lead to some interesting conclusions about work of the quadriceps muscles at different angles of the knee. If you go by EMG results, you would not conclude that there is a difference in quadriceps effort at different knee angles (except for one finding with the rectus femoris). But the study of oxygen consumption at different angles of the knee leads to a more solid conclusion.

Some exercisers report that they feel more of a ‘burn’ and failure in the mid-thigh and vastus lateralis region when they limit the range of motion of a leg extension exercise from full flexion to only partial extension. One idea being that the vastus medialis does not work as hard and the vastus lateralis — a larger muscle — works harder in the relatively flexed range of motion. There is also a common practice to work the Vastus Medialis harder by focusing on Terminal Knee Extensions or TKE with the rationale that the Vastus Medialis Oblique (VMO) muscle — a stabilizing protector of the knee joint and patellar tracking mechanism — gets more work in the relative terminus-end extension of the Leg Extension exercise.

Research results of EMG activity in the relative flexion and relative extension range of motion do not show more activity of the vastus lateralis in the relatively flexed ROM and do not show more activity of the vastus medialis in the relatively extended ROM. However, in a study of oxygen consumption, the knee extensor oxygen consumption was significantly lower during isometric contractions at 30 degrees than oxygen consumption at the 60 degrees and 90 degrees of knee angle. The oxygen consumption near extension (30 degrees) was about 70% of the oxygen consumption level at the relatively more flexed angles of 60 degrees and 90 degrees.

Note that none of the studies below, support the fact that the VMO gets more conditioned in the TKE range of motion. Also note that EMG results may NOT be a good indicator of actual activity of work or force production of the muscles in all cases. The EMG studies do not seem to discover, with any direct correlation, the greater oxygen consumption of the relatively flexed positions of leg extension (relatively flexed knee joint angles) when the quadriceps muscles are in a more lengthened position under tension.

The results of these quadriceps studies bring to mind, the results of abdominal exercise studies of EMG activity that are used to determine and proclaim what positions and angles of abdominal exercise are most effective for training abdominal muscles. Many studies of comparisons of abdominal exercises use EMG activity results to determine if a specific abdominal exercise or designated equipment is more effective for the rectus abdominis, or the external and internal oblique muscles. Also traditional abdominal exercises or different equipment-assisted abdominal exercises are compared for their effectiveness on a target abdominal muscle using the results of EMG activity. Is it possible that the abdominal muscles also need to be tested for their oxygen consumption under the stress of different abdominal exercises, trunk flexion angles, leg positions, equipment type, and range of motion? In other words, can we really say with certainty that higher EMG activity means that the respective muscle is working harder?

Here is the EMG study that did not show any greater fatigue or selective conditioning of the VMO with ‘short arc’ or TKE range of motion exercises:

Fatigue rates of vastus medialis oblique and vastus lateralis during static and dynamic knee extension.

Fatigue rates of the vastus medialis oblique and vastus lateralis muscles were compared from static and dynamic exercise. Based upon clinical observations, it was expected that the vastus medialis oblique would demonstrate greater fatigability than the vastus lateralis. However, based upon physiological and morphological considerations, it was expected that the vastus lateralis would exhibit greater fatigability than the vastus medialis oblique. In the static exercise condition, nine subjects–having no history of knee problems–maintained an isometric knee extension torque at 30 and 60% of their maximum value until exhaustion, during which time electromyography (EMG) data were collected every 10 s. The knee angle was 20 degrees flexion. In the dynamic exercise condition, a different group of seven subjects performed exercise sets consisting of eight cycles of concentric-eccentric knee extension, with a resistance equal to 40% of the maximum isometric value. The range of motion was restricted to the terminal 30 degrees of knee extension. Each set of eight repetitions was followed by a 1 s 50% maximum isometric knee extension, during which time EMG data were collected. The raw EMG data were numerically processed to extract the median frequency of the power density spectrum, which has been shown to reflect the metabolic processes associated with fatigue. Linear regression generated a slope coefficient representing the rate of change of the median frequency, with respect to contraction duration for each subject, muscle, and condition. Analyses of variance, with repeated measures from both exercise conditions suggest that “short-arc” quadriceps exercise did not selectively fatigue either the vastus medialis oblique or vastus lateralis, thereby supporting neither clinical nor physiological expected outcomes. Therefore, the clinical contention that short-arc quadriceps exercises selectively strengthen the vastus medialis oblique is questioned.

Grabiner MD, Koh TJ, Miller GF. Fatigue rates of vastus medialis oblique and vastus lateralis during static and dynamic knee extension. J Orthop Res. 1991 May;9(3):391-7.

Here is the study that showed more oxygen consumption of the quadriceps muscles at relatively flexed angles compared to the the relatively extended range of motion angle.

Knee angle-dependent oxygen consumption during isometric contractions of the knee extensors determined with near-infrared spectroscopy.

Fatigue resistance of knee extensor muscles is higher during voluntary isometric contractions at short compared with longer muscle lengths. In the present study we hypothesized that this would be due to lower energy consumption at short muscle lengths. Ten healthy male subjects performed isometric contractions with the knee extensor muscles at a 30, 60, and 90 degrees knee angle (full extension = 0 degrees ). At each angle, muscle oxygen consumption (m.VO2) of the rectus femoris, vastus lateralis, and vastus medialis muscle was obtained with near-infrared spectroscopy. m.VO2 was measured during maximal isometric contractions and during contractions at 10, 30, and 50% of maximal torque capacity. During all contractions, blood flow to the muscle was occluded with a pressure cuff (450 mmHg). m.VO2 significantly (P < 0.05) increased with torque and at all torque levels, and for each of the three muscles. m.VO2 was significantly lower at 30 degrees compared with 60 degrees and 90 degrees and m.VO2 was similar (P > 0.05) at 60 degrees and 90 degrees . Across all torque levels, average (+/- SD) m.VO2 at the 30 degrees angle for vastus medialis, rectus femoris, and vastus lateralis, respectively, was 70.0 +/- 10.4, 72.2 +/- 12.7, and 75.9 +/- 8.0% of the average m.VO2 obtained for each torque at 60 and 90 degrees . In conclusion, oxygen consumption of the knee extensors was significantly l
ower during isometric contractions at the 30 degrees than at the 60 degrees and 90 degrees knee angle, which probably contributes to the previously reported longer duration of sustained isometric contractions at relatively short muscle lengths.

de Ruiter CJ, de Boer MD, Spanjaard M, de Haan A. Knee angle-dependent oxygen consumption during isometric contractions of the knee extensors determined with near-infrared spectroscopy. J Appl Physiol. 2005 Aug;99(2):579-86. Epub 2005 Mar 17.

Here is the study that led researchers to conclude that muscle length-related differences in metabolic cost are more important in endurance of the quadriceps than central activation. Note that the vastus lateralis EMG signals were similar at muscle failure at the range of motions tested, but the rectus femoris showed more EMG activity at the relatively flexed position of ROM.

Muscle activation and blood flow do not explain the muscle length-dependent variation in quadriceps isometric endurance.

We investigated the role of central activation in muscle length-dependent endurance. Central activation ratio (CAR) and rectified surface electromyogram (EMG) were studied during fatigue of isometric contractions of the knee extensors at 30 and 90 degrees knee angles (full extension = 0 degree). Subjects (n = 8) were tested on a custom-built ergometer. Maximal voluntary isometric knee extension with supramaximal superimposed burst stimulation (three 100-mus pulses; 300 Hz) was performed to assess CAR and maximal torque capacity (MTC). Surface EMG signals were obtained from vastus lateralis and rectus femoris muscles. At each angle, intermittent (15 s on 6 s off) isometric exercise at 50% MTC with superimposed stimulation was performed to exhaustion. During the fatigue task, a sphygmomanometer cuff around the upper thigh ensured full occlusion (400 mmHg) of the blood supply to the knee extensors. At least 2 days separated fatigue tests. MTC was not different between knee angles (30 degrees : 229.6 +/- 39.3 N.m vs. 90 degrees: 215.7 +/- 13.2 N.m). Endurance times, however, were significantly longer (P < 0.05) at 30 vs. 90 degrees (87.8 +/- 18.7 vs. 54.9 +/- 12.1 s, respectively) despite the CAR not differing between angles at torque failure (30 degrees: 0.95 +/- 0.05 vs. 90 degrees: 0.96 +/- 0.03) and full occlusion of blood supply to the knee extensors. Furthermore, rectified surface EMG values of the vastus lateralis (normalized to prefatigue maximum) were also similar at torque failure (30 degrees : 56.5 +/- 12.5% vs. 90 degrees : 58.3 +/- 15.2%), whereas rectus femoris EMG activity was lower at 30 degrees (44.3 +/- 12.4%) vs. 90 degrees (69.5 +/- 25.3%). We conclude that differences in endurance at different knee angles do not find their origin in differences in central activation and blood flow but may be a consequence of muscle length-related differences in metabolic cost.

Kooistra RD, de Ruiter CJ, de Haan A. Muscle activation and blood flow do not explain the muscle length-dependent variation in quadriceps isometric endurance. J Appl Physiol. 2005 Mar;98(3):810-6. Epub 2004 Oct 15.

Here is an EMG study that found no difference in the degree of synchronization between the vasti at different knee angles

Effect of knee joint angle on motor unit synchronization.

Activity of the vasti has been argued to vary through knee range of movement due to changes in passive support of the patellofemoral joint and the relative contribution of these muscles to knee extension. Efficient function of the knee is dependent on optimal control of the patellofemoral joint, largely through coordinated activity of the medial and lateral quadriceps. Motor unit synchronization may provide a mechanism to coordinate the activity of vastus medialis (VMO) and vastus lateralis (VL), and may be more critical in positions of reduced passive support for the patellofemoral joint (i.e., full extension). Therefore, the aim of this study was to determine whether the degree of motor unit synchronization between the vasti muscles is dependent on joint angle. Electromyographic (EMG) recordings of single motor unit action potentials (MUAPs) were made from VMO and multiunit recordings from VL during isometric contractions of the quadriceps at 0 degrees, 30 degrees, and 60 degrees of knee flexion. The degree of synchronization between motor unit firing was evaluated by identification of peaks in the rectified EMG averages of VL, triggered from MUAPs in VMO. The proportion of cases in which there was a significant peak in the triggered averages was calculated. There was no significant difference in the degree of synchronization between the vasti at different knee angles (p=0.57). These data suggest that this basic coordinative mechanism between the vasti muscles is controlled consistently throughout knee range of motion, and is not augmented at specific angles where the requirement for dynamic control of stability is increased. Copyright (c) 2006 Orthopaedic Research Society.

Mellor R, Hodges PW.  Effect of knee joint angle on motor unit synchronization. J Orthop Res. 2006 Jul;24(7):1420-6.

So … do the quadriceps work harder when the knee is near full flexion compared to near full extension? If you go by EMG results, you probably would not be so sure. If you go by oxygen consumption analysis, you would say, ‘yes,’ the quadriceps work harder at greater muscle lengths when concentric flexion action, eccentric extension action or isometric action is happening in relatively flexed knee joint angles.