The relationship between torque and joint angle during knee extension in boys and men

The length-tension relationship of muscle contraction is well documented in adults. However, research on this relationship in children has been limited. The aim of this study was to compare differences in the torque-joint angle relationship of the quadriceps muscle in children and adults. Eight boys aged 8-10 years and eight men aged 20-26 years performed two maximal voluntary isometric contractions at six knee joint angles (20°, 40°, 60°, 80°, 90°, 100°). The mean of the two trials was used as the performance measure. Both groups demonstrated an expected increase in relative torque as the joint angle increased (P ? 0.05). The men produced significantly greater relative torque at 20°, 40° and 60° knee flexion (P ? 0.05). The percentage of maximal torque at these angles for the men and boys respectively were: 35.2 - 4.3 vs 15.2 - 12%, 63.6 - 9.1 vs 51.8 - 16.8% and 93.6 - 6.5 vs 84.4 - 14.4%. There were no group differences at 80° or 90°. Peak torque was attained at 80° in men, but decreased significantly (P ? 0.05) at 90° and 100°. For boys, peak torque was attained at joint angles of 80° and 90°. The reduction in peak torque at 100° was not statistically significant, but the relative torque at this angle was lower in men than in boys (77.9 - 13.7 vs 87.1 - 10.4%; P ? 0.05). In conclusion, the relationship between torque and joint angle appears to be affected by age.     

Variable-cam resistance training machines: do they match the angle - torque relationship in humans?

The aim of this study was to determine the extent to which the resistance provided by variable-cam resistance training machines match joint torque capability. Eight variable-cam knee extension machines from six different manufacturers were assessed. Resistive torque for a constant weight/load was measured at five knee joint angles. Knee extensor muscle torque capability--the angle-torque relationship--of 10 healthy young men was determined isometrically and dynamically at the same five angles. After normalization, the two relationships were compared. The angle - torque relationship of the knee extensors displayed an inverted "U" shape as expected, with dynamic torque changing by +40% on the ascending limb (1.75 - 1.40 rad) and -60% on the descending limb (1.05-0.35 rad) of the angle - torque relationship. The angle-torque relationship of the different training machines was highly variable, but consistently less curvilinear and significantly different from knee extensor capabilities, with changes in torque of +2.5 to +22.2% (ascending limb) and +37.6 to -20.5% (descending limb). It is recommended that variable-cam resistance training machines be designed with the angle - torque relationship of the relevant joint and the inertial profile of the lifting exercise in mind.     

Acute effects of static stretching on characteristics of the isokinetic angle - torque relationship, surface electromyography, and mechanomyography

The aims of this study were to examine the acute effects of static stretching on peak torque, work, the joint angle at peak torque, acceleration time, isokinetic range of motion, mechanomyographic amplitude, and electromyographic amplitude of the rectus femoris during maximal concentric isokinetic leg extensions at 1.04 and 5.23 rad x s(-1) in men and women. Ten women (mean +/- s: age 23.0 +/- 2.9 years, stature 1.61 +/- 0.12 m, mass 63.3 +/- 9.9 kg) and eight men (age 21.4 +/- 3.0 years, stature 1.83 +/- 0.11 m, mass 83.1 +/- 15.2 kg) performed maximal voluntary concentric isokinetic leg extensions at 1.04 and 5.23 rad x s(-1). Following the initial isokinetic tests, the dominant leg extensors were stretched using four static stretching exercises. After the stretching, the isokinetic tests were repeated. Peak torque, acceleration time, and electromyographic amplitude decreased (P< or = 0.05) from pre- to post-stretching at 1.04 and 5.23 rad . s(-1); there were no changes (P > 0.05) in work, joint angle at peak torque, isokinetic range of motion, or mechanomyographic amplitude. These findings indicate no stretching-related changes in the area under the angle - torque curve (work), but a significant decrease in peak torque, which suggests that static stretching may cause a "flattening" of the angle - torque curve that reduces peak strength but allows for greater force production at other joint angles. These findings, in conjunction with the increased limb acceleration rates (decreased acceleration time) observed in the present study, provide tentative support for the hypothesis that static stretching alters the angle - torque relationship and/or sarcomere shortening velocity.     

Effects of series elasticity on the human knee extension torque-angle relationship in vivo

The purpose of this study was to investigate the effects of series elasticity on the torque-angle relationship of the knee extensors in vivo. Forty-two men volunteered to take part in the present study. The participants performed maximal voluntary isometric contractions at eight knee-joint angles (40, 50, 60, 70, 80, 90, 100, 110 degrees). The elongation of the tendon and aponeurosis of the vastus lateralis muscle was directly measured by ultrasonography, while the participants performed ramp isometric knee extensions to the voluntary maximum at 800 of knee angle. There was no significant difference in the torque value between 50 degrees and 100 degrees, although there was variation in the shape of the "torque-angle" relationship. The variability in the torque-angle curve was not affected by the activation level of agonist and antagonist muscles and the moment arm length. The ratio of torque at 1000 compared to that at 50 degrees was significantly (r(2) = 18-23 %) correlated to the maximal elongation and strain (to initial length) of the tendon structures. These results suggest that increased compliance of tendon structures of the knee extensors may contribute to variations in the torque-angle curves of the knee extensors.     

Angle-specific impairment of elbow flexors strength after isometric exercise at long muscle length

In this study, we examined the long-term reductions in maximal isometric force (MIF) caused by a protocol of repeated maximal isometric contractions at long muscle length. Furthermore, we wished to ascertain whether the reductions in MIF are dependent on muscle length--that is, are the reductions in MIF more pronounced when the muscle contracts at a short length. The MIF of the elbow flexors of seven young male volunteers was measured at five different elbow angles between 50 degrees and 160 degrees. On a separate day, the participants performed 50 maximal voluntary isometric muscle contractions with the elbow flexors at a lengthened position; that is, with the shoulder hyperextended at 45 degrees and the elbow joint fixed at 140 degrees. Following this exercise, the MIF at the five elbow angles, range of motion, muscle soreness and plasma creatine kinase activity were measured at 24 h intervals for 4 days. On day 1, the decline in MIF was higher at the more acute elbow angles of 50 degrees (42 +/- 8%) and 70 degrees (39 +/- 8%; both P<0.01) than at 90 degrees (26 +/- 4%) and 140 degrees (16 +/- 3%; both P<0.01). No significant reduction in MIF was evident at an elbow angle of 160 degrees. Maximal isometric force at an elbow angle of 140 degrees was fully restored on day 3, whereas at an angle of 50 degrees it remained depressed for the 4 day observation period. Restoration of MIF was a function of the elbow angle, with force recovery being less at the smaller angles. The range of motion was decreased by 14 +/- 2 degrees on day 1 (P<0.01) and did not return to baseline values by day 4. Muscle soreness ratings remained significantly elevated for the 4 day period. Serum creatine kinase peaked on day 1 (522 +/- 129 IU, P<0.01) and decreased thereafter. We conclude that the disproportionate decrease in MIF at the small elbow angles and the length-specific recovery in MIF after repeated maximal isometric contractions at long muscle length may be explained by the presence of overstretched sarcomeres that increased in series compliance of the muscle, therefore causing a rightward shift of the force-length relationship.     

Intra-limb coordinative adaptations in cycling

This study aimed to establish the nature of lower extremity intra-limb coordination variability in cycling and to investigate the coordinative adaptations that occur in response to changes in cadence and work rate. Six trained and six untrained males performed nine pedalling bouts on a cycle ergometer at various cadences and work rates (60, 90, and 120 revolutions per minute (rpm) at 120, 210, and 300W). Three-dimensional kinematic data were collected and flexion/extension angles of the ankle, knee, and hip joints were subsequently calculated. These data were used to determine two intra-limb joint couplings [hip flexion/extension-knee flexion/extension (HK) and knee flexion/extension-ankle plantar-flexion/dorsi-flexion (KA)], which were analysed using continuous relative phase analysis. Trained participants displayed significantly (p < 0.05) lower coordination variability (6.6 +/- 4.0 degrees) than untrained participants (9.2 +/- 4.7 degrees). For the trained subjects, the KA coupling displayed significantly more in-phase motion in the 120 rpm (19.2 +/- 12.3 degrees) than the 60 (30 +/- 7.1 degrees) or 90 rpm (33.1 +/- 7.4 degrees) trials and the HK coupling displayed significantly more in-phase motion in the 90 (33.3 +/- 3.4 degrees) and 120 rpm (27.9 +/- 13.6 degrees) than in the 60 rpm trial (36.4 +/- 3.5 degrees). The results of this study suggest that variability may be detrimental to performance and that a higher cadence is beneficial. However, further study of on-road cycling is necessary before any recommendations can be made.     

Damage to the human quadriceps muscle from eccentric exercise and the training effect

Nine participants performed two bouts of a step exercise, during which the quadriceps muscle of one leg acted eccentrically. Before and after the exercise, isokinetic torque was measured over a range of knee angles to determine the optimum angle for torque. Immediately after the first bout of exercise, the quadriceps showed a significant (P < 0.05) shift of 15.6 +/- 1.4 degrees (mean +/-sx) of its optimum angle in the direction of longer lengths, suggesting the presence of damage. A drop in peak torque, together with delayed soreness and swelling, confirmed that damage to muscle fibres had occurred. After the second bout of exercise, 8 days later, the shift in optimum angle was 10.4 +/- 1.0 degrees, which was significantly less than after the first bout (P < 0.05). Other indicators of damage were also reduced. In addition, the muscle exhibited a sustained shift in optimum angle (3.4 +/- 0.9 degrees), suggesting that some adaptation had taken place after the first bout of exercise. We conclude that muscles like the quadriceps can show evidence of damage after a specific programme of eccentric exercise, followed by an adaptation response. This is despite the fact that the quadriceps routinely undergoes eccentric contractions in everyday activities.     

Changes in the relationship between joint angle and torque production associated with the repeated bout effect

A single bout of eccentric exercise induces a protective adaptation against damage from a repeated bout. The aim of this study was to determine whether this repeated bout effect is due to a change in the length-tension relationship. Twelve individuals performed an initial bout of six sets of 10 eccentric quadriceps contractions and then performed a repeated bout 2 weeks later. Eccentric contractions were performed on an isokinetic dynamometer at 1.04 rad x s(-1) with a target intensity of 90% of isometric strength at 70 degrees of knee flexion. Isometric strength and pain were recorded before and after both eccentric bouts and on each of the next 3 days. Isometric strength was tested at 30 degrees, 50 degrees, 70 degrees, 90 degrees and 110 degrees of knee flexion. On the days following the initial bout, there was a significant loss of isometric strength at all knee flexion angles except 110 degrees (bout x angle: P < 0.01). On day 2, strength averaged 86% of baseline for 30-90 degrees and 102% of baseline for 110 degrees. Strength loss and pain after the initial bout was contrasted by minimal changes after the repeated bout (pain: P < 0.001; strength: P < 0.01). The repeated bout effect was associated with a rightward shift in the length-tension curve; before the repeated bout, isometric strength was 6.8% lower at 30 degrees and 13.6% higher at 110 degrees compared with values before the initial bout (bout x angle: P < 0.05). Assuming that torque production at 110 degrees occurs on the descending limb of the length-tension curve, the increase in torque at 110 degrees may be explained by a longitudinal addition of sarcomeres. The addition of sarcomeres would limit sarcomere strain for subsequent eccentric contractions and may explain the repeated bout effect observed here.     

The effects of imagery training on fast isometric knee extensor torque development

Abstract

We hypothesized that imagery training would improve the fast onset of neuromuscular activation and thereby fast knee extensor isometric torque development. Forty young healthy participants, not involved in strength training, were assigned to one of four groups: physical training, imagery training, placebo training or control. The three training groups had three 15 min sessions per week for 4 weeks, with a 90° knee angle but were tested also at 120°. At 90° knee angle, maximal torque increased (～8%) similarly in all three training groups. The torque–time integral (contractile impulse) over the first 40 ms after torque onset (TTI40) increased (P < 0.05) after physical training (by ～100%), but only at 90°. This increase was significantly different from the delta values (change pre to post) in the control and placebo groups, whereas delta values in the imagery group were similar to those in the placebo group. The increases in TTI40 following physical training were related (r ² = 0.81, P < 0.05) to significant increases of knee extensor rectified surface EMG at torque onset (EMG40). In conclusion, only physical training led to a knee angle specific increase of contractile impulse that was significantly different from placebo and controls and that was related to improved onset of neuromuscular activation.     

The effects of imagery training on fast isometric knee extensor torque development

We hypothesized that imagery training would improve the fast onset of neuromuscular activation and thereby fast knee extensor isometric torque development. Forty young healthy participants, not involved in strength training, were assigned to one of four groups: physical training, imagery training, placebo training or control. The three training groups had three 15 min sessions per week for 4 weeks, with a 90 ° knee angle but were tested also at 120 °. At 90 ° knee angle, maximal torque increased (-8%) similarly in all three training groups. The torque-time integral (contractile impulse) over the first 40 ms after torque onset (TTI40) increased (P < 0.05) after physical training (by -100%), but only at 90 °. This increase was significantly different from the delta values (change pre to post) in the control and placebo groups, whereas delta values in the imagery group were similar to those in the placebo group. The increases in TTI40 following physical training were related (r (2) = 0.81, P < 0.05) to significant increases of knee extensor rectified surface EMG at torque onset (EMG40). In conclusion, only physical training led to a knee angle specific increase of contractile impulse that was significantly different from placebo and controls and that was related to improved onset of neuromuscular activation.     

Maturity status effects on torque and muscle architecture of young soccer players

ABSTRACT

This study investigated the effects of maturity status on knee extensor torque and vastus lateralis architecture of young soccer players. Thirty-four males aged 13–18 years were divided into two groups: pubescent (PUB, n = 15) and postpubescent (POSP, n = 19). Torque by angle interaction was established for absolute [F(2.649, 84.771) = 9.066, p < 0.05] and relative to body mass [F(2.704, 86.533) = 4.050, p < 0.05] isometric torque with the POSP group showing greater values. Muscle volume torque-angle relationship was similar between groups. Absolute, relative to body mass, and relative to muscle volume concentric and eccentric torque-velocity relationship showed a non-significant interaction but a significant group effect in favour the POSP group for absolute and concentric torque relative to body mass. Torque-angle and torque-velocity relationship normalized by body mass allometric exponents showed a non-significant interactions and group effects. Muscle thickness (3.6 ± 0.6 vs. 3.8 ± 0.6 cm), fascicle length (8.3 ± 1.4 vs. 8.9 ± 1.6 cm) and pennation angle (15.0 ± 2.3 vs. 14.3 ± 3.2 degrees) was similar between PUB and POSP groups, respectively. Maturity status did not show a significant effect on muscle architecture and on isometric and dynamic torques when allometrically normalized.     

Effects of muscle-tendon length on joint moment and power during sprint starts

The aim of this study was to examine the effects of muscle-tendon length on joint moment and power during maximal sprint starts. Nine male sprinters performed maximal sprint starts from the blocks that were adjusted either to 40 degrees or 65 degrees to the horizontal. Ground reaction forces were recorded at 833 Hz using a force platform and kinematic data were recorded at 200 Hz with a film camera. Joint moments and powers were analysed using kinematic and kinetic data. Muscle - tendon lengths of the medial gastrocnemius, soleus, vastus medialis, rectus femoris and biceps femoris were calculated from the set position to the end of the first single leg contact. The results indicated that block velocity (the horizontal velocity of centre of mass at the end of the block phase) was greater (P < 0.01) in the 40 degrees than in the 65 degrees block angle condition (3.39 +/- 0.23 vs. 3.30 +/- 0.21 m . s(-1)). Similarly, the initial lengths of the gastrocnemius and soleus of the front leg in the block at the beginning of force production until half way through the block phase were longer (P < 0.001) in the 40 degrees than in the 65 degrees block angle condition. The initial length and the length in the middle of the block phase were also longer in the 40 degrees than in the 65 degrees block angle condition both for both the gastrocnemius (P < 0.01) and soleus (P < 0.01-0.05) of the rear leg. In contrast, the initial lengths of the rectus femoris and vastus medialis of the front leg were longer (P < 0.05) in the 65 degrees than in the 40 degrees block angle condition. All differences gradually disappeared during the later block phase. The peak ankle joint moment (P < 0.01) and power (P < 0.05) during the block phase were greater in the 40 degrees than in the 65 degrees block angle condition for the rear leg. The peak ankle joint moment during the block phase was greater (P < 0.05) in the 40 degrees block angle for the front leg, whereas the peak knee joint moment of the rear leg was greater (P < 0.01) in the 65 degrees block angle condition. The results suggest that the longer initial muscle-tendon lengths of the gastrocnemius and soleus in the block phase at the beginning of force production contribute to the greater peak ankle joint moment and power and consequently the greater block velocity during the sprint start.     

基于人机工效学的健身动感单车鞍座高度设计研究

选取武汉体育学院普通男大学生11名,采用Vicon三维动作捕捉系统并结合表面肌电测试对受试者在不同的鞍座高度下骑行的运动学和肌肉力学指标进行同步采集,使用配对样本T检验分析比较不同高度骑行差异。结果：（1）膝关节活动范围在不同座高下呈显著变化,踝关节出现一定程度的变化,但个体差异较大。（2）鞍座高度增加,髋关节相对力矩下降,且90%座高与95%和100%座高相比存在显著性的差异（P<0.05）;膝关节相对力矩呈现出类似的变化且不同座高下存在显著差异（P<0.05）。（3）不同座高下,肌肉的放电量出现变化,但均无显著性差异（P>0.05）。其中股二头肌和胫前肌则是在95%座高时放电量达到最低。（4）随着鞍座高度的增加,髋关节和膝关节在90%、95%、100%大转子高的座高下相互之间均存在显著性的差异（P<0.05）,踝关节的角速度在90%与100%座高相比和95%与100%座高相比时差异明显（P<0.05）。结论：95%大转子高时,股二头肌和胫前肌的放电量最小,股直肌、股外侧肌和腓肠肌的放电量处于中间值,膝关节力矩和活动范围也处于中间值。选取95%大转子高的鞍座高度更利于骑行者的骑行。     

Analysis of elbow muscle strength parameters in Brazilian jiu-jitsu practitioners

Upper-body dynamic and isometric maximum strength are essential components for success in Brazilian jiu-jitsu (BJJ). This study was aimed at analysing strength parameters in the elbow flexor and extensor muscles of BJJ practitioners. Participants (n = 28) performed maximum isometric contractions of elbow flexors and extensors to determine peak torque (PT), rate of force development (RFD), and the torque–angle (T–A) relationship at elbow angles of 45°, 60°, 75°, 90°, 105°, and 120°. Additionally, concentric and eccentric PTs were measured at 1.04 rad·s^-1. Student t-test and ANOVA were performed using α = 0.05. Elbow flexors were stronger isometrically (P < 0.001, ES = 1.23) but weaker concentrically (P < 0.05, ES = 0.54) than extensor muscles, possibly because of the extensive grip disputes and pushing of opponents in BJJ. The T–A relationship had an inverted “U”-shape. Torque differences across elbow angles were moderate (ES = 0.62) for the extensor and large (ES = 0.92) for the flexor muscles. Isometric torque was greatest for elbow angles of 105° and 75° and smallest for 45° and 120° for extensor and flexor muscles, respectively. Elbow flexors had a greater RFD than extensors, regardless of elbow angle. The present study provides comprehensive results for elbow muscle strength in BJJ practitioners.     

Effects of low volume isometric leg press complex training at two knee angles on force-angle relationship and rate of force development

This study compared knee angle-specific neuromuscular adaptations after two low-volume isometric leg press complex training programmes performed at different muscle lengths. Fifteen young males were divided into two groups and trained three times per week for 6 weeks. One group (n?=?8) performed 5–7 sets of 3 s maximum isometric leg press exercise, with 4?min recovery, with knee angle at 85°?±?2° (longer muscle-tendon unit length; L-MTU). The other group (n?=?7) performed the same isometric training at a knee angle of 145°?±?2° (180°^?=?full extension; shorter muscle-tendon unit length; S-MTU). During the recovery after each set of isometric exercise, participants performed two CMJ every minute, as a form of complex training. Maximum isometric force (MIF) and rate of force development (RFD) were measured over a wide range of knee angles. Countermovement jump (CMJ) performance and maximum half-squat strength (1RM) were also assessed. Training at S-MTU induced a large increase of MIF (22–58%, p?p?p?=?0.001). In contrast, training at L-MTU, resulted in a moderate and similar (≈12.3%, p?=?0.028) improvement of force at all knee angles. CMJ performance and 1RM were equally increased in both groups after training by 10.4%?±?8.3% and 7.8%?±?4.7% (p?相似文献   

Acute effects of static stretching on characteristics of the isokinetic angle – torque relationship,surface electromyography,and mechanomyography

Abstract

The aims of this study were to examine the acute effects of static stretching on peak torque, work, the joint angle at peak torque, acceleration time, isokinetic range of motion, mechanomyographic amplitude, and electromyographic amplitude of the rectus femoris during maximal concentric isokinetic leg extensions at 1.04 and 5.23 rad · s^?1 in men and women. Ten women (mean ± s: age 23.0 ± 2.9 years, stature 1.61 ± 0.12 m, mass 63.3 ± 9.9 kg) and eight men (age 21.4 ± 3.0 years, stature 1.83 ± 0.11 m, mass 83.1 ± 15.2 kg) performed maximal voluntary concentric isokinetic leg extensions at 1.04 and 5.23 rad · s^?1. Following the initial isokinetic tests, the dominant leg extensors were stretched using four static stretching exercises. After the stretching, the isokinetic tests were repeated. Peak torque, acceleration time, and electromyographic amplitude decreased (P≤ 0.05) from pre- to post-stretching at 1.04 and 5.23 rad · s^?1; there were no changes (P > 0.05) in work, joint angle at peak torque, isokinetic range of motion, or mechanomyographic amplitude. These findings indicate no stretching-related changes in the area under the angle – torque curve (work), but a significant decrease in peak torque, which suggests that static stretching may cause a “flattening” of the angle – torque curve that reduces peak strength but allows for greater force production at other joint angles. These findings, in conjunction with the increased limb acceleration rates (decreased acceleration time) observed in the present study, provide tentative support for the hypothesis that static stretching alters the angle – torque relationship and/or sarcomere shortening velocity.     

The muscle activity paradox during circular rhythmic leg movements

A cyclist's legs make a simple 360 degrees circular and rhythmic movement, activated by a simple flexion-extension function in a sagittal plane. However, because of the simultaneous combination of leg rotation in the hip, knee and ankle joint with translation of the upper body, the general motion becomes quite complex. This complexity is increased by the anatomical interpretations of EMG readings taken during the pedalling cycle, indicating a high activity of 'flexor' muscles during the downward 'extension' of the leg (0-90 degrees propulsion phase of the pedalling cycle). This calls for an anatomical paradox. In order to verify these interpretations, the activity of six lower limb muscles was measured under field circumstances on nine elite cyclists using a portable EMG data acquisition system and active surface electrodes allowing remote (non-telemetric) monitoring of the cyclists' muscle activity patterns. Measurements were made during a 1000 m submaximal but constant effort and during a 200 m sprint. Confirmation of the anatomical paradox was found in both test circumstances. Analyses of the normalized EMG in combination with torque values of both hip and knee during the pedalling cycle indicate a zero torque at 135 degrees for the knee, while at this same angle the overall extensor activity ends in one leg and starts simultaneously in the other leg (at 315 degrees). Since the propulsion does not continue until 180 degrees, the flexor muscles have to be activated before the extension activity ends in order to generate the continuation of the circular motion until (and beyond) the bottom dead centre (180 degrees).(ABSTRACT TRUNCATED AT 250 WORDS)     

Stiffness,intralimb coordination,and joint modulation during a continuous vertical jump test

This study analysed the modulation of jump performance, vertical stiffness as well as joint and intralimb coordination throughout a 30-s vertical jump test. Twenty male athletes performed the test on a force plate while undergoing kinematic analysis. Jump height, power output, ground contact time, vertical stiffness, maximum knee and hip flexion angles, and coordination by continuous relative phase (CRP) were analysed. Analysis of variance was used to compare variables within deciles, and t-tests were used to compare CRP data between the initial and final jumps. Results showed reduction in jump height, power output, and vertical stiffness, with an increase in contact time found during the test. Maximum knee and hip flexion angles declined, but hip angle decreased earlier (10–20% of the test) than knee angle (90–100%). No changes were observed in CRP for thigh–leg coupling when comparing initial and final jumps, but the trunk–thigh coupling was more in-phase near the end of the test. We conclude that fatigue causes reduction in jump performance, as well as changes in stiffness and joint angles. Furthermore, changes in intralimb coordination appear at the last 10% of the test, suggesting a neuromotor mechanism to counterbalance the loss of muscle strength.     

The effect of knee angle on the external validity of isometric measures of lower body neuromuscular function

The aim of this study was to evaluate the effect of varying knee angle (120 degrees and 90 degrees) on the external validity of an isometric leg press test with reference to vertical jump performance. Isometric peak force (PF120 and PF90), rate of force development (RFD120 and RFD90), and maximum height reached with a squat jump and counter-movement jump were measured in 14 males. Although RFD120 was significantly correlated with squat jump and counter-movement jump performance (r = 0.71 and 0.69), and the correlations with PF120 approached statistical significance (r = 0.53 and 0.50), neither PF90 nor RFD90 was significantly related to vertical jump performance. Furthermore, although both RFD120 and PF120 were significantly different between the best five and the worst five jumpers, RFD90 and PF90 did not differentiate between individuals' vertical jump performance. We conclude that the choice of joint angle affects the external validity of isometric strength testing. Based on our results, we recommend accurate control of biomechanical specificity and assessment at different angles to find the position at which isometric strength testing is most comfortable.     

Angle-specific hamstring-to-quadriceps ratio: A comparison of football players and recreationally active males

It is currently unclear how football participation affects knee-joint muscle balance, which is widely considered a risk factor for hamstrings injury. This study compared the angle-specific functional hamstring-to-quadriceps (H:Q) ratio (hamstrings eccentric torque as a ratio of quadriceps concentric torque at the same knee-joint angle) of football players with recreationally active controls. Ten male footballers and 14 controls performed maximal voluntary isometric and isovelocity concentric and eccentric contractions (60, 240 and 400° s^?1) of the knee extensors and flexors. Gaussian fitting to the raw torque values was used to interpolate torque values for knee-joint angles of 100–160° (60° s^?1), 105–160° (240° s^?1) and 115–145° (400° s^?1). The angle-specific functional H:Q ratio was calculated from the knee flexors eccentric and knee extensors concentric torque at the same velocity and angle. No differences were found for the angle-specific functional H:Q ratio between groups, at any velocity. Quadriceps and hamstrings strength relative to body mass of footballers and controls was similar for all velocities, except concentric knee flexor strength at 400° s^?1 (footballers +40%; P < 0.01). In previously uninjured football players, there was no intrinsic muscle imbalance and therefore the high rate of hamstring injuries seen in this sport may be due to other risk factors and/or simply regular exposure to a high-risk activity.