Strength training effects in prepubescent boys

Possible changes in muscle size and function due to resistance training were examined in prepubertal boys. Thirteen boys (9-11 yr) volunteered for each of the training and control groups. Progressive resistance training was performed three times weekly for 20 wk. Measurements consisted of the following: 1 repetition maximum (RM) bench press and leg press; maximal voluntary isometric and isokinetic elbow flexion and knee extension strength; evoked isometric contractile properties of the right elbow flexors and knee extensors; muscle cross-sectional area (CSA) by computerized tomography at the mid-right upper arm and thigh; and motor unit activation (MUA) by the interpolated twitch procedure. Training significantly increased 1 RM bench press (35%) and leg press (22%), isometric elbow flexion (37%) and knee extension strength (25% and 13% at 90 degrees and 120 degrees, respectively), isokinetic elbow flexion (26%) and knee extension (21%) strength, and evoked twitch torque of the elbow flexors (30%) and knee extensors (30%). There were no significant effects of training on the time-related contractile properties (time to peak torque, half-relaxation time), CSA, or %MUA of the elbow flexors or knee extensors. There was, however, a trend toward increased MUA for the elbow flexors and knee extensors in the trained group. Strength gains were independent of changes in muscle CSA, and the increases in twitch torque suggest possible adaptations in muscle excitation-contraction coupling. Improved motor skill coordination (especially during the early phase of training), a tendency toward increased MUA, and other undetermined neurological adaptations, including better coordination of the involved muscle groups, are likely the major determinants of the strength gains in this study.     

Effects of long term Tai Chi practice and jogging exercise on muscle strength and endurance in older people

Objectives

To investigate the influence of regular Tai Chi (TC) practice and jogging on muscle strength and endurance in the lower extremities of older people.

Methods

Twenty one long term older TC practitioners were compared with 18 regular older joggers and 22 sedentary counterparts. Maximum concentric strength of knee flexors and extensors was tested at angular velocities of 30°/s and 120°/s. Ankle dorsiflexors and plantar flexors were tested at 30°/s and the dynamic endurance of the knee flexors and extensors was assessed at a speed of 180°/s.

Results

The differences in the muscle strength of the knee joint amongst the three experimental groups were significant at the higher velocity. The strengths of knee extensors and flexors in the control group were significantly lower than those in the jogging group and marginally lower than those in the TC group. For the ankle joint, the subjects in both the TC and jogging groups generated more torque in their ankle dorsiflexors. In addition, the muscle endurance of knee extensors was more pronounced in TC practitioners than in controls.

Conclusion

Regular older TC practitioners and joggers showed better scores than the sedentary controls on most muscle strength and endurance measures. However, the magnitude of the exercise effects on muscles might depend on the characteristics of different types of exercise.     

Effects of age on the reciprocal peak torque ratios during knee muscle contractions in elite soccer players

To investigate the effects of age on the reciprocal peak torque ratios during knee muscle contractions, 25 elite male soccer players, aged 22.3 +/- 3.8 yr (18-28), volunteered for the present study. The players were grouped as adult (> 21 years, n = 13) and young players (< or = 21 years, n = 12). Maximal concentric (CON) and eccentric (ECC) isokinetic thigh muscle strength was measured at angular velocities of 30 degrees, 180 degrees, 240 degrees and 300 degrees/s. ECC and CON peak torques of knee flexors (hamstring, HAM) and CON peak torques of knee extensors (quadriceps, QUA) in the dominant knee were greater (P < 0.05) in adult players than in young players at 180, 240 and 300 degrees/s. ECC HAM/CON QUA peak torque ratio at 300 degrees/s was greater (P < 0.05) for adult players compared to young players in the dominant knee but not in the nondominant knee. Furthermore, conventional HAM/QUA peak torque ratios of the dominant knee at all angular velocities for ECC contraction were higher (P < 0.05) in adult players than in young players. In conclusion, the findings of the present study indicate that the reciprocal torque ratio is influenced by age in the dominant knee of elite soccer players. Because there was no effect of age for the nondominant leg, the findings of the present study are more likely to be the result of the training background of the players than their age.     

Effects and mechanisms of strength training in children

It has been demonstrated that strength training can be organized in children in a safe and effective way. However, there is limited data regarding its impact on muscle hypertrophy. This study investigated the effects of a high-intensity strength training (HIS) on knee extensor/flexor strength, countermovement (CMJ) jumping height, postural control, soft lean mass and muscle cross-sectional area (CSA) of the dominant leg in prepubertal children. Thirty-two children participated in this study and were assigned to an intervention (INT; N=17) or a control class ( N=15). The INT participated in 10 weeks of weight-machine based HIS integrated in physical education. Pre/post tests included the measurements of peak torque of the knee extensors/flexors at 60 and 180°/s, CMJ jumping height, postural sway, soft lean mass of the leg by bioelectrical impedance analysis, and CSA (m. quadriceps) by magnetic resonance imaging. HIS resulted in significant increases in knee extensor/flexor peak torque (60°/s and 180°/s). HIS did not produce significant changes in CMJ jumping height, postural sway, soft lean mass, and CSA. Although HIS was effective at increasing peak torque of the knee extensors/flexors in children, it was unable to affect muscle size. It appears that neural factors rather than muscle hypertrophy account for the observed strength gains in children.     

Influence of acute eccentric exercise on the H:Q ratio

The purpose of the present study was to examine the effects of an acute bout of eccentric exercise on maximal isokinetic concentric peak torque (PT) of the leg flexors and extensors and the hamstrings-to-quadriceps (H:Q) strength ratio. Sixteen male (mean±SD: age=20.9±2 years; stature=177.0±4.4 cm; mass=76.8±10.0 kg) volunteers performed maximal, concentric isokinetic leg extension and flexion muscle actions at 60°·sec?-?1 before and after (24-72 h) a bout of eccentric exercise. The eccentric exercise protocol consisted of 4 sets of 10 repetitions for the leg press, leg extension, and leg curl exercises at 120% of the concentric one repetition maximum (1-RM). The results indicated that the acute eccentric exercise protocol resulted in a significant (P<0.05) decrease in isokinetic leg flexion (13-19%) and leg extension (11-16%) PT 24-72 h post-exercise. However, the H:Q ratios were unaltered by the eccentric exercise protocol. These findings suggest that an acute bout of eccentric exercise utilizing both multi - and single - joint dynamic constant external resistance (DCER) exercises results in similar decreases in maximal isokinetic strength of the leg flexors and extensors, but does not alter the H:Q ratio.     

Testing of isokinetic muscle strength in the ankle

The purpose of this study was to evaluate isokinetic strength measurement in ankle joint. The study was divided into three parts. A comparison of dorsal and plantar flexion with and without consideration to the biomechanical circumstances was done. The series comprised 15 men and 10 women. A comparison of strength measurements of dorsal flexion with and without fixation of the upper trunk. The series consisted of 10 men. A reference series for dorsal and plantar flexion comprising 15 men (mean age = 34 +/- 9 yr) was used for comparison. Strength was tested at 0 degree, 15 degrees, 30 degrees, 60 degrees, 120 degrees, 180, and 240 degrees/s. The readings made without biomechanical adjustment were significantly higher than connected readings. Mean difference at 30 degrees/s was 8.4 +/- 7.1 Nm for dorsal flexion and 18 +/- 3.1 Nm for plantar flexion. The torque values were significantly higher when the upper trunk was not immobilized, and higher standard deviations were obtained. The left (viz. dominant) leg produced significantly higher muscle torque at all velocities for plantar flexors and at all but 180 degrees and 240 degrees/s for dorsal flexors. The regression coefficient was used as a measure of peak torque velocity relation, the coefficient was -0.50 +/- 0.16 for plantar flexion and -0.22 +/- 0.06 for dorsal flexion.     

Isometric and isokinetic knee joint performance in Japanese alpine ski racers.

Isometric and isokinetic (concentric and eccentric, strength of alpine skiers with different performance levels were measured. Nine national (elite, EG) and 10 collegiate (trained, TG) female alpine skiers (16 to 23 years of age) performed maximal voluntary knee extension and flexion. Peak torque was measured at an angular velocity of 30 deg.s-1. The cross-sectional area (CSA) of thigh muscles (quadriceps and hamstring muscles) was determined by an ultrasonic method. No significant differences in anthropometric variables and CSA were observed between EG and TG. EG had significantly greater (p < 0.01 for extensors and p < 0.05 for flexors) eccentric knee extensor and flexor strength than that of TG whereas no significant differences were noted in isometric and concentric strength. Eccentric strength/CSA ratio was also higher for EG than for TG. It was suggested that knee extension and flexion strength during eccentric muscle action might be related to the performance level of alpine skiers.     

Isokinetic profile of dorsiflexors and plantar flexors of the ankle--a comparative study of élite versus untrained subjects.

A comparative study was made of the isokinetic characteristics of the ankle (plantar-flexion and dorsiflexion) in young men. Six cyclists, seven gymnasts, 10 soccer players and 25 non-athletic young men were tested on the Cybex II+ dynamometer. Peak torque, torque acceleration energy (TAE), total work and average power were measured. Cyclists had slightly higher (5%) mean plantar flexion than the others, but this was not significant. The situation was reversed for dorsiflexion. Moreover, the average dorsiflexion per unit of plantar flexion was significantly higher in the gymnasts than it was in the cyclists for both torque and work. This suggests that at a specific level of plantar flexion, the gymnasts had stronger dorsiflexion compared with the cyclists and that in sports involving jumping and running, increased attention should be given to strengthening the antagonist muscle groups (dorsiflexors) in order to achieve greater agonist-to-antagonist muscle balance thus preventing injury. The non-athletic subjects had substantially lower endurance capability in both flexors as measured by the endurance ratio. This implies that identifiable specialization in particular muscles results from training or participating in specialized sports.     

Characteristics and preliminary observations of the influence of electromyostimulation on the size and function of human skeletal muscle during 30 days of simulated microgravity

During 30 days (d) of bedrest, the practicality of using electromyostimulation (EMS) as a deterrent to atrophy and strength loss of lower limb musculature was examined. An EMS system was developed that provided variable but quantifiable levels of EMS, and measured torque. The dominant leg of three male subjects was stimulated twice daily in a 3-d on/1-d off cycle during bedrest. The non-dominant leg of each subject acted as a control. A stimulator, using a 0.3 ms monophasic 60 Hz pulse waveform, activated muscle tissue for 4 s. The output waveform from the stimulator was sequenced to the knee extensors (KE), knee flexors (KF), ankle extensors (AE), and ankle flexors (AF), and caused three isometric contractions of each muscle group per minute. Subject tolerance determined EMS intensity. Each muscle group received four 5-min bouts of EMS each session with a 10-min rest between bouts. EMS and torque levels for each muscle action were recorded directly on a computer. Overall average EMS intensity was 197, 197, 195, and 188 mA for the KE, KF, AF, and AE, respectively. Overall average torque development for these muscle groups was 70, 16, 12, and 27 Nm, respectively. EMS intensity doubled during the study, and average torque increased 2.5 times. Average maximum torque throughout a session reached 54% of maximal voluntary for the KE and 29% for the KF. Reductions in leg volume, muscle compartment size, cross-sectional area of slow and fast-twitch fibers, strength, and aerobic enzyme activities, and increased leg compliance were attenuated in the legs which received EMS during bedrest.(ABSTRACT TRUNCATED AT 250 WORDS)     

优秀击剑运动员下肢三关节等速肌力测试分析

目的:研究我国优秀击剑运动员下肢髋、膝、踝三关节肌群等速肌力表现,找出薄弱肌群。方法:国家击剑队备战2008年奥运会重点队员26名,男女各13名,年龄23.1±2.36岁。在60°/s、240°/s速度下测定运动员双侧下肢髋、膝、踝屈伸肌群等速向心峰力矩。结果发现:(1)髋关节:在240°/s下双侧伸肌峰力矩均明显低于60°/s(P<0.05),60°/s和240°/s下双侧髋关节屈肌峰力矩无显著差异(P>0.05);男运动员两个速度下前腿伸肌峰力矩均显著高于后腿(P<0.05),女运动员无显著差异(P>0.05);两个速度下,运动员双腿屈伸肌峰力矩比值低于正常。(2)膝关节:60°/s时男运动员双腿屈伸肌峰力矩有显著差异(P<0.05),240°/s时双侧伸肌峰力矩有显著差异(P<0.05),女运动员两个速度下均无显著差异(P>0.05);60°/s时,女运动员前后腿屈伸肌峰力矩比值分别为0.56和0.54,男运动员分别为0.63和0.67;240/°s时男女前后腿屈伸肌峰力矩比值均在正常范围。(3)踝关节:60°/s时男运动员前腿踝关节屈伸肌峰力矩显著大于后腿(P<0.05),240°/s时无显著差异(P>0.05);两个速度下,女运动员踝关节屈伸肌峰力矩均无显著性差异(P>0.05);运动员踝关节屈伸峰力矩比值在两个速度下均低于正常。结论:优秀男子击剑运动员下肢关节等速肌力双侧不对称,女运动员不对称表现不明显;击剑运动员下肢薄弱肌群为后腿髋关节伸肌群、股后肌群、前腿股后肌群及踝关节背伸肌群。     

Knee flexor strength and rate of torque development deficits in women with patellofemoral pain are related to poor objective function

BackgroundDeficits in knee flexor strength and rate of torque development (RTD) might be present in women with patellofemoral pain (PFP). In addition, maximal strength and RTD of the knee flexors and extensors might be related with subjective and objective function in women with PFP. However, both conjectures are still poorly understood.Research questionDo women with PFP have deficits in the maximal strength and RTD of the knee flexors and extensors during isometric, concentric, and eccentric contractions? Is there a relationship between maximal strength and RTD of the knee flexors and extensors with subjective and objective function in women with PFP?MethodsFifty-six women with, and 46 women without, PFP participated. Maximal strength and RTD (to 30% and 60% maximal torque) during isometric, concentric, and eccentric contractions of the knee flexors and extensors were assessed using an isokinetic dynamometer. Objective assessment included single leg hop test (SLHT) and forward step-down test (FSDT). Subjective assessment involved the anterior knee pain scale.ResultsWomen with PFP had small to large deficits in maximal strength and RTD of the knee flexors and extensors during isometric, concentric and eccentric contractions (Effect sizes: -0.43 to -1.10; p ≤ 0.016). Small to moderate correlations of maximal concentric and eccentric knee flexor strength and RTD with SLHT and FSDT (r = 0.28 to 0.41; p ≤ 0.037) were identified. Subjective or objective function were not correlated with maximal isometric knee flexor strength and RTD, or any knee extensor measures (p > 0.05).SignificanceMaximal strength and RTD deficits of the knee flexors and extensors were identified in this female PFP cohort, but they were unrelated to subjective function. The relationship of concentric and eccentric knee flexor strength and RTD deficits with poor objective function should be considered in future exercise trials for women with PFP.     

Effect of hamstring flexibility on isometric knee flexion angle-torque relationship

The purpose of this study was to examine the relationship between hamstring flexibility and knee flexion angle–torque relationship. Hamstring flexibility was assessed in 20 subjects (10 men, 10 women) using the straight leg raise (SLR) and active knee extension (AKE) tests. Isometric knee flexion strength was measured at five knee flexion angles while subjects were seated with the test thigh flexed 40° and the trunk flexed 80°. Lower extremities were classified as tight or normal based on the SLR and AKE tests. Peak knee flexion torque, angle of peak torque, and angle–torque relationship were compared between flexibility groups. Peak knee flexion torque was not different between tight and normal groups (SLR P =0.82; AKE P =0.68) but occurred in greater knee flexion (shorter muscle length) in the tight group compared with the normal group (SLR P <0.01; AKE P <0.05). The tight group had higher torque than the normal group at the shortest muscle length tested but lower torque at longer muscle lengths (SLR P <0.001; AKE P <0.001). In conclusion, the angle–torque relationship was shifted to the left in less flexible hamstrings such that knee flexion torque was increased at short muscle lengths and decreased at long muscle lengths when compared with more flexible hamstrings.     

Velocity‐specific and time‐dependent adaptations following a standardized Nordic Hamstring Exercise training

The Nordic Hamstring Exercise (NHE ) is effective for selective hamstring strengthening to improve muscle balance between knee flexors and extensors. The purpose of this study (within subject design of repeated measures) was to determine the effects of a standardized 4‐week NHE training on thigh strength and muscle balance with concomitant kinetic and kinematic monitoring. Sixteen male sprinters (22 years, 181 cm, 76 kg) performed a standardized 4‐week NHE training consisting of three sessions per week (each 3×3 repetitions). Six rope‐assisted and six unassisted sessions were performed targeting at a constant knee extension angular velocity of ~15°/s across a ~90‐100° knee joint range of motion. Kinetic (peak and mean moment, impulse) and kinematic parameters (eg, ROM to downward acceleration, ROM_DWA ) were recorded during selected sessions. Unilateral isokinetic tests of concentric and eccentric knee flexors and extensors quantified muscle group‐, contraction mode‐, and velocity‐specific training adaptations. Peak moments and contractional work demonstrated strong interactions of time with muscle group, contraction modes, and angular velocities (η²>.150). NHE training increased eccentric hamstring strength by 6%‐14% as well as thigh muscle balance with biggest adaptations at 150°/s 2 weeks after NHE training. Throughout the training period significant increases (P <.001) of peak (η²=.828) and mean moments (η²=.611) became apparent, whereas the impulse and the ROM_DWA of unassisted NHE repetitions remained unchanged (P >.05). A 4‐week NHE training significantly strengthened the hamstrings and improved muscle balance between knee flexors and extensors. Despite the slow training velocity, biggest adaptations emerged at the highest velocity 2 weeks after training ended.     

Facilitation of quadriceps activation is impaired following eccentric exercise

Contracting the knee flexor muscles immediately before a maximum voluntary contraction (MVC) of knee extension increases the maximal force that the extensor muscles can exert. It is hypothesized that this phenomenon can be impaired by muscle fiber damage following eccentric exercise [delayed onset muscle soreness (DOMS)]. This study investigates the effect of eccentric exercise and DOMS on knee extension MVC immediately following a reciprocal‐resisted knee flexion contraction. Electromyography (EMG) was recorded from the knee extensors and flexors of 12 healthy men during knee extension MVCs performed in a reciprocal (maximal knee extension preceded by resisted knee flexion), and nonreciprocal condition (preceded by relaxation of the knee flexors). At baseline, knee extension MVC force was greater during the reciprocal condition (P < 0.001), whereas immediately after, 24 and 48 h after eccentric exercise, the MVC force was not different between conditions. Similarly, at baseline, the EMG amplitude of the quadriceps during the MVC was larger for the reciprocal condition (P < 0.001). However, immediately after, 24 and 48 h postexercise the EMG amplitude was similar between conditions. In conclusion, eccentric exercise abolished the facilitation of force production for the knee extensors, which normally occurs when maximum knee extension is preceded by activation of the knee flexors.     

Isokinetic peak torque and hamstring/quadriceps ratios in young basketball players. Effects of age, velocity, and contraction mode

AIM: The purpose of this study was to provide an extensive isokinetic profile, including peak torque and "functional" and "conventional" muscle group ratios, of basketball players throughout the developmental years 12-17. Furthermore, we investigated the effects of age and angular velocity on peak torque and reciprocal muscle group ratios, and observed the force-velocity pattern. METHODS: Basketball players (n=180) 12 to 17 years. were divided according to their age in 6 equal (n=30) groups aged 12, 13, 14, 15, 16, 17 years. All subjects performed maximum knee extension and flexion efforts at 60 inverted exclamation mark /s and 180 inverted exclamation mark /s angular velocities under eccentric and concentric conditions. The absolute (Nm) peak torque was measured, and the relative peak torque (PT/BW) and "functional" and "conventional" reciprocal muscle group ratio (%) were calculated. RESULTS: PT/BW values of knee flexors ranged from 0.98 to 2.29 Nm/kg and of extensors from 1.53 to 3.69 Nm/kg, across all age, velocity, and muscle action levels. PT/BW was higher at slow speed and during eccentric contraction irrespective of age and angular velocity/muscle action. The results indicated a significant (p<0.05) age-related increase in peak torque independent of changes in BW. The force-velocity patterns were similar across ages. The "functional" and "conventional" ratios were not affected by age, but increased with increase in angular velocity, except the CON(KF)/ECC(KE) ratio that was higher (p<0.05) at slow speed. CONCLUSION: The present study provides an extensive isokinetic profile of a knee joint in basketball players throughout developmental years 12-17. Coaches and physical therapist may use these information when evaluating and planning exercise programs during training and rehabilitation.     

Calf muscle strength in humans

In an effort to measure strength characteristics of the calf muscles, 18 subjects (14 male, 4 female, age =34.3+/-2.4yrs) were tested using a specially designed torque velocity device (TVD). This TVD is a hardware interface with the subject's lower leg which stabilizes the leg for calf muscle strength measurements. Calf muscle strength measurements consisted of 1) isometric force production at ankle angles of 80, 90, and 100 degrees of plantar flexion, 2) peak torque at six isokinetic angular velocities 0.52, 1.05, 2.09, 3.14, 4.19, and 5.24 rad x s(-1), and 3) a fatigue test consisting of 30 maximal contractions at 3.14 rad x s(-1). The greatest force production occurred at 80 degrees of ankle plantar flexion (148.5 +/- 40.2 Nm). Isokinetic force production ranged from 114.1 +/- 24.7 Nm at 0.52 rad x s(-1) to 16.8 +/- 6.5 Nm at 5.24 rad x s(-1). A fatigue test consisting of 30 maximal repetitions at 3.14 rad x s(-1) resulted in a 61 +/- 15% decline in force production. To assess reproducibility and day to day variation, measurements at 1.05 and 2.09 rad x s(-1) were made during five different trials in a single day and one trial per day for three days, respectively. The within subject coefficient of variation was 2.6 to 6.5% for reproducibility and 1.9 to 7.4% for day to day variation. Magnetic resonance imaging (MRI) of the lower limb and muscle biopsy specimens from the gastrocnemius (lateral head) and soleus muscles were obtained to examine the relationship between strength and morphological characteristics of the calf muscles. Cross-sectional area of the primary plantar flexors (gastrocnemius and soleus) was 47.9 +/- 1.3 cm2 while muscle volume was 642 +/- 16 cm3. Muscle fiber composition of the gastrocnemius and soleus averaged 57 +/- 2 and 85 +/- 3% type I fibers, respectively. A poor correlation was found between fiber type and maximal isometric force production (r =0.38; p>0.05). However, calf muscle strength and muscle size was positively correlated (r = 0.76; p < 0.05). These data indicate that using the TVD interface to stabilize the lower leg is a reliable and reproducible procedure for the measurement of calf muscle strength.     

Muscle strength and fatigue during isokinetic exercise in individuals with multiple sclerosis

PURPOSE: To compare muscle strength and muscle fatigue of the knee extensors and flexors in individuals with multiple sclerosis (MS) and non-MS control subjects and to evaluate the reliability of muscle strength and muscle fatigue testing in these individuals. METHODS: Thirty individuals (13 women and 2 men for both MS and control groups), age (mean +/- SD) 38.8 +/- 10 for MS and 33.1 +/- 7.6 yr for controls, participated in this investigation. Peak torque was measured on two occasions separated by approximately 7 d at 30, 60, 90, 120, 180 degrees.s(-1) with 2 min of recovery between each bout. The nondominant leg was tested followed by the dominant leg after 10 min of recovery. Subjects then performed three bouts of 30 flexions and extensions of the dominant leg at 180 degrees.s(-1) with 1 min of recovery between bouts. RESULTS: The reliability of muscle torque was very high for individuals with MS (only 1 of 20 measurements with an ICC below 0.900). Total work was also highly reliable for MS, but the Fatigue Index (work during the last 15 contractions/work during the first 15 contractions) x 100 was not. Peak torque adjusted for age, body mass, and fat free mass (measured by whole body plethysmography; the Bod Pod; Life Measurement Instruments; Concord, CA) was significantly greater for controls than for MS for three of four lower body muscle groups tested. For the muscle fatigue test (3 bouts of 30 knee extensions and flexions at 180 degrees.s(-1)), the Fatigue Index was greater (less fatigue) for the knee extensors for controls than MS for the third bout. For flexion, the Fatigue Index was greater for controls than MS over the three bouts (group effect). Total work was significantly greater for controls than MS for the flexors (group effect) and approached significance for the extensors. CONCLUSIONS: Individuals with MS were weaker than controls when data were adjusted for age, body mass, and fat free mass. This latter finding (force relative to age and fat free mass) suggests that there is a reduced ability to activate muscle mass in MS and/or the muscle mass of individuals with MS is of lower quality (i.e., reduced force/unit muscle mass) than controls.     

Muscle determinants of bone mass, geometry and strength in prepubertal girls

PURPOSE: The aim of this study was to compare the relative contribution of peak muscle force (isokinetic peak torque) with surrogate estimates of muscle force, including leg lean tissue mass (LTM) and vertical jump height (VJH), on bone mass, geometry and strength in healthy prepubertal girls (n = 103). METHODS: Total leg and FN BMC and leg LTM were measured by DXA; the hip strength analysis program was used to assess FN diameter, cross-sectional area (CSA) and section modulus (Z). Isokinetic peak torque of the knee extensors and flexors (60 degrees .s) were used as direct measures of peak muscle force. VJH was measured as an estimate of neuromuscular function. Total leg length or femoral length was used as a surrogate measure of moment arm length. RESULTS: All estimates of muscle function, except VJH, were positively associated with leg BMC (r = 0.72 - 0.90) and FN BMC, geometry and strength (r = 0.35-0.65) (all, P < 0.001). Multiple linear regression analyses revealed that leg LTM and isokinetic peak torque were independently and equally predictive of leg BMC and FN BMC, bone geometry and strength, explaining 8 to 28% of the variance in each of the bone traits after accounting for moment arm length. When isokinetic peak torque was corrected for both leg LTM and moment arm length, it remained an independent predictor of BMC, CSA and Z, but only accounted for an additional 2 to 5% of the variance. CONCLUSION: These data suggest that DXA-derived leg LTM can be used as a reasonable surrogate for isokinetic peak muscle forces when assessing bone strength in relation to muscular function in healthy pre-pubertal girls.     

Gain in strength and muscular balance after balance training.

The isolated effect of balance training on muscle strength of the flexors and extensors of the knee, without accompanying strength training, has not been addressed in the past. Effects of a balance training program alone were compared to a strength training program. Balance and strength training were performed by 15 persons each for 6 weeks including 12 training units of 25 min. Balance training was performed on instability training devices such as rolling board, mini trampoline and large rubber ball. The 15 persons of the strength training group trained on machines for leg curls and on leg presses for 25 min per unit. Measurements for balance were performed with one-leg balance on a narrow edge and a tilting stabilometer for 30 s; maximum isometric strength was measured using an isokinetic device for each leg separately. The muscular balance between dominant and non-dominant leg was calculated. Strength gain was similar for the flexors and extensors in both groups. One-leg balance improved after balance training (P< 0.01) with a 100% increase over the strength training group (P < 0.05) and the stabilometer test for each person in the balance (P < 0.01), but not in the strength training group. In the balance group the initial difference between right and left diminished. The results indicate balance training to be effective for gain in muscular strength, and secondly, in contrast to strength training, equalisation of muscular imbalances may be achieved after balance training.